Mineral Oils and Waxes.
This class of bodies is totally different in chemical constitution from the true oils and waxes, containing neither glycerides, fatty acids nor alcohols, but consisting of carbon and hydrogen only, approximately in the proportion of one atom of the former to two of the latter. They occur in underground lakes, from which they are obtained by springs or borings; or in shales, from which they are separated by distillation. It is commonly supposed that they have been formed, at some remote period of the earth’s history, by the decomposition of animal and vegetable matters, at a high temperature and under great pressure.[167]
[167] Oils from wells or springs are technically called “petroleum oils,” those from shale, “paraffin” oils, but chemically, there is no definite distinction.
The mineral oils and waxes are largely capable of being distilled without decomposition, but if heated to high temperatures, are readily “cracked” or broken up into simpler and generally more volatile compounds—a fact which is employed in the production of gas, and the utilisation of some of the heavier products.
They differ greatly in their gravity and boiling-point, but not much in their ultimate composition, consisting largely of saturated or nearly saturated hydrocarbons (cp. [p. 354]), and hence are little liable to oxidation, and acted on by few chemical reagents. From their constitution they are of course unsaponifiable, and in this way can be separated from fats and oils with which they have been mixed. (For particulars of the method see L.I.L.B., [p. 178].)
The heavier mineral oils are a good deal used in mixture with other oils and fats, for stuffing leathers, those of a specific gravity of 0·880-0·900 being usually most suitable. They are quite incapable of “spueing,” and are useful in lessening that tendency in other oils with which they are mixed. They have not, however, the same affinity for the leather fibre as some of the true oils, and are to a certain slight extent volatile, and should generally be used in mixture, rather than alone.
Most mineral oils, when held so that a strong light (daylight or electric light rich in ultra-violet rays) falls upon them, show a green or violet fluorescence or “bloom.” This is very persistent, even when the oil is mixed with a large volume of other oils, and is often relied upon as a means of detecting them when used as adulterants. The test is, however, not infallible, since the effect is due to impurities which may be removed by purification, or masked by the addition of such substances as nitrobenzene or nitronaphthalene, and it also occurs in the hydrocarbon products produced in the distillation by steam of animal oils, and is occasionally seen to some extent even in oils which have not undergone distillation.
Vaseline and Vaseline Oil are the most viscous and densest of the petroleum oil products. They probably differ from the solid paraffins in chemical constitution, though their ultimate composition is almost the same. They are often useful constituents of stuffing greases.
Paraffin Wax consists of a mixture of hydrocarbons similar in chemical constitution to the paraffin and petroleum oils, but of higher boiling point, and solid at ordinary temperatures. Its hydrocarbons are mostly saturated, and hence very stable bodies, and little liable to oxidation. They are completely unsaponifiable, and unaffected by boiling with alcoholic potash, and in most cases by boiling with strong sulphuric acid, by which they may be separated from animal and vegetable waxes or fats with which they have been mixed. They are quite incapable of resinising by oxidation, or of causing “spueing” in leather. They are soluble in petroleum spirit, carbon disulphide and most of the ordinary solvents of fats, but insoluble in alcohol.
Paraffin wax separates from the liquid oils by crystallisation on cooling, and the remaining liquid which adheres is removed by hydraulic pressing, as in the case of tallow. The hardness and melting point vary according to the extent to which the pressing has been carried, and the temperature at which it has been done. The paraffins of higher melting point are as a rule the more costly.
Pure paraffin wax is a white, more or less hard and brittle substance which does not melt so easily as ordinary fats, and is on this account used in stuffing certain kinds of leather, hardening the stuffing grease, and making the leather feel less oily. When melted, paraffin wax forms a thin liquid, more resembling an ordinary petroleum lamp oil than the viscous vaselines and leather oils. On ignition it burns with a bright somewhat smoky flame, and leaves no ash behind. It is found on analysis when mixed with other waxes or oils in the “unsaponifiable matter” (see L.I.L.B., p. 178).
Ozokerit is a natural paraffin material used for the manufacture of cerasin candles, which sometimes occurs in the vicinity of petroleum springs, especially in Galicia. It is of pale yellow colour when pure, and has then a melting point of about 70° C. Its chief impurities are petroleum oils, water and clay. These are removed by melting the ozokerit, decanting off the clear oil, and filtering it through fine animal charcoal. If liquid oils are present the material is treated with alkali or with strong sulphuric acid, and is pressed before filtering through charcoal. The refined product is termed “cerasin,” and is of a more waxy and less crystalline texture than ordinary paraffin wax.
The Resin Oils are derived from resins, and mainly from colophony or common pine rosin, by destructive distillation. Their specific gravity ranges from 0·96 to 0·99, but their chemical composition is very imperfectly understood, and appears to be by no means constant. Like the mineral oils they are “unsaponifiable,” but often contain small amounts of soap-forming material (resin acids).
The detection and estimation of resin oils is often a matter of considerable difficulty, but further particulars on this point will be found in L.I.L.B., p. 180. From their cheapness, they are considerably employed as adulterants of other oils, and their high gravity makes them convenient to adjust the gravity of mineral oils when used for this purpose, as the latter are usually lighter than the fatty oils. As currying oils, they are not particularly suitable, though often employed in stuffing picker bands, and other heavily greased leathers. They have considerable antiseptic powers, and for this reason are useful in leather greases, preventing heating, and checking mildews.
Resin itself is occasionally used as an addition to stuffing greases, and is said to increase the waterproofness of the leather, and to give it a drier feel. In mixture with about half its weight of paraffin wax, and with a little grease if necessary to soften the mixture, it is often used in waterproofing mixtures, which can be made to melt at 50° to 60° C. Leather will bear immersion in the melted mixture without scalding if thoroughly dried in a hot stove at a temperature of not less than 50° C. before dipping. Any great increase of the proportion of paraffin wax causes the rosin to separate. Rosin consists mainly of free acids which easily combine with alkalies and alkaline carbonates in boiling. It is hence largely used in the manufacture of soaps on account of its cheapness, and to render them more soluble in water. The rosin acids are not so strong as many of the fatty acids, and rosin soaps are therefore somewhat strongly alkaline. Rosin soap, precipitated among the ground paper pulp in the rag engine, by addition of alum or sulphate of alumina, is largely used as a sizing for common papers.
CHAPTER XXIV.
OIL TANNAGES, AND THE USE OF OILS AND FATS IN CURRYING.
The conversion of skin into leather by the agency of oils and fats is probably one of the most primitive methods, and is used in different ways suited to the skins and fats which are available, by savage races in all quarters of the globe. In its simplest form, it consists merely in oiling or greasing the wet skin, and kneading and stretching it as it slowly loses moisture and absorbs the fat. Under these conditions, the fibres become coated with a greasy layer, which prevents their adherence after they are once separated by the mechanical treatment. At the same time some chemical change takes place in the fibre itself, which has a part in its conversion into leather varying in importance according to the method and fat employed, and of which the chemistry will be best discussed after some slight sketch has been given of the methods themselves.
The most complete sort of oil-leather is that produced by “chamoising,” or oil-dressing with marine oils, a process applied to the ordinary “chamois” or “wash-leathers” (now made from the flesh-split or “lining” of the sheep-skin), and to the manufacture of “buff-leather” for military purposes. The process varies somewhat according to the character of the leather, but the manufacture of the common wash-leather may be taken as a type. For this purpose the sheep-splits are freed from the loose and fatty middle layer ([p. 51]) by “frizing” with a sharp knife on a beam similar to that used for fleshing ([Fig. 30], [p. 147]), but much more steeply inclined. The process is rather one of scraping than cutting, and was originally adopted to remove the grain from the deer-skins which were largely used for glove-leathers, since oil-dressing does not easily penetrate a skin with the grain surface intact. The fleshes are usually delimed by drenching, but removal of fat is unimportant. After being well drained, they are “stocked” for some time with sawdust till they become partially dry and porous, the common “faller” stocks shown in [Fig. 22], [p. 116], being generally employed. During the stocking, care must be taken that the goods are not overheated by the friction produced. When the skins have become opaque from the inclusion of air between the fibres, they are, according to the Continental method, shaken out and oiled on the table, and after folding into bundles, are put back in the stocks. In England, the oil is usually added to them during the stocking, in small quantities, which become rapidly and evenly distributed by the motion of the skins. In England, cod oil is almost exclusively employed, but on the Continent, a considerable proportion of seal and whale oils is used. As the goods are apt to heat, not only from friction, but from the oxidation of the oils employed, they are removed from the stocks at intervals, and allowed to cool, usually hung on hooks exposed to the air. In France this exposure to the air is much more considerable than in England, the skins being hung for eight or twelve hours after each stocking. The drying rooms are kept moderately warm, and a good deal of oxidation of the oil takes place in them, which materially affects the character of the product, and especially of the residual oil or dégras, which is afterwards squeezed out of the skins and used for currying ([p. 368]). Great care is required to prevent any parts of the skins becoming dry before they are completely saturated by the oil, which causes hard and transparent patches which the oil will not afterwards penetrate. After each exposure to the air, the skins are oiled on the table and returned to the stocks. The stocking has to be continued for many hours, even for wash-leather; and as it proceeds, the skins lose the smell of limed skin, and acquire a peculiar mustard-like odour from the volatile products of oxidation of the oils. When the skins are completely saturated, they are, according to the English method, packed in boxes, and allowed to heat spontaneously by oxidation of the oils, during which great care is required, especially at the outset, that the heat does not rise so high as to destroy the skins. To prevent this, they are removed at intervals from the boxes and spread on the floor to cool, and then re-packed, and this treatment is continued until the oxidation is complete, and the skins cease to heat. During the heating, large quantities of volatile and very pungent products are given off, and especially acrolein (acrylic aldehyde, from the dehydration of the glycerine), which is excessively irritating to the eyes. The German method is not unlike the English, but in France, the packing in boxes is omitted, and the oxidation is completely effected in warm stoves in which the goods are hung on hooks. The heating in this case is much more moderate, and the oil less thickened, a result which may be partly due to the different oils employed, and which leads to differences in the subsequent treatment of the leather.
In the French process, the oily skins are dipped in hot water and wrung or hydraulic pressed, the expressed oil constituting moellon or dégras ([p. 368]), and the skins are afterwards washed in a hot soda or potash solution, from which a further portion of an inferior dégras is recovered. In the old-fashioned English method, the oil became so thickened that it could not be pressed out, and the whole was removed by washing with soda or potash solution, from which it was recovered by the use of acid, constituting “sod oil” ([p. 369].) Now many English manufacturers adopt a modified method, and remove a good deal of their oil by pressure.
Buff leather, much used for military accoutrements, is made in a similar manner to chamois, from ox or cow hides, the grain of which is frized off. The bleaching, both of buff and chamois, is done by exposing to the sun in a damp condition, the skins being watered as required with water or fat-liquor, or the alkaline emulsion of dégras obtained in washing the skins. It may also be bleached by oxidising agents, such as permanganate of potash or acidified sodium peroxide. If permanganate is used, the leather is treated in a solution of perhaps 5 grm. per liter till of a deep brown colour, and then in a solution of sulphurous or oxalic acid till the colour is removed.
Messrs. J. and E. Pullman, of Godalming, make a species of buff leather, which they style “Kaspine” leather, by treating limed and drenched hides or skins in a drum with a very dilute solution of formaldehyde (“formalin”) rendered alkaline with sodium carbonate (Eng. Pat. 2872, 1898). The change to leather takes place very rapidly, and the leather is afterwards treated with soap solutions of fat-liquors, to feed and soften it. It is almost indistinguishable from genuine buff leather, except from the fact that it is white throughout, and needs no bleaching. It is finding considerable application for military purposes.
A type of leathers which bear a close chemical relation to oil-leathers, is that including “Crown,” “Helvetia,” and fat-tanned leathers. The first leather of the sort was invented by a German cabinet-maker named Klemm, by whom the secret was sold to Preller, who manufactured it in Southwark, under the name of “Crown” leather. Klemm used flour, ox-brains, butter, milk, and soft fat, which was made into a paste with water, and spread on the limed, drenched and partially dried skins, which were rolled into bundles, and drummed in slightly warmed drums for some hours; taken out, again dried slightly, and coated with the mixture, and again drummed. For thick hides the process was repeated a third time, drumming in each operation for about eight hours. The leather was used for laces, picker-bands, light belts, and other purposes where great toughness and flexibility were required. It was found by further experience (if indeed, it was not known to Klemm himself) that the only really essential ingredients of the mixture were the soft fats and flour; and even the latter could, for some sorts of leather, be dispensed with. It was further ascertained that only the gluten or albuminous part of the flour was absorbed by the leather, the starch serving mainly to facilitate the emulsification of the fats. The proportions used in the paste are about seven parts of flour, seven parts of soft fat such as horse grease, two parts of tallow, four parts of water, and a little salt or nitre to act as an antiseptic. Other greases, such as mixtures of tallow and oil, can be substituted for the horse grease, and pipe-clay or ochre may to some extent take the place of the flour, while soap may also be added. The similarity of the mixtures used to the tawing paste in calf- and glove-kid dressing ([pp. 191], [196]) is obvious, and Klemm had an earlier process in which the operation just described was preceded by a slight alum tannage, and which was almost identical in its detail with the methods now in use for the production of so-called “raw-hide.” On the other hand it is nearly allied to the production of “Riems,” or raw-hide straps in South Africa, for which a long thong is cut spirally from a hide, and wound into a sort of skein which is suspended from a crossbar, with a heavy weight at its lower end, and oiled and twisted, with frequent changes of position, until the water is dried out, and the thong is saturated with fat, forming a very tough and durable leather. A similar material can be made by fulling or otherwise working grease into a raw hide prepared for tanning. Eitner examined samples of “Crown-leather” chemically, by removing the gluten of the flour with an alkaline solution, and found that an imperfectly chamoised leather remained, which when restuffed with fat, was much less full, and carried a much smaller quantity of grease than before.[168]
[168] Gerber, 1878, p. 2.
Various theories have been proposed to explain the reaction which takes place in the production of oil-leathers. Knapp supposed that it was merely a case in which the smallest fibrils of the hide were coated with the products of the oxidation of oils, and so prevented from adhering together, and protected from the action of water by the sort of waterproof coating which was formed. This explanation is scarcely feasible in the face of the fact that chamois leather can be treated even with hot dilute solutions of the caustic alkalies without destruction, while cotton fibres waterproofed by treatment with drying oils have their coating entirely removed by treatment with alkalies. Lietzmann supposed that the whole of the gelatinous fibres were removed in the liming and subsequent treatment, and that the finished leather consisted only of the skeleton of yellow or elastic fibre which exists in the skin, and which is remarkable for its resistance to heat, acids and alkalis. Unfortunately the proportion of these fibres does not exceed about 6 per cent. of the total, so that they are quite insufficient to account for the production of the leather. We now know, however, that aldehydes, including the acryl-aldehyde, which is evolved in the oil oxidation of chamoising (and which is covered by Messrs. Pullman’s patent) are capable in themselves of converting gelatinous substances into a material identical in its properties, and especially in its power of resisting hot water and alkaline solutions, with the fibre of chamois leather. In all cases where perfect chamoising is produced, intense oxidation takes place, and oxidisable oils are used which will evolve acrylic and other aldehydes. Where oils of little drying power are employed, as in the case of Crown- and other fat-leathers, only an imperfect chamoising occurs, and we are therefore justified in attributing the special qualities of chamois leather to a natural aldehyde tannage. On the other hand, there is no doubt that the coating of the fibres with oxidised oil-products really occurs, and is probably a powerful factor in the leathering of Crown-leather, and other similar products which are not washed out with alkaline solutions. Knapp proved by treating raw pelt which had been dehydrated with alcohol ([p. 74]) with a very dilute alcoholic solution of stearic acid, that a thin coating of stearic acid on the fibres would confer great softness and considerable resistance to water. Even where no stearic or other fatty acid is purposely added to alcohol used for dehydrating pelt, traces are present from the decomposition of the natural fat of the skin, and there is little doubt that this is the cause why such alcohol-leathers are much more difficult to wet back again to the state of pelt than would a priori be expected; and why hide-powder dehydrated in this way is unsuitable for use in the hide-powder filter ([p. 311]) from its non-absorption of water.
Fig. 85.—Scouring large Seal-Skins by Hand.
It is not within the scope of the present volume to describe in detail the processes used in currying, many of which are purely mechanical, and of no theoretical interest, whatever their practical importance; and with which the writer hopes to deal fully in a future book. The leather is usually scoured with stone, brush and sleeker to free it from “bloom” and loose tan ([Fig. 85]); or by machines such as [Fig. 86]; and is often reduced in thickness by shaving by hand ([Fig. 87]), or by machine ([Fig. 88]). In place of shaving, hides and skins are frequently split into two or more thicknesses. This is done by various machines, of which the “bandknife” shown in [Fig. 89] is the most important; the cutting tool being a thin steel belt stretched like a bandsaw and sharpened on one edge by an emery-wheel.
Fig. 86.—Scouring Machine.
Currying Shop, Leather Industries Department, Yorkshire College.
Something must be said here about the function of the oils and fats used in currying, and their general method of application. It is obvious that the possibility of coating the finest fibrils of leather with a fatty layer is not restricted to raw hide, but is present, sometimes even in a higher degree, in tanned or tawed leathers, in which the fibres are already so far isolated as to make the access of the fat easy. Even the possibility of an aldehyde-tannage is not excluded, where the fibre is not already completely saturated with other tanning agents or where these agents, from their nature, have not so firm a hold on the fibre as to be incapable of being displaced by the action of aldehydes. It is therefore obvious that we may apply some of the ideas which we have formed with regard to oil-tannages to the action of fats upon tanned leather. In the first place, it must be remembered that gelatinous matters are as a rule insoluble in fats; and vice versa, that fats are incapable of penetrating dry and solid gelatinous fibres. If the skin becomes dry in the chamoising process, that part remains raw. It may therefore be concluded that fats and oils have little power in themselves of isolating the fibrils, and that this must be accomplished by other agencies, since if they are still adhering together, the fats cannot penetrate them. Hence the necessity of moisture, which keeps the fibres soft and divisible; and with raw hide, the importance of powerful mechanical treatment, which will work the minute globules of fat between the fibrils. In the case of tanned leathers, the last condition is less important, since the fibres are already isolated by the tannage, and capillarity assists the penetration. Even in this case the distribution of the fat is much assisted if it is already in a state of fine division (emulsification), and if the surface-tension ([p. 76]) between it and water is low, as is the case with dégras and other partially oxidised oils. On this rather than on any special chemical affinity probably depends the importance of the “dégras-former” and other products of oxidation which are present in dégras; and the difference in penetrating power of different oils. So long as oil remains in an undivided condition, so long can it be squeezed out, and the leather will feel and appear greasy; while, when it is thoroughly emulsified, and adherent to the fibre, it can no longer be expelled by mechanical means. No doubt the different power of different tannages to “carry grease” without appearing greasy, is also related to the degree of isolation of the fibrils, and their surface tension with regard to fats. We may judge that the more readily an oil can be emulsified, the more freely and completely it is likely to fix itself on the leather fibre.
Fig. 87.—Hand Shaving.
Fig. 88.—Shaving Machine.
It is a practically invariable rule that the leather-fibre must be wet when it is stuffed. The surface-tension between the water and the fats is less than that of either with regard to air; and therefore, as the water dries out of the small interstices of the leather, the fat follows it in, and gradually takes its place. Generally speaking, the amount of water should be such that some exsudes in minute drops when the leather is pinched, that is, that not only the minutest spaces between the fibrils are filled, but even the larger ones between the fibre-bundles to a considerable extent.
Fig. 89.—Band-Knife Splitting Machine.
In “hand-stuffing,” the leather is now coated on the flesh side, or occasionally on both sides with “dubbing,” which is a pasty mixture of fats usually mainly composed of cod-oil and tallow, which is applied rather thickly with a brush and smoothed down with the fleshy part of the forearm. When such constituents are melted together, the harder fats dissolve in the oils, and as the mixture cools, much of the hard fats again crystallise out. To make a good dubbing, the cooling fats must be stirred continuously till this has taken place, as otherwise the mixture separates into little globular masses of crystals with liquid oil between them, instead of forming a uniform body of salve-like consistency. The proportions of the hard and soft constituents of the dubbing should be adjusted to the season, and to the temperature at which the drying of the stuffed leather is to take place, so that on the one hand, the dubbing will not melt and run off, and on the other, that it should not solidify more than is necessary, as only the liquid solution which remains entangled among the crystals can be absorbed by the leather. The solid crystalline fats remain on the surface, and are scraped off by the sleeker in finishing, as “table-grease,” which is generally re-melted and used over again. It does not answer, in hand-stuffing, to carry this re-use too far, as the table-grease contains only the harder parts of the fat, with a continually increasing proportion of stearic acid, so that if a dubbing be made continuously of table-grease and oil, in the end little but the latter will be absorbed by the leather; while where fresh tallow is used, a portion of its softer constituents remains dissolved in the oil. The principal function of the harder fats is the mechanical one of retaining the oil on the surface of the leather; and to a certain extent they may be replaced by other solids, such as steatite (“French chalk”), or perhaps other pulpy materials. The use of a portion of soft fat, such as bone-fat, or the better sorts of glue-grease, is quite practicable, especially if mixed with the harder table-grease.
The drying of hand-stuffed leather should be slow, to allow time for the absorption of the grease; and the temperature should be so regulated as to keep the dubbing in a soft but not liquid condition. In winter, if the temperature of the outer air be raised sufficiently for this, the drying will be too keen (cp. [p. 426]) and the water will be dried out before the grease is properly absorbed. It is therefore best, in cold weather, to maintain the ventilation mainly by circulating the air in the room, with little admission from the outside, and in extreme cases even artificial damping of the air may be advantageous. Sometimes the tendency to mildew during slow and warm drying is very troublesome. This may be prevented by the addition of antiseptics to the stuffing grease. Carbolic acid and creasote are effective, but generally objectionable from their smell; rosin oil has considerable antiseptic power, and mineral oils also in a less degree. Probably α-naphthol would prove an efficient remedy, as it has little odour, and its antiseptic properties are very strong, but it has not been tried by the writer. (Cp. [Chapter V.])
Fig. 90.—Haley’s Injector Stuffing Drum.
In drum-stuffing the conditions differ materially from those of hand-stuffing. The goods, in a damp condition, are placed in a drum ([Fig. 90]), which has been heated by steam to as high a temperature as the leather will safely stand. Cold damp leather may be stuffed in a drum heated to 60° C. and the grease may be run in at the same temperature. The grease should generally be melted and mixed at a somewhat higher temperature. Sometimes steam is merely blown into the drum before introducing the leather, to heat it to the required temperature; sometimes a steam-coil is placed in the drum itself. A more modern method, which is now largely used in the United States, is to heat by hot air, which is circulated by a fan over an external steam heater and through the drum. The drum is set in rotation, and the stuffing grease in a melted condition is run in through the hollow axle, or if this is not provided, it is introduced through the door, and the rotation is maintained for twenty to thirty minutes. During the last few minutes the door is frequently replaced by an open grating or cold air is drawn through the drum by means of the fan, in order to cool the goods, which are set out with the sleeker on the table while yet somewhat warm, and dried under much the same conditions as have been described with regard to hand-stuffed goods.
In drum-stuffing, the hardness of the grease is limited by its melting-point, which must not be so high as to damage the leather, but it may be soft as is desired. As the grease is forced by mechanical means into the interior of the leather, there is no danger of its running off, but the drying must take place at such a temperature as to keep it at least in a partially soft condition, as the drumming only forces it into the coarser spaces of the leather, and does not complete its distribution on the fibre. By the use of exceedingly hard greases, such as “stearin” ([p. 359]) and oleo-stearin ([p. 356]), sometimes with additions of paraffin wax, it is possible to introduce immense quantities of grease, and yet to obtain a leather which will board up to a good colour. In America, it is not unusual to reckon 100 or even 115 lb. of greases to 100 lb. of leather weighed dry after scouring, or estimated from its wet weight; and the whole of this is absorbed, scarcely anything coming off in “setting.” The leather, as it comes from the drum, is dark brown, but when bent sharply in “boarding” to form the grain, after cooling and drying, the very hard and crystalline fats crumble into white powder, and the leather takes a light and pretty colour. Such leather would of course darken at once if it were held to the fire, but would again brighten on cooling and breaking up with the “board.” Some portion of liquid fats, such as dégras or fish oil, should be contained in the stuffing grease, as the solid fats alone will not penetrate to the heart of the fibres, but will leave the leather dry and harsh.
By drum-stuffing, it is possible to incorporate solid matter with the leather, and barytes (ground heavy-spar or barium sulphate) was formerly much used for this purpose, but has now been nearly abandoned. Glucose is still used as an adulterant of leather, but is not introduced in the drum, but by painting the goods with syrup before stuffing. It not only adds weight, and gives the leather a lighter colour than an equivalent quantity of grease, but at the same time lessens its toughness, and ought to be prohibited in England, as it already is in Germany. On the detection of adulteration of leather, see L.I.L.B., p. 212. Drum-stuffing is in this country mainly applied to shoe-leathers, but in America, with the hot-air drum, is coming into increasing use for harness, and even belting.
A method of stuffing is used in Germany for heavy belting and the like, which appears at first glance to contradict the axiom that leather must be stuffed wet. It is called Einbrennen (to burn in), and consists in first drying at a high temperature (50° C.), to ensure the absence of all moisture, and then either pouring hot melted tallow over the leather on a table, and holding it over a brazier, to allow the grease to sink in, or dipping it completely in a bath of melted tallow. The exception is only apparent, because, though the leather is at this stage completely saturated with tallow, it is only after wetting and drumming that it attains the flexibility due to true stuffing. Similar methods are applicable to alumed leathers, and even to chrome-leather; and so-called “waterproof” or “anhydrous” leather is made by immersing thoroughly dried leather in a bath of 2 parts of resin and 1 of paraffin, or some similar mixture. If the leather is not first thoroughly dried, it is scalded and destroyed by the hot grease.
The most troublesome defect to which stuffed leathers are liable, is known as “spueing,” and is of two kinds, of which the first and less serious (perhaps more properly distinguished as “striking out”) consists of a white efflorescence rather like incipient mould, which is easily wiped off, but generally reappears. This is due to the crystallisation of the harder fats, and especially of the free fatty acids, on the surface of the leather, and is almost sure to occur in greater or less degree when the hard fats such as tallow or stearine are combined with a non-drying oil such as neatsfoot, or when soft fats are present in the leather. It is sometimes combined with actual mildew, from which it is rather difficult to distinguish, even under the microscope, and may even be caused by fungoid plants, which not only mechanically expel the fats by their growth, but probably promote their rancidity and the separation of the crystalline fatty acids. It is at most only a defect of appearance, and does not in any way injure the leather. It is constantly present in calf-kid, from the neatsfoot oil used in finishing, and is in this case rather liked by the buyers, who for some reason regard it as a proof of quality. A very similar appearance may be caused by the use of solutions of barium chloride, alum or other mineral salts, for weighting or other purposes; but is persistent when the leather is held to the fire, while the crystallised fatty acids at once melt and disappear. The fatty acids are at once removed by a drop of benzene or petroleum spirit; but unaffected by water, while with water-soluble salts the reverse occurs.
The second form of spueing is of a much more troublesome character, and makes its first appearance as minute spots or pimples of resinous matter, raised above the surface of the leather, which if removed, generally reappear, and which may become so bad as to form a sticky resinous coating over the whole surface. The exsuded matter consists of the oxidised products of oxidisable oils, but the cause of its appearance is not always easy to explain. The currier generally attributes it to adulterated oils, and it must be admitted that some oils almost invariably produce it, but it appears occasionally when only the purest and absolutely genuine cod-oil has been used. It can only be produced from drying or semi-drying oils, which include all the ordinary fish oils and most of the vegetable seed oils, but can never arise from tallow or stearine, from mineral oils or vaseline, or from genuine non-drying oils, such as tallow, neatsfoot, sperm, or mineral oils, nor, probably, from rosin oil. It is favoured by causes which promote the oxidation of oils, such as moist heat with limited access of air, and by the presence of oxygen-carriers, such as iron-salts in blacks, and possibly also by the presence of free acids. A large amount of free fatty acid in the oils themselves is suspicious, not only because the free acids oxidise more freely than the neutral fats, but because their presence is an evidence of the tendency to rancidity and change in the oil. It is also said to be caused by previous mildewing of the leather, and certainly often occurs where the grain has been rendered porous by bacterial action in the soaks, limes, or bates, probably from the greater quantity of oil absorbed by these parts. While it is easy to say which oils may possibly spue, there is no known chemical test which will foretell whether a given sample is likely to do so under ordinary conditions. Eitner[169] states that seal oil extracted at a low temperature is very liable to spue, but that when heated for a considerable time to a temperature of 250°-290° C. it darkens in colour and loses the tendency. This is probably true of many other marine oils; and may be one cause of the frequent trouble with modern oils, many of which, especially the lighter coloured kinds, are extracted by steam at a temperature below boiling point. It is very probable that one effect of heating to a considerable temperature is to dehydrate and separate albuminous or gelatinous matters which are present in the fresh oils, and which probably increase their tendency to decomposition. Many of these substances separate as “foots” from oils during long storing, and such old oils are said to be less liable to spue than those of recent manufacture.
[169] Gerber, 1880, p. 243.
If oxidisable oils are used upon leather, they “dry” upon the fibre, and if a sufficiency of non-drying constituents are not present at the same time, the leather will ultimately become hard, and may even crack from hardening of the fibre. Mineral oils are not liable in this way to form a hard coating on the fibre, but as they are slightly volatile, though of very high boiling point, they may ultimately evaporate, and leave the leather insufficiently nourished. From their low surface-tension, they have great powers of capillary penetration, as is witnessed by the way that lamp oils “creep” over the surface of the lamp, but they have less affinity for water than the more oxidisable oils, and probably do not combine so intimately with the leather-fibre. They are probably better used in combination with other greases than alone. The admixture of solid paraffin with stuffing greases has the tendency to make the leather feel less greasy and drier than it otherwise would; and crude turpentine and rosin are said to have a still greater effect in this direction.
The water which is required for satisfactory stuffing may in some cases be introduced into the stuffing grease as well as into the leather. The effect of dégras is largely due to the water with which it is intimately mixed, and when dégras or sod-oil is deprived of that which it naturally contains, by heating it to too high a temperature, either before or after its mixture in a stuffing grease, its efficacy is greatly lessened.
Fat-liquoring ([pp. 217], [239]) may be considered a special case of stuffing, in which the oil is very perfectly emulsified with a large quantity of water. In this way, very considerable quantities of oil may be introduced into leather without giving it the least greasy feel. Egg-yolk contains about 30 per cent. of an oil chemically very like olive, but with a larger proportion of palmitin, and may be considered as a very perfect natural fat-liquor, containing also some albumen which serves as “nourishment” for the leather. If a means of emulsifying olive, lard, or tallow oil (with the addition of a little palm oil) with albuminous matter as perfectly as in the egg could be discovered, the problem of an egg-yolk substitute would in all probability be solved. Milk and cream are also natural fat-liquors.
CHAPTER XXV.
DYES AND DYEING.
Before the discovery of artificial organic dyestuffs, the only colouring materials known to industry were those of mineral and direct organic origin; and on this account the dyeing of leather was formerly subject to great difficulties and limitations.
The discovery of the means of artificially preparing an organic dyestuff (mauve) by Perkin some forty-five years since, opened up a new field for research, and since that time, the list of commercial dyes has so increased that there is now scarcely a tint or shade which cannot be accurately matched and reproduced by the coal-tar colours. These colours are often spoken of as “aniline dyes” owing to the fact that many of them, and especially the earlier ones, have been derived from aniline, one of the products of coal-tar; but more recently, a considerable number of important colours have been prepared from other constituents of the tar, and it is therefore more correct to term the whole of the dyes obtained, either directly or indirectly, from coal-tar, the “coal-tar colours.”
The coal-tar colours are generally soluble in water, or mixtures of water and alcohol, and the majority of them combine with the fibre of the leather without the use of any mordant, so that in most cases it is only necessary to apply a solution of the dye direct to the leather, though their suitability for the purpose varies considerably. A few which are only soluble in oils or hydrocarbons, are not suitable for leather-dyeing, though they may sometimes be utilised in conjunction with fats in currying; and there are also certain colours which are not applied to the fibre ready formed, but are developed on it by subsequent chemical treatment, and which have only been applied to a limited extent to leather.
A number of the coal-tar dyes, which are produced in the crystalline form, have a totally different colour when solid to that of their solutions, and to the colour they produce when dyed. A well-known instance of this is magenta or fuchsine, which forms glistening green crystals, while in solution it is a brilliant red dye. The colours of the crystals are usually complementary to those of the solution, thus several blues have the appearance of metallic copper, and violets, such as methyl-violet, are greenish-yellow, generally with a pronounced metallic lustre. This peculiarity is the cause of the defect in dyeing known as “bronzing,” in which the dye, when applied in too concentrated a form, takes a surface-shimmer of its complementary colour.
The coal-tar colours are mostly either “acid” or “basic.” The former are the salts of organic colour-acids with inorganic bases (generally sodium) and are usually readily soluble in water, but frequently do not fix themselves on the fibre till the colour-acid is set free by the addition of some stronger acid to the bath, and in many cases the free colour-acid is of different colour to its salts. The “basic” colours are salts of colour-bases (organic bases of the nature of very complicated ammonia-derivatives) with acids (mostly hydrochloric, sulphuric or acetic). Most of those in commercial use are soluble in water, though a few require the addition of alcohol. The colour-bases themselves are usually insoluble in water, and therefore precipitated by alkalies, and in some cases they are also colourless. The basic dyes have generally greater intensity of colour than the acid dyes, but large classes of them are very fugitive when exposed to light, and in strong solution many others are very liable to “bronze,” a defect which is generally less marked with the acid colours.[170]
[170] It has recently been shown by Lamb (see [App. D], [p. 498]) that many basic colours are much faster to light on leather than on textiles.
As it is not obvious at first sight whether a given dye is acid or basic, a reagent to distinguish them is useful. For this purpose a solution of 1 part of tannic acid and 1 part of sodium acetate in 10 parts (by weight) of water is conveniently employed, which gives coloured precipitates with basic dyes, but is not affected by acid ones. The fact that basic dyes are precipitated by tannins influences their use in leather dyeing, not only as regards their fixation on the leather-fibre by the tannin which it contains, but as the cause of their precipitation in the dye-bath if great care is not taken to avoid the presence of tannins in a soluble form. The use of the sodium acetate is to combine with the mineral acid of the colour-salt, which if left free would prevent complete precipitation, substituting for it acetic acid, which is much weaker, especially in presence of excess of sodium acetate (cp. [p. 81]).
In using the terms “acid” and “basic” with regard to dyes, it is not to be understood that the dyestuffs as employed are acid or alkaline in the sense that vinegar is acid, and soda basic, but merely that the actual colour-constituent of the salt is in the one case of an acid nature, and set free by stronger acids, and in the other case is basic, and liberated (and often precipitated) by stronger alkalies.
There are several general theories with regard to the fixation of colours in dyeing organic fibres, and it is probable that no one of them affords a complete explanation in all cases. One holds that the action of dyeing is mechanical rather than chemical, the colour adhering to the fibre by surface-attraction; another, that an actual chemical compound is formed between the dye and the dyed material or one of its constituents; and a third, the “solid solution” theory of Witt, is in a sense intermediate, holding that the colouring matter is actually dissolved in the dyed fibre. The idea of a solid solution, strange at first, offers little difficulty on consideration. The colouring metallic salts in tinted glasses exist obviously in solution in the melted glass, and can hardly be said to change their condition in this respect when the glass becomes solid. Gelatine, indiarubber, and perhaps all other colloid bodies, absorb water or other liquids without losing their solid form, and these liquids may fairly be said to be dissolved in the solid. All animal and vegetable fibres are in this respect like gelatine, and during the process of dyeing are swollen with water. It is quite easy to dye a mass of gelatine throughout with most water-soluble dyestuffs. (Compare on these points what is said in [Chapter IX.] on the physical chemistry of hide-fibre.) The distinctions between solution and molecular surface-attraction on the one hand, and certain forms of chemical combination on the other, are not wide ones, and probably all three theories are true in different cases, and shade off into each other by imperceptible gradations. The subject of leather-dyeing is, in fact, a very complicated one, since we are not dealing with a fibre of uniform composition, but with one which has had its structure (both chemical and physical) altered by the processes to which it has been subjected during its conversion into leather.
Although, strictly speaking, the constitution of the gelatinous fibre of the skin is unknown, we are quite justified in stating[171] that, like the amido-acids which are important proximate products of its decomposition, it contains both acid and basic groups, and is therefore capable of attracting both bases and acids. It is well known, for instance, that the neutral fibre is capable of withdrawing sulphuric acid from a decinormal solution with such vigour that the residual liquid is neutral to litmus paper; and it will also absorb caustic alkalies with perhaps equal avidity.[172]
[171] Procter, Jour. Soc. Chem. Industry, 1900, p. 23.
[172] Cp. [Chap. IX.]
It is thus readily dyed by colouring matter of either basic or acid character, and in many cases will even dissociate their salts, dyeing the characteristic colour of the free dyestuff, but possibly at the same time fixing the liberated base or acid with which the colouring matter has been combined. Many tanning processes consist in a somewhat analogous fixation of weak bases and acids, and it is, therefore, to be anticipated that they will profoundly modify the colour-fixing properties of the original fibre, as indeed proves to be the case. Exactly what the result of a particular tanning process in this respect will be is less easy to foresee.
In the ordinary vegetable tanning process, the tannins, which are of acid nature, are freely fixed by the fibre. It is, therefore, not surprising that vegetable-tanned leather most readily fixes the basic colours, especially as these form insoluble compounds with the tannic acids, so that it is quite probable that the dyeing is mainly effected by the formation of tannin-colour-lakes on the fibre, rather than by actual fixation of the colour-base in combination with the original matter of the skin. It is noteworthy, however, that even fully tanned skin has by no means lost its attractions for acid colouring matters, many of which will dye it even without the presence of free acid, though it is possible that the tannic acid performs the function of saturating the alkaline base with which the colour acid has been combined.
It should be pointed out that while the substance of animal skin consists practically of gelatinous fibres, it is covered on the outer surface with a thin membrane of extreme tenuity, called the hyaline or glassy layer ([p. 50]) which, in the living animal, separates the true skin from the epidermis. This layer, the chemistry of which is quite unknown, reacts to colouring matters differently from the gelatinous fibres, and probably is less absorbent for basic colours, and more so for the coloured anhydrides of the tannins, and perhaps for acid colours generally, than is the true skin. As a result, it colours more darkly in tanning, and less so in dyeing with basic colours, and as it is extremely liable to damage in the preliminary operations of removing hair and lime by the tanner, this irregularity of colouring is a serious disadvantage which is most marked with the basic colours. Small quantities of lime left in the skin are also probably important causes of irregular dyeing.
Mordants are chemicals used to enable the fibre to fix dyes for which it would not otherwise have sufficient attraction, and hence are generally substances which have affinity both for the fibre and the dye. Thus cotton, which does not itself attract the basic colours, is mordanted for them by a solution of tannin, which it attracts, and which, in its turn, attracts and fixes the colours. In many cases, however, the function of mordants is more complex, not merely fixing the dyestuff, but often modifying, or even producing its colour. Thus tannin dyes black on an iron mordant, though it is itself colourless. Such mordants may be applied after the colouring matter, where it has sufficient attraction for the fibre to be taken up alone, but does not produce the required colour. This process is often called “saddening,” as the colour is generally darkened. A familiar instance is the use of iron solutions to darken or blacken tannin or logwood. There is scarcely any distinction in theory between mordants of this class and the constituents of dyes which are successively applied to the leather in order to produce the colouring matter on the fibre. Among these may be mentioned several mineral salts which were formerly employed in leather dyeing, though their use is now nearly obsolete. Iron salts are easily fixed by leather, whether tanned or tawed, and in the former case produce a dark colour by action of the tannin. On subsequent treatment with a solution of potassium ferrocyanide, a deep blue is formed (Prussian blue). If copper acetate or ammoniacal solution of copper sulphate be substituted for the iron salt, a deep red-brown ferrocyanide is produced. Yellows are sometimes dyed by first treating tanned leathers with lead acetate, which is fixed by the tannin, and then with potassium bichromate, by which yellow lead chromate is produced. A more important use of lead is in the so-called “lead-bleach,” which is really a white pigment-dyeing with lead sulphate. The tanned leather, after washing, is first treated with a solution of lead acetate (usually “brown sugar of lead” of about 4 grm. per liter), and subsequently with a dilute sulphuric acid of about 30 grm. of concentrated acid per litre, and then thoroughly washed to free it from acid. The process is often used as a preparation for dyeing pale shades, as many of the aniline dyes are easily fixed on the bleached leather, but is subject to the disadvantage attendant on all pigments containing lead, of becoming rapidly darkened by traces of sulphur or sulphuretted hydrogen, such as are constantly contained in lighting gas, or arise from the putrefaction of organic matters. The use of acid is also liable to cause early decay of the leather.
A large proportion of the coal-tar colours contain amido-groups (NH2 groups) which, when treated on the fibre with nitrous acid (or an acidified solution of sodium nitrite), become “diazotised” (converted into —N : N— groups with elimination of OH2). On further treating the diazo-compound with solutions of amines or phenols, combination takes place, and new azo-colours are formed in or on the fibre, often remarkably fast to washing or rubbing. Since these qualities are less important in leather than in textiles, and the process is moreover somewhat delicate, and the nitrous acid is apt to injuriously affect the leather, these processes have been little used in leather-dyeing, and are only mentioned here for the sake of completeness.
The use of the natural polygenetic colours in dyeing leather of vegetable tannage, which was once universal, is gradually disappearing, except for the production of blacks. Leather cannot be very satisfactorily mordanted for these colouring matters; but they have some natural attraction for the leather itself, and are generally dyed first, and their colours afterwards developed by metallic mordants such as iron, chrome, tin salts, and alum, which act not only on the absorbed dyestuff, but frequently on the tannin and colouring matters derived from the tanning materials. For black-dyeing, the use of coal-tar colours, either alone, or to deepen the colours produced by iron, is gradually extending. Claus and Rée’s “Autho-black,” the “Corvolines” of the Badische Co., and Casella’s “Naphthylamine Black,” “Aniline Grey,” and “Naphthol Blue-black” may be mentioned as useful colours. As coal-tar blacks are mostly dark violets rather than dead blacks, their colour may be deepened by the admixture of suitable yellows or browns, and this has already been done in one or two of the colours named. Apart from the coal-tar colours, black dyeing is generally produced by the action of iron (and chrome), either on the tannin of the leather itself or on logwood. As the leather is frequently greasy, and the satisfactory formation of a tannin- or logwood-lake can only take place in presence of a base to absorb the liberated acid of the iron salt, the skins are either brushed with, or plunged in, a logwood infusion, rendered alkaline with soda or ammonia, or the tanned leather receives a preliminary treatment with weak soda or ammonia solution. As such solutions act powerfully on tanned leathers, rendering them harsh and tender, great care must be taken to avoid excess. The effect of this alkaline treatment is not only to assist the wetting of the greasy surface, but to prevent too deep penetration of the dye, by causing rapid precipitation of the colour-lake. In recent times, however, leathers are sometimes demanded in which the colour goes right through, and in this case it might be well to reverse the treatment, beginning with a weak solution of a ferrous salt, perhaps with addition of sodium acetate or potassium tartrate, and finishing with alkaline logwood, as without alkali the full colour is not developed. The use of iron salts is not very satisfactory in regard to the permanence of the leather; and in this respect it is of great importance that they should not be used in excess, and that any strong acids they contain should be saturated with permanent bases, and if possible washed out. Leather-surfaces blacked with iron almost invariably ultimately lose their colour, becoming brown if tannins, and red if logwood has been employed, and at the same time the leather surface usually becomes brittle or friable. This is to a large extent due to the effect of iron oxides as oxygen-carriers. Exposed to light, they become reduced to the ferrous state, oxidising the organic matters with which they are combined, and in the dark they re-oxidise, and the process is repeated. It is therefore of the first importance that excess of the organic colouring matter should be provided, and that the quantity of the iron should be as small as possible, and in stable combination. These points are greatly neglected in practice, especially where blacking is done by the application of iron salts without logwood, when the evils mentioned are intensified by the actual removal of part of the tannin of the leather, and perhaps by the combination of ferric oxide with the skin-fibre itself, forming a brittle iron-leather. Treatment with alkaline sumach-, gambier- or logwood-solutions, both before and after the application of the iron, would lessen the evil. Iron-logwood blacks are much less permanent, and fade more rapidly under the influence of light and air than iron-tannin blacks. The use of iron-blacks on curried leathers seems considerably to increase the tendency to “spueing,” a defect due to oxidation of the oils (see [p. 390]). Copper salts mordant logwood a very dark blue, which is much more stable than the iron compound, and hence are often used advantageously in mixture with iron salts. In practice, iron blacks are generally oiled in finishing, and this renders them more permanent, both by protecting the lake from air and by forming iron soaps which are stable. The use of actual soaps in blacking and finishing is not unknown, and probably deserves more attention. Hard soaps of soda and stearic acid,[173] form an excellent finish where a moderate glaze is required, the soap jelly being applied with a brush very thinly, allowed to dry thoroughly, and polished with a flannel or brush, or glassed. Many acid colours are soluble in such soap jellies, which may thus be employed for staining. Similar but harder finishes, and capable of being glazed to a high polish, are made by dissolving shellac with dilute borax or ammonia solutions.[174] Both of these finishes are useful in lessening the tendency of iron blacks to smut or rub off, a failing which is due to the precipitation of loose iron-lakes on the surface, instead of in combination with the fibre, and is particularly obvious where “inks” or one-solution blacks are employed, or where the mordant and the colouring matter solutions are allowed to mix on the surface of the leather. Such “inks” are generally made with a ferrous salt and logwood or tannin, together with some aniline black, and the colour-lake should only be formed on oxidation. Chrome is not much employed in blacks with vegetable tannages, as it only produces blacks with logwood, the chrome compounds of tannins having no colouring value; and bichromates used at all freely being very injurious to the leather.
[173] 1 of caustic soda in 10-15 of water, boiled with 8 of stearic acid till clear, cooled to 25° C. and diluted with 400-800 water, with constant stirring, till the white jelly of suitable consistence is obtained. Somewhat similar, but harder preparations may be made with waxes, or fatty acids still higher than stearic.
[174] 5 parts of shellac digested warm with 100 water and 3 of ammonia fort., or 1 of borax. If the solution is used as a “seasoning” for glazing, the waxy matter which separates on standing should be mixed by shaking before use. As a varnish, a stronger solution should be used and the wax skimmed off.
In dyeing blacks on other than vegetable tannages, however, chrome becomes of importance, as logwood is principally employed, though sometimes in conjunction with tannin, and often with addition of quercitron or fustic, to correct the bluish shade of the logwood-chrome or logwood-iron lake. It must not be overlooked in practice, that if ferrous salts are mixed with bichromate solutions, the latter are reduced, and the iron is oxidised to the ferric state.
In alumed leathers the fixing power of the original hide-fibre is much less affected than in vegetable tannages. Whatever may be the truth with regard to the latter, there is little doubt that physical influences are at least as important as chemical ones in the production of mineral tannages. The amount of the tanning agent absorbed is greatly influenced by the concentration of the solutions, and in ordinary alum tawing much of the alumina may again be removed by free washing. In this case, the sulphate of potash present takes no part in the operation, but the alumina salt is absorbed apparently as a normal salt. Alum or alumina sulphate alone is incapable of producing any satisfactory tannage without the assistance of common salt, the quantity absorbed being small, and the fibre becoming swollen by the action of the acid. In presence of salt the absorption is greater, and the swelling is prevented. The explanation of this is not to be found in the formation of aluminium chloride, for though this undoubtedly takes place, it has been shown that the action of aluminium chloride without salt is not more satisfactory than that of alum. It has long been known that salt prevents the swelling action of acids on skin, although it does not lessen the absorption of acid; and the fact is capable of explanation on modern osmotic theories (cp. [p. 89]). The skin so treated is found to be converted into leather, but if the salt be washed out, the acid is retained by the skin, which returns to the state of acid-swollen pelt. It is probable, therefore, that although the acid and alumina are absorbed in equivalent proportions to each other, they are really dissociated, and attached to different groups in the gelatine molecule, and that the effect of the salt is to allow the absorption of the acid without swelling, and, osmotically, to increase the dissociating power of the pelt. If, in place of a normal alumina salt, a basic salt is employed, such as may be obtained by partial neutralisation of the sulphuric acid with soda, satisfactory tannage may be accomplished without salt, a basic compound is absorbed, and the leather is much less affected by washing. In the analogous case of chrome tannage, this basic compound may be still further deprived of its residual acid, by washing the tanned skin with alkaline solutions, leaving a leather which is extremely resistant even to hot water; and a somewhat similar result may be obtained with alumina, though with more difficulty, as apparently a very small excess of alkali destroys the qualities of the leather. (Cp. [p. 187].)
The results on dyeing are almost what might have been foreseen. While ordinary alumed leather absorbs both acid and basic dyes readily, the basic chrome leather has practically lost its affinity for the latter. Both chrome and alumina leathers readily absorb vegetable tannins, thus supporting the view that the acid-fixing groups of the gelatine molecule are still unsaturated (tannins are capable of tanning pelt swollen with sulphuric acid and apparently of expelling the acid). In the case of chrome leather the effect of re-tanning with tannins is greatly to lessen its stretch, and if carried too far, to destroy its toughness, but it at once becomes capable of fixing basic dyestuffs. This property is frequently made use of in dyeing, but the effect on the leather must not be disregarded where softness and stretch are important, as in the case of glove-leathers. Polygenetic dyes are, of course, fixed on alum- or chrome-leathers by the alumina- or chrome-mordant, though apparently the bases are not present in the most favourable condition for fixing colours. Thus logwood extracted without alkali dyes tanned leather yellow, alumed leather violet-blue, and chrome leather blackish-violet, and some of the alizarine group dye very well on chrome as its resistance to hot water allows much higher temperatures to be used than with most other leathers. The tannin contained in dyewoods has the effect of lessening the stretch of chrome leathers.
Something should perhaps be said on the dyeing of oil and aldehyde leathers, but the subject has as yet been scarcely treated scientifically, and our practical knowledge of the subject is insufficient to justify theorising. (See, however, [p. 496].)
Defects in the colour of the finished leather are due to a variety of causes, but many are produced by want of cleanliness and system during the dyeing itself. The greatest care is needed in this respect, and in brush-dyeing a different brush should be used for each different colour, as it is impossible to thoroughly remove all traces of dye by the ordinary methods of cleansing.
Irregular and surface dyeing sometimes occurs owing to too rapid fixation of the colours; while in other cases the affinity of the dye is too small to allow of reasonable exhaustion of the bath. Addition of salts of weak acids, such as potassium hydrogen tartrate (tartar), or of those like sodium sulphate, which form hydric salts, lessen rapidity of dyeing with acid colours; while acids generally increase it, and it is also often increased by addition of common salt, which lessens the solubility of the dye. Weak acids, such as acetic or formic, or acid salts, such as sodium bisulphate, are generally to be preferred to sulphuric acid as an addition to the dye-bath; and if the latter is used, great care is desirable in its complete removal. There is no doubt that the rapid decay of leather bookbindings and upholstery is largely due to the careless use of sulphuric acid in “clearing” and dyeing the leather;[175] and even if it is fully removed, it has saturated all bases such as lime, which are naturally present in leathers in combination with weak acids, and which would otherwise act as some protection from the sulphuric acid evolved in burning coal gas.
[175] See Report of Committee of Society of Arts on Bookbinding Leathers, 1901.
“Bronzing,” the dichroic effect produced by light reflected from the surface of many colouring matters, complementary to that transmitted by them and reflected by the surface of the dyed material, is not peculiar to basic colours, but is generally more marked in them than in acid ones. Basic colours, from their great affinity for tannins, and consequent rapid dyeing, are apt to dye irregularly, and without sufficiently penetrating the leather, and if the soluble tannin is not wholly washed out of the skins previously to dyeing, it bleeds in the dye-bath, and precipitates insoluble tannin-lakes, which waste colour and adhere to the surface of the leather. The inconvenience of basic colours due to their too rapid fixation may sometimes be lessened by slight acidification of the dye-bath with a weak acid, such as acetic or lactic. The acid may be still further “weakened” if desired, by the addition of its neutral (sodium) salt. The precipitation of tannin-lakes in the bath may be prevented by previous fixation of the tannin with tartar emetic, titanium potassium oxalate or lactate, or some other suitable metallic salt.
The fading of the colours of dyed goods by exposure to light is a defect which has been much more investigated in the textile industries than in leather manufacture, though in the latter case, and especially with regard to bookbinding and furniture leathers, it is of even greater importance. It is probable that no colours are actually unaffected by strong sunlight, but in many cases the action is so slight that it may practically be disregarded; some of the coal-tar colours, and especially some of the alizarines, being practically permanent, while others, and particularly the aniline colours belonging to the triphenylmethane group, such as magenta, are so fugitive as to be practically bleached by a week of strong sunlight. Chrysoidine and the eosins are also very bad in this respect. The fastness of colours to light is a good deal influenced by the material on which they are dyed, and but little has yet been published of the results of direct experiments on leathers, but Mr. M. C. Lamb has been for some time engaged in a research of this nature,[176] and the subject is now receiving a good deal of attention in other quarters. Experiments are easily made by exposing samples to sunlight under glass or in a south window, a part of the leather being covered with wood or thick brown paper for comparison. The results are often complicated by the tendency of all leathers tanned with tannins of the catechol group, and especially with turwar bark ([p. 298]), mimosa and quebracho, to darken and redden in sunshine, or even by exposure to diffused light. Pure sumach tannages are nearly free from this defect, and are also much less easily destroyed by the action of gas fumes (sulphuric acid), and the other injurious influences to which books and furniture are often subjected.[177]
[176] See [App. D.], [p. 488], [498], and Journ. Soc. Chem. Ind., 1902, pp. 156-158.
[177] Cp. Report of Society of Arts Committee on Bookbinding Leathers, 1901.
Want of fastness to friction or rubbing is a defect generally more important in textiles than in leather, where it is often prevented by glazings or other finishes applied to the surface; but in some cases, and, especially in black leather, it is apt to be annoying. If suitable colours are used, the defect is generally due to the precipitation of loose colour on the surface, either by the too free use of mordants, or the dyeing of basic colours on leathers which have not been sufficiently freed from loose tannin. It is also often caused by “flaming” or the application of colour mixed with the “seasoning” used in glazing, to hide imperfections in the dyeing, or vary its colour. Colour applied in this way is only mechanically fixed on the leather, and is easily removed by moisture, staining articles with which it comes in contact.
A very similar defect may be caused by incomplete washing of the dyed leather, which leaves loose dye from the dye-bath in the goods. To avoid it in glove-leathers, where its occurrence would be particularly annoying, the natural mordant colours are still largely in use, which being precipitated on the fibre in an insoluble form by the mordant or “striker” (generally a metallic salt) are little liable to come off. Basic colours may be fixed by a subsequent treatment with tannin, or by topping with certain acid colours such as picric acid. Some few colours, and especially Martius or “Manchester” yellow (dinitronaphthol) are volatile at a low temperature, and therefore liable to “mark off” or stain any materials with which the dyed fabric, even in a dry state, is placed in contact.
Fig. 91.—Dyeing in the Tray.
The practical dyeing of leathers varies considerably according to whether they are tanned with vegetable materials, chrome, alumina salts, or chamoising. Vegetable-tanned leathers are dyed either by hand in the “dye-tray,” or in the drum or paddle, the two latter methods being now largely employed. The dye-tray is a shallow vat, about 10 inches deep, and large enough for the goods to be laid flat in it. In the English method, one or two dozen skins, or even more, are dyed at a time, being turned over in the tray by hand, the undermost pair being drawn out and placed on the top ([Fig. 91]). The method is convenient where only a small number of skins are to be dyed to one particular shade, which is more easily matched as the goods are always under observation, and it has the further advantage that, if desired, the grain sides only of the skins can be coloured, by “pairing” or “pleating” them before dyeing. For this purpose two skins of equal size are laid together flesh to flesh (pairing), or each skin is doubled down the back, flesh side in (pleating), and pressed firmly together with a sleeker on the table, when the skins adhere so closely that if carefully handled, no colour penetrates between them during the dyeing, except a little round the edges. This effects considerable economy of dye-stuff, as the fleshes would absorb a good deal, and for some purposes, an undyed flesh is preferred. In dyeing in the paddle or drum, the skins are merely placed loose in the dye-liquor, so that the fleshes are dyed equally with the grain sides. Paddle-dyeing has the advantage of effecting a considerable saving of labour, as compared with the dye-tray, in which constant handling, which often lasts an hour or more, is required. It also allows of almost equal facility in examining the colour of the skins, which is very important when dyeing to shade; but it is less economical in dye-stuff, as not only the flesh sides are dyed but a much larger volume of liquor is used, and as the dye-bath can never be entirely exhausted, more dye is run away in the used liquor. Drum-dyeing is much less expensive in this respect, as the volume of liquor may be very small, and from the efficiency of the motion, the dyeing is very thorough, and penetrates deeply into or through the skin, which in many cases is advantageous, but it is difficult to dye to exact shade, since the skins can only be examined by stopping and opening the drum. Most dyes are more readily fixed at high temperatures, and in this respect the drum has an advantage over all other methods, as once heated it retains its heat with very little loss to the end of the operation, while both in the paddle and the dye-tray the liquor is rapidly cooled, and special methods of maintaining the temperature complicate the apparatus, and require great care to avoid overheating. It is usually best to work at the highest temperature which the goods will safely bear, and this varies to some extent with the class of goods, chrome tannages and chamois leather being peculiar in standing almost any temperature short of boiling. With vegetable tanned leather 50° C. may be taken as a maximum; but cold wet skins may safely be introduced rapidly into a liquor heated to 60°, as they will cool it sufficiently.
The Continental method of dyeing in two trays may be mentioned here, as it produces very rapid and even dyeing, with considerable economy of dye-stuff, and the principle is capable of application to other methods where a large number of skins have to be dyed to the same colour. As generally carried out, two trays are employed, each about 4 feet long, 18 inches wide, and 10 inches or a foot deep, and these are usually made with a sloping bottom, or propped up in such a way that the dye-liquor all runs to the further side of the tray. A single pair of skins is usually dyed at once (in about 6 liters (5 qt.) of liquor for sheep and goat). To begin with, the first tray is filled with a very weak liquor, and the second with one of about half strength. The goods are entered in the first tray, turned a few times, and passed into the second; the liquor in the first is run away, and it is re-filled with one of the full strength, to which the goods are then transferred, and dyed to shade. The second tray is much reduced in strength by the skins, and now serves as the weak liquor for a fresh pair, which in its turn passes into that from which the goods have been dyed out, and then into a new liquor; each pair of goods thus passing through three baths, of which the last is of full strength, and which quickly brings up a full and even colour. In the ordinary English method, the goods must, for the sake of economy of dye-stuff, be dyed out in a nearly exhausted bath, which is a tedious operation, the last stage of dyeing often taking a time far longer than that required to bring the goods nearly up to shade, and even then failing to produce a good and full colour. This evil may be lessened by adding the dye-stuff in several successive portions, as the bath becomes exhausted, but cannot be altogether avoided with a single tray, if any reasonable exhaustion of the bath is to be attained. At first sight it seems a very slow process to dye the goods in single pairs, but this is to a great extent compensated by the rapidity with which they take on colour. In the Continental system, the dyes, mostly of the coal-tar series, are used as strong solutions, and each new dye-bath is made up by filling the tray with a definite volume of hot water and adding a measured quantity of the dye-solution.
The re-use of partially exhausted dye-baths is generally limited to cases where either single dyes, or mixtures of very equal affinity for the leather are employed, since where dyes of unequal affinity are employed, one is more rapidly removed than the other, and the shade of the dye-bath is altered. Many dyes sold as single colours are really mixtures,[178] and alter in shade if successive quantities of leather are dyed in their solutions. Basic dyes are also apt to be precipitated by traces of tannin washed out of the goods, and thus rendered unfit for use a second time. This may be avoided by suitable preparation of the goods (see [p. 411]).
[178] Such mixtures may often be detected by putting a drop of their solution on blotting-paper, when the dyes form differently coloured rings according to their more or less rapid fixation by the paper, or by dusting the dry dye very thinly on wet blotting-paper, when each particle produces its separate spot.
Much of the success of practical leather-dyeing depends on proper selection and preparation of the goods. Sound uninjured grain is a matter of first importance; no satisfactory dyeing can be expected on skins which through carelessness in soaks, limes, or bates, are tainted by what is known as “weak grain,” caused by destruction or injury of the delicate hyaline layer, which forms the natural glaze and outer surface of the skin ([p. 50]). For such goods, “acid” are to be preferred to “basic” dyes, the latter having an especial tendency to dye darker and deeper where the grain is imperfect. Goods of different tannages and colours should never be dyed together, as they are certain to produce different shades in the same dye-bath. Tanned skins which have been dried, especially if they have been in stock for some time, should be thoroughly softened by soaking in tepid water and drumming, a temperature of between 40° and 45° C. being most advantageous. Skins, such as calf of mixed or bark tannage, must now be freed from all bloom by scouring with brush and if necessary with slate or stone, but great care is requisite to avoid injury to the grain. A little borax or other weak alkaline solution assists in removing bloom. Fresh sumach-tanned skins merely require setting out with a brass sleeker, but those which have been long dried often dye more evenly and readily if they are re-sumached.
Dark coloured tannages, such as Australian bazils, and East India sheep and goat tanned with cassia bark, are always improved by sumaching, and if for light colours, by first stripping a portion of the original tan by drumming for a quarter of an hour with a weak (1⁄4 per cent.) solution of soap powder or borax at a temperature of 30° to 35° C. and then passing (after well washing in warm water, but with as little exposure as possible to the air) through a weak sour of sulphuric acid of 1-2 per cent. The acid should now be as thoroughly removed as possible by washing in water, and the goods should be sumached. The process, and especially the use of sulphuric acid, is always deleterious to the skins, and is one of the causes of the early decay of coloured bookbindings and furniture leathers. Lactic, formic, or acetic acid may be substituted for sulphuric with safety, and the risk of injury from sulphuric, which generally is only apparent after the lapse of a considerable time, is a good deal lessened by adding to the sumach liquor a small quantity of potassium tartrate, sodium acetate or lactate, or some other salt of a weak organic acid, which is thus substituted for the much more dangerous sulphuric. Except in cases of absolute necessity for the production of light shades, the use of sulphuric acid should not be resorted to, and then only for goods which are not expected to possess great permanence. For light shades for bookbinding and upholstery, good sumach-tanned leathers and organic acids only should be employed. Alkaline treatment also demands great caution, as excess of strong alkalies is very injurious to the leather. Another objectionable method for the preparation of leather for very light shades, is the use of the lead-bleach described on [p. 399].
The sumaching is best done in a drum, at a temperature of about 40°. Lamb advises that 1 to 2 lb. of sumach per dozen is sufficient for calf, and recommends running in this liquor for two or three hours. The skins are then rinsed in water to free them from adhering sumach, and set out on a table with a brass sleeker, and are now ready for dyeing with “acid” dye-stuffs. If “basic” dyes are used, thorough washing in several tepid waters is necessary to free them from the loose tannin; and if deep colours are to be dyed, it is better, instead of too much washing, to fix the tannin, which then serves as a mordant for the colour. For blues, blue-greens, or violets, this is done with a solution of “tartar emetic” (antimony potassium tartrate, of 5 to 20 grm. per liter according to the amount of tannin to be fixed, often with addition of some common salt), which produces no alteration in the colour. For browns, yellows, deep reds, or yellow-greens, it is advantageous to use titanium-potassium lactate or oxalate (2 grm. per liter), which in combination with the tannin produces a very permanent yellow coloration on which the basic colours dye freely. In many cases the titanium salt is best applied after dying with one of the dyewoods (Dreher).
The basic colours usually require simple solution in hot water before adding to the dye-bath, and are used in quantities of 0·5 to 2·5 grm. per liter of dye-bath, according to their colouring power, which varies a good deal, and to the depth of shade required. The solutions should not be boiled, and some colours are injured by too high a temperature. Some colours dissolve incompletely, and require filtration through a cotton cloth. As basic colours are precipitated by calcium carbonate, it is important that “temporary” hard waters should be neutralised with acetic or lactic acid till they faintly redden litmus; and in the case of colours which, from their attraction for the leather fibre, dye too rapidly, and consequently unevenly, better dyeing is often obtained by the use of a small excess of acetic acid, which also increases the solubility of the colour. Too much acid, however, will prevent the proper exhaustion of the bath. Some few colours, now little used, require to be dissolved in the first instance in a little methylated spirit; and the addition of spirit will often assist dyeing and staining where the leather is slightly greasy, though considerations of cost generally prevent its use. Sodium sulphate is not unfrequently added to dyeing baths to improve equality of dyeing; and with some of the cotton dyes common salt is used to lessen their solubility and facilitate the exhaustion of the dye-bath.
“Acid” colours usually dye better if acid is added to the bath, to liberate their colour-acids, and for this purpose sulphuric acid is generally used in weight about equal to that of the colour used. Its use is, however, objectionable, in this case, for the same reasons as in bleaching, since it is impossible by mere washing to remove it entirely from the leather, which it ultimately rots when concentrated by exposure to a dry atmosphere or high temperature; and it is better to use formic or acetic acid to the extent of two or three times the weight of the dye-stuff. Sodium acid sulphate may also be used, but is probably more objectionable than an organic acid. Many acid colours, however, dye quite satisfactorily from a neutral bath. The acid colours are used in somewhat similar quantities to the basic, but are generally inferior in colouring power, though they dye more evenly, especially on defective grain, and are often more permanent to light.
Mention has already been made of the polygenetic or mordant dye-stuffs, which are still used to some extent for dyeing glove-leathers, and of which logwood is important in dyeing blacks. Fustic and Brazil-wood (peach-wood) are not quite gone out of use among old-fashioned dyers, even for dyeing moroccos and other coloured leathers of vegetable tannage. Peach-wood, with a tin mordant (generally a so-called “tin spirits” made by dissolving tin in mixtures of hydrochloric and nitric acid) was formerly much used in dyeing cheap crimsons, but is now quite displaced by the azo-scarlets. The acid tin-solutions were frequently very injurious to the leather.
The wood-infusion, rendered slightly alkaline with soda, ammonia or, formerly, with stale urine, is usually dyed first on the leather, and followed by the mordant “striker”; ferrous or ferric solutions, and potassium bichromate being used for dark colours, and tin salts, or sometimes alum, for the brighter ones. The mordant is sometimes added to the dye-bath towards the end of the operation, but is better used as a separate bath, as it is apt to produce a precipitate of colour-lake on the surface of the skin, which rubs off on friction. In some cases, and especially in black dyeing, the strong infusion of dye-wood, and the necessary “striker” are successively applied by brushing instead of in the dye-tray.
Logwood and Brazil wood are both Cæsalpinias closely allied to divi-divi. Logwood is Cæsalpinia (see [p. 287]) Campechianum. Its colouring matter is hæmatoxylin, a substance nearly allied to tannins, and almost colourless; which on oxidation gives hæmatin, which dyes a yellow-brown, only developing other colours by the aid of mordants. Logwood chips are extracted by boiling or heating under pressure for some time with water; and as hæmatin gives dark purplish-red compounds with alkalies, soda or stale urine is frequently added under the mistaken belief that it produces a better extraction, but really leads to waste of colouring matter by oxidation. It is best to extract with water alone, and add any necessary alkali to the infusion before use. 1-2 lb. of wood per gallon is frequently employed in making the infusion, and as this proportion of water is quite insufficient to properly extract the wood, the residue should be boiled with one or more further quantities, which are employed in turn for extracting fresh portions of wood. Logwood dyes best at high temperatures, and especially in the case of chrome leather with which a temperature of 80° C. may be safely used. The presence of a trace of a salt of lime is advantageous, and with very soft waters a little lime water or chalk may be added to the logwood liquor.
In blacking skins, the strong infusion is rendered slightly alkaline with sodium carbonate or ammonia, and brushed undiluted on the leather. If employed as a bath, a somewhat weaker infusion is used, and the leather is frequently treated first in an alkaline bath, to which a small quantity of potassium bichromate is often added. The object of the alkali is not only to assist in the formation of the colour-lake, by saturating the acid set free from the iron-salt used as a striker, and thus to prevent the colour from penetrating the leather too deeply, but, at the same time, to overcome the resistance to wetting caused by grease or oil which the leather may contain. It must thus be used more freely when stuffed leather is to be blacked, but excess should be carefully avoided, as it easily renders the leather tender and brittle. The potassium bichromate oxidises the hæmatoxylin, or the ferrous salt subsequently applied, and forms a nearly black chrome-logwood lake.
The iron solution is generally either of ferrous sulphate of perhaps 5 per cent. strength, or commercial “iron-liquor,” which is a “pyrolignite” or crude acetate of iron, containing catechol-derivatives and other organic products from the distillation of wood, which act advantageously, both as antiseptics, and in preventing the rapid oxidation which occurs when pure ferrous acetate is used. Iron-liquor is generally to be preferred to ferrous sulphate (“green vitriol”), as the sulphuric acid of the latter, unless completely neutralised by the alkali employed in preparation, acts in the end disastrously on the leather. Commercial iron-liquor is often adulterated with ferrous sulphate, which may be detected by its giving a precipitate with barium chloride. Great care should be taken not to use iron in excess of the logwood or tannin present, as it otherwise takes tannin from the leather itself, making it hard and liable to crack, while any uncombined iron acts as a carrier of oxygen, giving up its oxygen to the colouring matter or tannin with which it is in contact, and again oxidising from the air, and so causing “spueing” or oil-oxidation, and other evils.
Good blacks which are more permanent than those with logwood, may be obtained by merely treating leather containing an excess of oak-bark tannin or sumach, first with an alkaline solution (not at the most stronger than 21⁄2 per cent. of liquid ammonia, or 5 per cent. of soda crystals), and then with iron-liquor. If it is not certain that the leather contains excess of a suitable tannin, a tannin-solution must be employed like the logwood infusion, or the leather must be sumached. The addition of some sumach to logwood liquor is often advantageous, and a blacker (i.e. less blue) black, especially on alumed leathers, is obtained by using a proportion of fustic. Solutions made by boiling 10 per cent. of cutch with 5 per cent. of sodium carbonate give good blacks with iron-liquor, and do not make the leather tender, and they can be used in mixture with logwood. Many commercial logwood extracts contain chestnut-wood extract as an adulterant.
Instead of dyeing in the bath, it is very common, especially for the cheaper leathers such as linings, and coloured leathers of the commoner sort, to apply the colour by brushing (commonly called “staining”). Many colours, however, which dye well with time and warmth, are inapplicable in this way, and only those should be used which have a strong attraction for the leather, and hence go on well in the cold. If “acid” colours are employed, it is essential to select those which can be used in neutral solution, or at most with addition of some mild organic acid such as formic or acetic, since, as the leather is not washed after staining, the sulphuric acid would remain in it, and would ultimately destroy it. Where leathers have a hard and repellent surface, the addition of a little methylated spirit to the dye is often very useful. The colours are used in solutions of from 1⁄4 to 1 per cent., which should be quite clear and free from sediment. Difficultly soluble colours must be used in weak solution, or the dye kept warm while in use. Dye-solutions will not generally keep for any great length of time without change.
Before staining, the leather must be carefully “set out,” or otherwise made as smooth as possible, and the staining is generally done after most of the other operations of currying or dressing have been completed. Staining is best begun with the leather in a slightly damp or “sammied” condition, and the colour is applied evenly with a softish brush in two or three coats, the leather being slightly dried after each. As a rule the more coats are applied, the more even is the work; but to save cost of labour it is common on cheap goods to be content with two, of which the first is given, preferably with a weaker solution, to the dry leather. Where the leather is “weak-grained” it is sometimes advantageous to size it first with a weak solution of gelatine, gum tragacanth, or linseed mucilage, and similar solutions are often used to fix the colour and give a higher gloss. The stearine-glaze mentioned on [p. 401] may also be used for this purpose, and a weak solution of it is sometimes employed as a vehicle for the acid colours. Acid yellows and browns may also be dissolved in the undiluted glaze where only a pale colour is required, or to heighten the colour of leather already stained. A list of suitable colours for staining is given in the Appendix, [p. 486].
It rarely happens in leather dyeing that the required colour can be given by the application of a single dye, most of the shades now required being produced by mixtures. It is, therefore, necessary to say a few words on the theory of colour combinations.
White light is of course composed of a mixture of all the spectrum-colours, and can be separated into them by the prism. It is probable, however, that the eye is only capable of three distinct colour-sensations, and that all the colours we perceive are represented by the excitement of these in different proportions, the actual colour-sensations being red, blue-green, and violet.[179] If we interpose a piece of yellow glass between the eye and white light, the violet and blue are absorbed, and the remaining red and green rays combine to produce the sensation of yellow. If pure blue glass is used, the red is absorbed, and we have blue as the result of the remaining mixture of green and violet. Red glass absorbs the whole of the green, and greenish-blue, allowing red and much of the violet to pass. Thus, if we combine blue and yellow glass, only the green is allowed to pass, and similarly with red and blue glass, green and blue is cut out, and only the violet remains. Thus red, yellow, and blue are frequently called the primary colours, and by combining all three in equal proportions all colours are cut out, and black or grey results. The blue and violet which are stopped by yellow glass are those colours which would produce the sensation of violet-blue, and hence the latter is called the “complementary colour” of yellow, and so on with the rest. It will be noted that all the colours of coloured objects are produced by absorption of a part of the light, and therefore coloured bodies are always darker than white ones, and where a colour is mixed with its complementary in suitable proportion, all colours are absorbed and black or grey is produced.
[179] The subject of colour is too complicated to be adequately treated here; and for fuller information, readers are referred to Abney’s ‘Colour Measurement and Mixture,’ S.P.C.K., London, 1891. It may, however, be pointed out that, while the true primary colour-sensations are unquestionably red, blue-green and violet, and by mixture of light of these colours, all other colours, including white, can be produced; the primary pigments or dyes are red, yellow, and blue; the effect being produced in the former case by the addition of colours, and in the latter by their subtraction.
Colours which are made by mixing two primary colours are generally called “secondary”; while the duller tints made by the addition to these of black, or of a complementary colour which produces black, are called “tertiary.” Any primary colour is complementary to the secondary colour produced by mixing the other two primaries and vice versa. The following tabular arrangement shows at once the effect of colour mixing.
| Pri- mary. | Secondary. | Tertiary. | |||
|---|---|---|---|---|---|
| Red | |||||
| - | Orange | with Black. | Brown. | ||
| Yellow | |||||
| - | Green | „ | Olive, Sage. | ||
| Blue | |||||
| - | Purple (Violet). | „ | Puce, Maroon. | ||
| Red | |||||
Theoretically, any colour may be obtained by mixture of the primaries, and that this is possible to a great extent is shown in the success of modern “three colour” printing, by which pictures are obtained in natural colours by the use of three primaries only; but in practice few colours are quite pure, and if two very different colours are mixed, it is difficult to avoid the production of tertiaries. The most brilliant colours are generally produced by dyeing with the nearest colour which can be obtained to that required, and shading with another which is near, but on the other side of the desired tint.
Thus if we want to produce bright shades in dyeing, we must avoid the introduction of complementary colours. A bluish red mixed with a reddish blue will produce a bright shade of violet, but if we mix an orange-red with a greenish-blue, we introduce yellow into the mixture, and obtain a dull maroon or puce according to the proportion of the other colours. In a similar way, the introduction of a blue dye will dull a bright orange to a brown, and a little of a yellow dye will dull a bright purple to a maroon. This fact is frequently used in producing the quiet shades of colour often required from the most brilliant dyes. If to a bright orange we add black, or a blue dye which as its complementary produces black, we convert it into a brown. If instead of blue we use green for dulling, we give the brown a yellower shade, since the green produces black at the expense of the red of the orange. Violet similarly used gives a redder brown, since it produces black by combination with the yellow. This shading, if small in amount, is frequently done by direct mixture of a suitable dye, but if considerable, it is generally better to top one colour with another. Thus a blue, topped with a powerful orange, will produce a Havanna brown. For dark colours, it is frequently convenient to produce a dark ground with some cheap dye, such as logwood and iron or chrome, and to top it with a bright shade of the colour required. In this way cheap dark blues and greens can be easily produced. For reds and browns, mixtures of logwood and Brazil-wood, or Brazil-wood and fustic may be used, topped with coal-tar colours. Tanning materials, such as quebracho and mangrove extracts, which give browns with bichromate, are also employed on cheap goods. It is also frequently wise to dye with a basic colour and top with an acid one, or vice versa; as in many cases the one fixes and combines with the other, and an increase of fastness is obtained.
Morocco and many other coloured leathers are finished by damping the surface of the dried leather with a very dilute “seasoning” of water, milk, and blood or albumen, allowing the leather to become quite or nearly dry, and polishing by friction under a cylinder of agate, glass, or wood in the glazing machine. Many leathers are also grained by printing from engraved or electrotype rollers, or by “boarding,” or a combination of the two. “Boarding” consists in pushing forward a fold in the leather on a table with a flat board roughed underneath, or lined with cork, in a way which is difficult to describe, but which in skilful hands wrinkles or “grains” the skin in a regular pattern.
The colour of a dyed skin is much altered by finishing and especially by glazing, which always darkens and enriches the colour. In dyeing to pattern, it is useful to glaze a little bit of the rapidly dried skin by friction with a smooth piece of hard wood for comparison, and a portion of the pattern may also be wetted for comparison with the wet skin. Colours which look full and even in the dye-bath, often go down in a most disappointing manner on drying, though to some extent they regain intensity on finishing.
In comparing the dyeing value of colours, the most practical way is to make actual dyeing trials with equal or known quantities of the colours and of water. Such trials may be made, either by “turning” the samples in photographic porcelain trays, kept warm in a water-bath (a “dripping tin” may be used for the purpose, the trays being supported a little above the bottom on tin supports soldered to the tin), or the leather may be hung from glass rods, by hooks of copper wire, in glass vessels (square battery jars), also placed in a water-bath. The leather samples should be of equal surface in every case; for suspension, pieces of “skiver” (sheep-grain) of 8 by 4 in. or 20 by 10 cm. are very convenient. These may either be “pleated” or suspended by the two ends grain side out, with a short glass rod to weight the fold, and keep them flat. The weight of colour used for a sample 8 in. by 4 in. multiplied by 54 times the area of a single skin in feet, will give approximately the weight of colour needed per dozen; which is, however, a good deal influenced by the mode of dyeing, and the quantity of water used.
In dyeing on the large scale, iron, zinc and even copper are to be avoided, the latter acting very injuriously on many colours, and on the whole wooden vessels are to be preferred. Though these become deeply dyed, they become very hard, and if well washed with hot water, and occasionally with dilute acid, they may be cleansed so as to give up no colour in subsequent dyeing operations, though of course it is not desirable, if it can be avoided, to use the same vessel for very different colours. Zinc rapidly bleaches many colours, especially while wet and slightly acid, and discharge-patterns may often be produced by pressing the wet leather on perforated zinc plates.
CHAPTER XXVI.
EVAPORATION, HEATING AND DRYING.
Questions of evaporation, whether for raising steam, or for the concentration of tanning extracts and other solutions are of considerable importance in the tanning industry, and as the same natural laws which apply to these equally govern the drying of leather, it is convenient to study the theory of the whole subject in one chapter, rather than to divide it, and place each part in a different portion of the book.
The modern conception of evaporation and vapour pressures has been described on [page 75], but it will be necessary to recapitulate a little. It is a well-known fact that most liquids, if left exposed in an open vessel, gradually disappear by evaporation into the air, even at ordinary temperatures. If the vessel is heated sufficiently, the liquid “boils”; that is, bubbles of vapour are formed in it, and escape, and the evaporation is therefore much more rapid. To avoid complication, let us first imagine a liquid sealed in a glass flask, which contains no air, but which is only partially filled by the liquid. It has been pointed out that the motion of heat by which the molecules of the liquid are agitated, enables some of them to break away from the attraction by which liquid particles are held together, and pass into the form of gas or vapour, which will fill the empty part of the flask. This evaporation will, however, soon reach a limit, since the vapour cannot escape from the flask. The flying molecules of vapour produce pressure by striking the walls of the flask, while a proportion of them will strike the surface of the liquid, and again be caught and retained by its attraction; and as the pressure rises, the number of these necessarily increases till a point is reached when as many fall back and are retained (or “condensed”), as those which evaporate, and the pressure will then remain constant. The amount of the pressure will vary with the nature of the liquid, and will be the greater the more volatile it is, or, in other words, the less the power of its internal attraction. It will also increase with rising temperature, which, by increasing the velocity of motion of the molecules, renders their escape from the liquid easier, and their recapture more difficult. It will not be at all affected by the volume of vapour or the size of the flask, but so long as any liquid is present, it will depend merely upon the nature of the liquid, and the temperature. If the flask is large, more of the liquid will evaporate till the same pressure is reached. If at the outset the flask is not empty, but filled with air, it will make no difference to the pressure or quantity of the vapour in it, which will be added to that of the air, whatever that may be. If the sealing of the flask is broken so that it is open to the atmosphere, air and vapour will escape, or air will pass in, till the total pressure is equal to the atmospheric pressure outside, (about 15 lb. per square inch). As, however, the vapour in the flask is always renewed by evaporation, so that the full vapour-pressure of the liquid is maintained, the “partial” pressure (as it is called) of the air in the flask will be less than that of the outer atmosphere by the amount of the vapour-pressure, which makes up the difference. Once this balance is attained, evaporation will go on very slowly in the flask, as it can only replace the small quantity of vapour which escapes. If, however, the vapour is removed by blowing fresh air into the flask, it will rapidly be replaced in the old proportion by fresh evaporation. Thus goods in a close room will dry only very slowly, even if the temperature is high, unless the moistened air is replaced by dryer air from the outside by some effective system of ventilation. In absence of this, evaporation only becomes rapid when the temperature of the liquid is raised to its “boiling point,” that is, when the vapour-pressure becomes slightly in excess of that of the atmosphere, so that the freshly formed vapour can push out that already in the flask or chamber into the outer air, and at the same time, bubbles can be formed in the interior of the liquid by the escaping vapour. As the vapour-pressure of a liquid rises continuously with increasing temperature, and its boiling point is defined as that temperature at which it is equal in pressure to the air (or vapour) in contact with it, it is evident that the boiling point must entirely depend on the pressure. Thus the boiling point of water in a boiler at a pressure of 55 lb. per square inch above the atmosphere is 150°C., and in a partial vacuum equal to 5·8 inches of barometric pressure, is only 60° C., a fact which is made use of in the concentration of extracts and other liquids at a low temperature in the vacuum-pan. (Atmospheric pressure is taken at 30 inches or 760 millimeters of the barometer or 14·7 lb. per inch, or 1·033 kilos per square centimeter.)
If a piece of iron is placed over a powerful gas-burner, it will go on getting hotter till its temperature is nearly or quite equal to that of the gas-flame. On the other hand, a pan of water, in the same condition, once it has reached its boiling point, becomes no hotter till all the water is evaporated. It is evident that the whole available heat or energy of the gas-flame is consumed in converting the water into steam. We might convert a proportion of this energy into mechanical work, by using the steam in a steam engine; but even without this, work is actually being done by the escaping steam in raising the weight of the atmosphere, and in overcoming the attractive force which holds the particles of water together in the liquid form. It is of course known to everyone, that energy may change its form, as from heat to work, but that it cannot be destroyed, diminished or increased; and therefore the whole of the work performed in converting the water into steam is again recovered as heat when the steam is condensed. In this connection a clear distinction must be made between quantity of heat, and temperature, which in popular language are often confused. It is for instance obvious that if we mix a pound of water at boiling temperature with another pound at freezing point, the temperature is altered to 50° C., but the total quantity of heat is unchanged. It is equally clear that no change in quantity of heat takes place when 1 lb. of mercury at 100° is mixed with 1 lb. of water at 0°, though in this case, owing to the small capacity of mercury for heat, the common temperature would only be raised to about 3°. We must therefore have some measure of heat apart from the mere direct indications of the thermometer, and that most generally used is the quantity of heat required to raise 1 kilo of water 1° C. (kilogram-calorie).[180] In England the heat required to raise 1 lb. of water 1° F. is also in use as a unit. The k.-calorie is equal to 3·97 (very approximately 4) lb. × F. units. For our purpose it may be taken that 100 k.-calories of heat are required to raise 1 kilo or liter of water from freezing to boiling temperature. If, however, the water is actually frozen, we require 80 k-calories merely to melt the kilogram of ice without perceptibly raising its temperature, and when the water is raised to 100°, 536 calories of heat are still necessary merely to convert it into steam at the same temperature. To melt 1 lb. of ice requires 144 lb. × F. units, to raise it to boiling point 180 more, and to evaporate it 965 additional. The quantity of heat required for actual evaporation varies a little at different temperatures, being somewhat larger at lower temperatures, but the total heat required to raise water from the freezing point, and convert it into steam at any pressure is nearly constant, being 635 calories at atmospheric pressure, and only about 650 calories, or 1180 lb. × F. units at 50 lb. per sq. inch. The quantity of heat evolved by the combustion of 1 lb. of good coal is 13,000 to 15,000 lb. × F. units; or of 1 kilo, 7200 to 8300 k-calories, but in raising steam in a good boiler coal will only evaporate 10 times its weight of water at 100° (5360 calories or 9650 lb. × F. units), the remaining heat being lost. 1 horse-power (33,000 foot-pounds per minute)[181] in the best engines requires about 11⁄2 lb. of coal or 15 lb. of steam per hour, but in those of worse construction may run up to many times that amount. As, even theoretically, not 20 per cent. of the total heat can be converted into mechanical work in a “perfect” engine working at 75 lb. pressure, it is often economical to use waste steam for heating or evaporation, and where this can be done profitably, the additional cost of the mechanical power is very small.
[180] A gram-calorie of one-thousandth part of the above is also in use for some scientific purposes, but the kilogram-calorie only is used in the following pages.
[181] This is equal to 76·04 kilogrammeters per sec., but the metrical horse-power is only taken at 75 kilogrammeters in France and Germany.
In evaporating liquids in the open pan 536 calories is required to evaporate 1 kilo of water already raised to boiling temperature, and a larger amount for salt-solutions, and it makes comparatively little difference whether this is done at 100° or at a lower temperature. Where, however, evaporation is done in vacuo, considerable economy can be effected by what are known as multiple “effects,” in which the steam from one vacuum-pan is employed to boil a second under a reduced pressure, and consequently boiling at a lower temperature. This principle can be practically applied to as many as five or six successive “effects,” the weaker liquor being usually evaporated at the highest temperature and lowest vacuum in the first “effect,” by the exhaust steam of the engine used for the vacuum pumps, while the steam from the first effect heats that of the next higher concentration, and so on. In the Yaryan evaporator ([p. 339]), the boiling liquid is sprayed through coil-tubes, thus exposing an enormous surface to evaporation, and the whole concentration of any given portion of liquid takes place as it passes through the apparatus, which does not, even in multiple effects, occupy more than 4 or 5 minutes; and without the temperature of the liquid ever rising above 60° or 70° C. In the case of liquids, like sugar- and tannin-solutions which are liable to chemical change from continued heating, the shortness of the time is a very great advantage. The number of effects which it is desirable to use depends greatly on the cost of fuel as compared to the largely increased cost of the apparatus. 1 lb. of coal employed in raising steam will evaporate 81⁄2 lb. in a single-effect Yaryan, 16 lb. in a double-effect, 231⁄2 lb. in a triple, 301⁄2 lb. in a quadruple, and 37 lb. in a quintuple-effect apparatus.
Where liquids are evaporated in the open air at temperatures below boiling, it is advisable by some means to spread the liquid in a thin film, so as to expose a large surface, which must be continuously removed by agitation, so as to prevent the formation of a skin. A good apparatus for this purpose is the Chenalier evaporator ([Fig. 92]), which consists of steam-heated copper discs rotating in a trough containing the liquid, which is taken up by buckets attached to the rims of the discs, and poured over their heated surfaces. In other forms, the liquid is allowed to trickle over steam-heated pipes or corrugated plates. Such evaporators should be placed in a current of air so as to rapidly carry off the vapour formed. Their use is very objectionable for liquids, like tannin-liquors, which are injured by oxidation, and they are not nearly so economical as vacuum-pans.
The drying of leather depends on the same laws as the evaporation of liquids, but demands special consideration from its very different conditions of temperature and supply of heat. It is important to remember that evaporation cannot go on unless the vapour-pressure of the liquid to be evaporated is higher than that of the vapour in contact with it, and that air-pressure does not prevent evaporation, so that if we sweep away the stagnant vapour with dry air, evaporation will go on as quickly as in vacuo, except that the liquid cannot boil. We must also bear in mind that evaporation consumes quite as much heat at low temperatures as in a steam boiler, and that this heat must generally come from the surrounding air, the temperature of which it reduces.
Fig. 92.—Chenalier Evaporator and Glue Coolers.
The rapidity of evaporation, and the quantity of moisture which can be taken up by a given volume of air depends on the vapour-pressure, which increases with temperature. The relation between the two, and the weight of water in grams per cubic meter which can be dissolved in dry air is given in the following table. (Grams per cubic meter is practically equivalent to ounces per 1000 cubic feet. Vapour-pressure is given in millimeters of mercury of the barometer, [p. 422].)
Vapour Pressure of Water.
| Temperature, | °C | -10 | -5 | 0 | 5 | 10 | 15 | 20 | 25 | 30 | 35 | 40 |
| „ | °F | 14 | 23 | 32 | 41 | 50 | 59 | 68 | 77 | 86 | 95 | 104 |
| Pressure, mm. | 2·2 | 3·2 | 4·6 | 6·5 | 9·1 | 12·7 | 17·4 | 23·5 | 31·5 | 41·9 | 54·9 | |
| Grams per cb. m. | 2·4 | 3·4 | 4·9 | 6·8 | 9·3 | 12·8 | 17·2 | 22·8 | 30·1 | 39·2 | .. | |
Air is practically never dry, and in damp weather is frequently saturated with moisture to the full extent corresponding to its temperature. In England the average quantity of moisture contained in the air throughout the year is 82 per cent. of the total possible, and even in the driest summer weather it is never less than 58 per cent. So long as the water is in the form of vapour, the air remains quite clear and does not feel damp; in fogs, the air is not only saturated with moisture, but contains small liquid particles floating in it. Of course when the air is really saturated with moisture, it has no drying power whatever.
As is evident from the table, the amount of water which can be dissolved in a given volume of air rapidly increases with temperature. Air at 0° C. is only capable of containing 4·9 grams per cubic meter, or not much more than 20 per cent. of what it can contain at 25° C. It hence rapidly increases in drying power as it is warmed, and consequently the air in a warm well-ventilated drying room in winter is generally much drier, and has greater capacity for absorbing moisture than the open air in the driest summer weather. This is the principal cause of the tendency to harsh and irregular drying by the use of artificial heat; and may be remedied by a proper circulation of the air by a fan without too frequent change with the colder air outside. On the other hand the use of a little artificial heat in damp summer weather, when the air is saturated with moisture, may be quite as necessary as in winter. The amount of moisture in the air is most easily ascertained by a device known as the “wet and dry bulb thermometers.” This consists of two thermometers mounted on a board; one of which has the bulb covered with muslin, and kept moist by a lamp-wick attached to it, and dipping in a vessel of water. The temperature of the wet bulb is lowered by the heat consumed in evaporation, and the difference of its temperature from that of the dry bulb is proportionate to the drying power of the air. This may be approximately calculated in grams per cubic meter by multiplying the difference by 0·64 for Centigrade or 0·35 for Fahrenheit degrees; and if deducted from the total capacity for moisture corresponding to the temperature of the wet bulb as given in [table], [p. 426], will give the actual moisture in grams contained in a cubic meter of air; but for practical purposes, all that is necessary is to find by experience the temperature and difference between the wet and dry bulbs, which gives the best result for the drying required, and to maintain it as nearly as possible by regulation of the heating and ventilation. Cheap forms of the instrument are made for use in cotton-mills, where it is necessary to maintain a certain degree of moisture; or it may be improvised from two chemical thermometers which agree well together. Distilled (rain or steam) water should be used to moisten the bulb, or it will quickly become coated with lime salts, and it should be placed in a draught, or its indications will not be accurate.
It is of course obvious that not only the wet thermometer, but the wet hides or skins are cooled by evaporation, and they, in their turn, cool the air with which they are in contact, which not only becomes moistened, but is lessened in its capacity for moisture by cooling, and thus rapidly reaches a condition when it can absorb no more moisture. It is thus necessary to maintain its temperature by artificial heat, or to replace it constantly by fresh air from the outside, and which of these expedients is most economical will depend on the temperature of the air outside as compared with that which it is required to maintain. If the outside air is sufficiently warm, and not saturated with moisture, it is generally best to use it in large quantities without artificial heat, wind usually supplying the necessary motive power for its circulation. Wet goods from the pits may thus be dried to a “sammed” condition by any air which is not saturated, and above freezing point; though the drying will often be slow. For drying “off,” artificial heat is generally necessary, since the attraction of the fibre for the last traces of moisture is very considerable, and to remove it the drying power of the air must be considerably higher than that required for the evaporation of free water.[182] In drying stuffed leather a temperature must generally be maintained sufficient to keep the fats employed in partial fusion, and so permit their absorption by the leather, while at the same time the drying must be gradual, or the water may be dried out before the fats have time to take its place. This is generally best attained by the use of artificial heat, and ventilation by circulating the air by a fan without its too frequent renewal, especially in cold weather. Frequently air which has been heated and used for drying off finished goods, and so partially saturated with moisture, may be used with advantage for wet goods, or for other purposes where a more gentle drying is required. If the temperature is low outside, the amount of heat consumed in heating cold air to the temperature required may be very considerable. The weight of a cubic meter of air at 0° C. and atmospheric pressure is 1·293 kilos, and its specific heat at constant pressure is 0·2375 of that of water. Therefore to heat a cubic meter of air at ordinary pressure and temperature 1° C. will require the same amount of heat as that used to heat 0·3 kilo of water to the same extent, or in other words 0·3 of a k.-calorie. If steam-heating is used, 1 kilo of good coal burnt under the boiler should heat about 1800 cubic meters 10° C., or 1 lb. should heat 52,000 cubic feet 10° F., assuming that the condensed water is not cooled below 100° C. These seem large volumes, but if we reflect that a 48-inch Blackman fan may move 30,000 cubic feet per minute, we shall realise that the cost of coal in heating air is not inconsiderable.
[182] Commercially-dry leather generally, if unstuffed, contains about 15 per cent. of residual moisture, which varies in amount with the weather, and can be more or less completely removed by drying at high temperatures. If leather has been over-dried, it only slowly regains its weight on exposure to cold air. Commercial disputes not unfrequently arise on the dryness of leather. In the opinion of the writer, a customer can only claim that the leather should be sufficiently dry not to lose weight when exposed to dry air at the ordinary temperature and degree of dryness of a warehouse or factory, and claims based on re-drying in hot drying rooms are distinctly fraudulent.
We must now consider the heat consumed by the actual evaporation of the water in the leather. The actual evaporation of water already raised to 100° C. consumes 536 k.-calories, but the evaporation of water which has not previously been heated so far consumes more heat, and we may take that required at ordinary temperatures as in round numbers 600 k.-calories per kilo, or 1080 lb. × F. units per lb. Disregarding small fractions, this is equivalent to the cooling to the same temperature of an equal weight of steam in the heating pipes, and this, as we have seen, demands about 1⁄10 of its weight of coal for its production from water already heated to 100° C.
The cooling takes place, in the first instance, in the leather, the temperature of which is reduced like that of the wet-bulb thermometer; and this in its turn cools the air in contact with it. Thus in air-drying without artificial heat, the whole heat must be supplied by the air and the loss reduces its capacity for moisture, greatly increasing the volume required. This is not of much consequence in open-air drying, since even a light wind will supply air in enormous volume. A moderate breeze of ten miles an hour moves about 15 feet or 41⁄2 meters per second. When, however, the air must be moved by fans, the power required becomes important. The evaporation of 1 kilo of water at summer temperature will cool about 2000 cubic meters, and that of 1 lb. 32,000 cubic feet of air 1° C.
In calculating the ventilating and heating power required in fitting up drying rooms, it is usually necessary to ascertain that required under the most unfavourable circumstances, and then add a liberal margin to cover errors and accidents. As the calculations are, in consequence of the many varying conditions, somewhat complex, it may be convenient to give as examples the quantities of air and heat required to evaporate 1 kilo (2·205 lb.) of water under different ordinary conditions, and these may serve as a basis of calculation of the drying power which must be provided for different tanneries.
1. Indifferent Open-Air Drying.—Air at 10° C. (50° F.), wet-bulb thermometer 7° C. (44·3° F.), indicating a total capacity for moisture of about 2 grm. per cubic meter; air not to be cooled beyond 7·75° C. (46° F.), leaving a residual capacity for moisture of 0·5 grm. per cubic meter. Each cubic meter will therefore take up 1·5 grm. of moisture, and as 1 kilo contains 1000 grm. we have 10001·5 = 666 cubic meters per kilo required to absorb moisture; and 6002·25° × 0·3 = 888 cubic meters reduced 2·25° to furnish the 600 cal. required for evaporation. Total air used 1554 cubic meters or 54,900 cubic feet.
2. Drying with Heat.—Outside-air at 10° saturated with moisture, heated to 20° C. (68° F.) acquires a capacity for 7·9 grm. per cubic meter. If we assume that a drying capacity of 2 grm. per meter is required to complete the drying, we have an effective capacity of 5·9 grm.
10005·9 = 170 cubic meters or 6000 cubic feet, and to heat this 10° C. will require 510 cal. Evaporation of 1 kilo will consume 600 cal. Total heat 1110 cal.
3. Drying with Heat.—Outside-air at 10° as above, heated to 25° C., giving an effective capacity for moisture of 13·5 - 2·0 = 11·5 grm. per cubic meter.
100011·5 = 87 cubic meters or 3070 cubic feet. To warm this 15° requires 391 cal.; and 600 cal. added for evaporation gives a total of 991 cal.
Comparing 2 and 3 we see that the higher temperature is more economical, where it can be allowed, than the lower, both in air and heat, though this is partly compensated by the greater loss of heat by cooling of the building, etc., which it entails.
4. Air at 0° C. heated to 20° requires about 97 cubic meters, or 3430 cubic feet of air, and a total of 1180 cal.
5. Air at 0° C. and heated to 25° C. requires 63 cubic meters or 2230 cubic feet, and a total of 1075 cal.
6. Air at -15° C. (5° F.) requires 4·5 cal. per cubic meter to raise it to 0° C., and acquires a capacity for drying of about 2 grm. per meter.
We will apply these figures to a drying room arranged with a screw-fan with a central division, or two floors, so that the air can be either circulated or replaced with fresh air from the outside at will (see [Fig. 94], [p. 435]). Such a room with 100 feet of length clear of space required for fans, air passages, and heating pipes, and 20 feet × 8 feet in section, should hang about 800 medium butts, weighing say 121⁄2 kilo (27 lb.) each, and when wet from the yard, containing the same weight of water. A 48-inch Blackman fan, under these conditions would probably move say 20,000 cubic feet (565 cubic meters) of air per minute, at the cost of 2 or 21⁄2 horse-power. This, in a room of the section named, would give an average velocity of 125 feet per minute or rather under 11⁄2 miles an hour; not at all too much to keep the air freely circulating among closely hung leather. If we assume that these butts are to be dried in a week (practically 10,000 minutes) under the conditions of No. 2, the 10,000 kilos of water they contain will require 1,700,000 cubic meters of air, or about 170 cubic meters per minute, or about 3⁄10 of the air must be fresh every time it passes through the fan. 1 kilo of water requiring 1110 cal. must be evaporated per minute.
Under the conditions of No. 4, only 97 cubic meters of air per minute would be required, or about 5⁄6 might be circulated without change, but the total heat required would be about the same, 1180 cal. Under the conditions of Nos. 4 and 6 some 1620 cal. per minute would be employed. It is hardly necessary to provide for the full amount of heat required by No. 6, since in this country such conditions occur but seldom, and never for more than a few days at a time, and during such a period, much less heat would suffice to carry on the drying at a slower rate, and keep out the frost.
Beside the heat required for actual drying, it is necessary to provide for that lost by the building during cold weather, and this is much more difficult to calculate. If, by arranging the outlet for moist air on the pressure side of the fan, the internal pressure of the building be kept a little lower than the outside, there can be no loss by escape of hot air, any leakage being inwards, and supplying a part of the change of air which, we have seen, is necessary. In a brick building with glass windows, the loss of heat is far less than in the old-fashioned wooden louvre-boarded structure, and where fan-drying is in constant use, the brick structure is much to be preferred. Frequent windows, with casements horizontally pivoted at the centre, will supply enough air for favourable conditions of air-drying, and when the weather is bad, resort is had to the fan. Most modern drying rooms in the Leeds district are built upon this plan. Where louvre-boarded structures must be used for fan-drying, the sides should be made as tight as possible in winter by sheets of canvas or sail-cloth nailed on, for which purpose old sails can be bought in seaport towns at reasonable rates, a few louvre-boards only being kept open for the admission of air in suitable positions.
Box, in his ‘Practical Treatise on Heat’[183] puts the loss through walls in brick buildings for a difference of 30° F. (16·6° C.) between inside and outside temperatures, at the approximate amounts shown in the following table.
[183] E. & F. N. Spon, Ltd., London.
Loss of Heat through Walls.
| Thickness of Wall in Inches. | K.-calories per Sq. Foot per Hour. | — |
|---|---|---|
| 4·5 | 1·76 | Stone walls must be about one-half thicker, to afford equal warmth with brick ones. |
| 9 | 1·44 | |
| 14 | 1·20 | The loss from glass windows amounts to 3 or 4 k.-calories per square foot per hour. |
| 18 | 1·06 |
If the building is of several stories, the loss to the roof in the intermediate ones need hardly be taken into account, but if the ceiling is not tight, and open to the roof, the loss may be great, but difficult to estimate. If we consider the drying room already described, the total area of the walls and ceiling is about 4000 feet, and to maintain its temperature 30° F. above the atmosphere at 1·2 cal. per sq. foot would require 4800 cal. per hour or 80 cal. per minute, a very small amount compared to that consumed in drying.
The following table calculated from data given by Box will give some idea of the amount of steam or hot-water piping required for heating. The sizes given are for the internal diameter of the pipe, allowance being made for the increased heating surface of pipes of ordinary thickness. Small pipes are considerably more effective in proportion to their surface than large ones, and for high-pressure heating 11⁄2 or 2-inch wrought-iron pipes are to be recommended as in many ways preferable to cast iron. The gilled or ribbed pipes now often used are also advantageous as giving a greatly increased heating surface.
Heat given by Steam-pipes.
| Steam Pressure, lb. per sq. in. | Temperature of Pipe. | K.-calories per hour per foot run of Pipe. | ||||
|---|---|---|---|---|---|---|
| °F. | 2 in. | 3 in. | 4 in. | |||
| 52 | 300 | 102 | 137 | 169 | ||
| 35 | 280 | 92 | 121 | 148 | ||
| 21 | 260 | 81 | 106 | 130 | ||
| 10 | 240 | 68 | 92 | 113 | ||
| 2·5 | 220 | 59 | 81 | 97 | ||
| 210 | 54 | 72 | 89 | |||
| 200 | 49 | 66 | 81 | |||
| 190 | 45 | 60 | 74 | |||
| 180 | 40 | 54 | 67 | |||
| 170 | 36 | 49 | 60 | |||
The temperature of the air to be heated is understood to be 60° F.; at lower temperatures the quantity of heat given off by the pipes would be greater, and at higher temperatures less; the amount being approximately proportional to the difference of temperature between the air and the hot pipes. It is also important to note that the table refers to steam-pipes in still air, and that if placed in a powerful draught, (as immediately before or behind the fan), their heating effect may be at least doubled. This has not been considered in the following calculations.
Applying these figures to the estimate of 1110 calories per minute required for drying in our building, and assuming 80 calories per minute for the loss of heat through the walls, we have a total of about 71,400 calories per hour, and to obtain this would require 736 feet of 4-inch pipe at 220° F. (heated by exhaust steam) or 700 feet of 2-inch pipe heated to 300° F. by steam at 52 lb. pressure.
If we adopt the estimate of 1620 calories of No. 5 and 6, we shall require 1050 and 1000 feet of the two pipes respectively, and this covers approximately the worst conditions. We must, however, remember that these estimates are made for continuous drying during the twenty-four hours, and that if the fan and steam are only applied during a portion of this time, the supply both of air and steam must be proportionately increased, or the time of drying correspondingly lengthened.
It is very desirable, however, that the fan should be driven by a small separate engine, the steam for which will only form a small proportion of that required for heating, and of which the whole of the heat will be recovered, since even that utilised in driving the fan will again be converted into heat by the friction of the air, and will therefore cost nothing. This arrangement will enable the drying to proceed so long as the necessary steam is maintained, which in bad weather can easily be done by the night watchman. It may also be pointed out that, during a great part of the year, the goods can be dried to a “sammied” condition without heat, or in the open air, or in the case of dressing leather, a considerable part of the water can be removed by pressing or squeezing, effecting a further economy.
Fig. 93.—Blackman Fan.
It must be left to the reader to apply the same calculation to other sorts of leather than sole, but it may be pointed out that the essential point, as regards heating and ventilation, is the weight of water to be evaporated in a given time, and that the actual size and shape of the drying room is unimportant, so long as adequate heating and circulation of the air between the leather is secured; and these remarks also apply to the particular form of fan or other ventilation employed, and to the means of heating. As the quantity of heat consumed is very considerable, it is well to look out for sources of waste heat which can be employed, or for means by which the heat of the fuel can be more directly and completely utilised than it is in raising steam. Thus a large amount of heat can sometimes be obtained by passing air through pipes or “economisers” fitted in a chimney-flue;[184] or gilled stoves or “calorifers” may be used in a separate chamber to directly heat the air which is drawn in by the fan.
[184] These pipes should be provided with scrapers to remove soot as in Green’s economiser, or their efficiency will be much diminished.
Fig. 94.—Section of Drying Rooms with Fan.
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[Figs. 93] and [94], furnished by the James Keith and Blackman Co., Ltd., give a good idea of the construction of screw fans, and the general principle of arrangement of fan drying rooms, the air in this case being circulated in opposite directions on two floors, and the amount of change being regulated by the shutters at A, etc. The grouping of pipes at the ends of the two floors which it shows is in general a good arrangement, but the length between them should not be too great, or the drying will be unequal in different parts of the room. Sometimes this is convenient; thus if most of the heat be supplied to the air coming fresh from the inlet on the upper floor, the damper and colder air of the lower room can be continuously used for drying wet goods from the yard, and the upper reserved for drying off the finished leather. A disadvantage of this plan is that open air drying can seldom be utilised except in an elevated building; and even when it is adopted, means should be provided for heating the lower room in cold weather. In place of two floors, it is obvious that a single floor may be divided into two compartments by a longitudinal partition. Whatever pipes are grouped at the ends of the building, it is advisable to arrange sufficient to prevent frost, against the walls, or in the old-fashioned way on the floors beneath the leather, but not too close to it, and protected by a wooden lattice on which the workmen can stand, which removes the risk of accident from wet leather falling on the hot pipes. The latticed space should be open at the end facing the air current, so as to receive a portion of the draught, which will become heated and ascend, its place being taken by damp and cold air from the leather, to be re-warmed. Water-vapour in itself is lighter than air, but the contraction produced by the cooling of evaporation more than compensates this, and the damp air is therefore heavier than the dry. The arrangement of hot pipes near the ceiling of a drying room, which has been borrowed from some American tanneries, is wrong in principle, unless the air is forced in at the upper part of the room, or the upper floor is latticed, and only acts in other cases when the air is thoroughly mixed and circulated by mechanical ventilators; while pipes near the floor will continue to produce a certain amount of circulation of the air, even when the fan is not running. In protecting pipes by lattices care should be taken not to confine them too closely, or their heating effect will be seriously diminished. In fan-drying, leather should be hung edgeways to the current of air, so as to allow of its free and uniform passage between. In the case of sole leather the butts or bends are conveniently suspended by S-hooks of brass or iron wire, to hooks or nails fixed in the joists. If gangways between the leather must be left in the direction of the draught, they should be closed at intervals in the length of the room by curtains or shutters, so as to deflect the air-current into the leather.
Screw fans like the Blackman can be used either to suck or to blow the air, though the former is preferable where it can be arranged, because it produces a more uniform current in the room. On the blowing side the air issues with considerable velocity in a sort of cone, but little coming through the centre of the fan, while that near the edges spreads rapidly from its centrifugal motion. This is rather advantageous where the fan blows into an open room, but involves waste of power where it discharges into narrow and square air-ways. The ends of the vanes of the Blackman are turned in at the rim of the fan to prevent this tangential discharge, but it is probable that where a fan is to blow into a room, it would be more advantageous to put it on the inner side of the wall, and without curved ends to the vanes, so as to distribute the air as widely as possible. A somewhat similar result would be attained with a Blackman, by placing it in a position the reverse of that for which it is intended, and running it also the reverse way; but its “efficiency” might possibly be lessened.
Screw-fans are good for moving large volumes of air at comparatively low velocities, and against little or no resistance, but they are quite unsuitable for forcing air against high resistance, or through narrow channels, and for this purpose centrifugal fans like the Capel ([Fig. 95]) are much more suitable, and mechanically more efficient. In any case there is much loss of power in forcing air through narrow airways, and if a screw fan must be employed for the purpose, the channel should be as large in section as the area of the fan, and all sharp angles in its course should be avoided. There is great loss of power where a current of air or water has to pass suddenly either from a wider to a narrower channel, or the reverse, and in both cases the resistance is diminished by making the enlargement or contraction gradual or “bell-mouthed.” Thus a pipe conveying water at a given head into or out of a cistern will discharge a much larger quantity, if the ends are bell-mouthed, than if it terminates abruptly. For the same reasons, air suffers considerable resistance if it has to pass suddenly into, or out of a larger space, such as a drying room; and unnecessary partitions, and other abrupt changes of dimension in the current should be avoided. Curves should also take the place of angles as much as possible.
Fig. 95.—Capel Centrifugal Fan.
Systems in which air is drawn or forced over systems of heating pipes by a centrifugal fan, and then distributed through comparatively small airways among the leather which is to be dried are in some cases convenient and advantageous. Among these may be mentioned the Sturtevant and the Seagrave-Bevington. There can be no valid patent on the general principle of heating by distributing air in this way, but only on the particular arrangement or appliances used in the special case. Centrifugal fans should be considerably larger in diameter than in axial length, those with long vanes of small radius being wasteful in power from the insufficient supply of air to the centre. There is also no reason why, in some cases, centrifugal fans should not be substituted for screw-fans in drying on the system which I first described, especially in cases where the air has to encounter considerable resistance, as for instance in traversing a filter to remove dust. One of the best filters for this purpose is a table of wire-gauze covered to a depth of 3 or 4 inches with loose wool. Hair or cheaper fibrous materials may be substituted for the wool, but are less efficient. The air must of course be sucked downwards through the gauze. When the wool becomes dirty, it may be washed, if possible in a wool- or hair-washing machine, and again spread on the table in a damp condition, as it will quickly be dried by the current of air. Flannel is also useful where the wool-filter is impracticable, but requires frequent washing.
Apart from wind, natural ventilation is seldom to be relied on for drying on any considerable scale. Heated air is, of course, lighter than cold, and this is the cause of chimney-draught, but to get a good circulation in this way, a high shaft, and high temperature is required. Nevertheless, in one of its best forms, the method has been a good deal used in America, in the so-called “turret-dryer,” a building of seven or eight stories in height, constructed of wood with latticed floors, and heated by steam-piping at the bottom, where the air is admitted. The method is not likely to be much used in this country, as apart from the questions of cost of building, fire-risk, and trouble of raising and lowering the leather, a good draught will only be obtained when the outer temperature is low in comparison to that inside, and in our milder and moister climate the conditions are not nearly so favourable as in the United States. As the air is rendered heavier by the cooling of evaporation to a larger extent than it is lightened by the water vapour, there is a tendency in drying by upward ventilation for the warm air to form local upward currents, while the cold and damp air falls back; and from this irregularity of flow, it is difficult to saturate the air equally. This may be avoided by downward ventilation, in which the warm air is admitted at the top of the drying room and the cold and damp air allowed to escape at the bottom. This fact suggests that in using systems of drying such as the Sturtevant, it would be better to place the distributing pipes at the top rather than the bottom of the room, but in this case care would have to be taken that there were no openings left by which the air could escape at the top of the room without descending through the leather. If this be avoided, the warm air will float on the top of the colder and damper, and press it uniformly down and out. I believe the merit of first having applied the principle of downward ventilation to leather-drying is due to Edward Wilson of Exeter. It is necessary that the hot air should be forced in at the top, or the cold air sucked out from the bottom; and the mere placing of hot pipes near the top of the room ([p. 436]) will not cause the required circulation. Wilson placed his heating pipes in a partitioned space at the side of the room, at the bottom of which cold air was admitted from the outside, which escaped into the room at the top. As the temperature of this side chamber was high and the air consequently light, an upward current was produced in it, though probably somewhat inefficiently, as the height of the column of heated air could only be small. Assisted by a fan, and circulating a part of the air, the method should give good results, especially over two (latticed) floors. As the air could not be satisfactorily heated in its downward course, the method would not be suited for more than about two floors, and the drying in the lower room would be cool and gentle.
One or two points in the practical arrangement of steam-pipes may be mentioned, as they are often overlooked even by professional engineers. The steam must always be admitted at the highest point in the system, and there must be a steady descent, without hollow places where condensed water can accumulate, to the steam-trap by which it is removed. In horizontal pipes, about 1 inch descent in 100 is sufficient. If water accumulates, there is not merely serious danger in case of frost, but during use, by the sudden condensation of the steam, a vacuum is frequently formed, into which the water is shot like the liquid in a “water hammer,” producing violent and noisy concussions, and in some cases even fracture of the pipes, or loosening of their joints. If high-pressure steam is used, a very small supply-pipe will feed a considerable system of heating pipes or radiators, but with exhaust steam, great pains should be taken to have pipes of ample size, to avoid back-pressure on the engines. In both cases it is often convenient to arrange the pipes, not as a continuous line, in which drainage is generally difficult, but in parallels like the bars of a gridiron. With high-pressure steam, there need be no fear, if the pipes are kept clear of air by allowing a little escape through small air-taps, of the steam failing to find its way to all parts of the pipe, as a vacuum is produced by condensation in proportion to the heat given off. With exhaust-steam, no steam-trap is desirable, but any steam not condensed should escape freely into the open air or a chimney (after separating condensed water), and it is well to render the resistance in all the pipes of a gridiron approximately equal, which may be done by admitting steam at one corner, and allowing it to escape at the opposite (diagonal) one. In the arrangement of steam-pipes in parallels, the practicability of repair to one pipe or joint without interfering with the others must always be considered. If screwed wrought-iron pipes are used, each parallel must be provided with a bolted flange, or “running socket,” to permit of unscrewing. The difficulty of accurately adjusting the lengths of the several parallels must be considered, especially with flanged metal pipes, and also their motion by expansion when hot, which amounts to 1 or 2 parts per 1000 of length according to the temperatures of steam and air. Expansion-joints with stuffing boxes are costly and troublesome, and apt to leak, and may in many cases be avoided by suitable arrangement of the pipes. Thus instead of having the pipes rigidly fixed at both ends, one end of the system may be left free to move, each pipe being separately returned to an exit pipe at the same end but lower in level than the supply; or a single exit pipe may be thus returned, its expansion and contraction being practically the same as that of the heating pipes. In moderate lengths of wrought-iron pipe, sufficient relief may often be obtained from the flexure of the pipe, if in some part of its course it is carried at right angles to its general direction, which is often necessary for other reasons. If pipes are laid in long lengths, the loose end should be supported on rollers or short pieces of pipe, so as to avoid moving the supports or straining the pipe in expansion.
It is useless to attempt to regulate the temperature of low pressure steam-pipes by turning down the steam, since, so long as the pipe is supplied with sufficient steam to fill it, its temperature cannot be less than 100°, and even with high-pressure pipes, the power of regulation by altering the steam-pressure is very limited. It is far better to arrange the pipes or radiators in groups, from some of which the steam can be turned off entirely when less heat is needed. It must not be forgotten that if these discharge into a common steam-trap, it will be necessary to turn off their exits as well as their steam supply, or steam will come back into them from the other pipes, and probably prevent the escape of condensed water. In some cases it is more convenient to give the several sections independent exits or steam-traps.
Many good steam-traps are now on the market, depending either on the expansion and contraction of metals, or on floats in a closed box, which open a valve as the water accumulates. Traps of the latter class with closed copper balls are to be avoided, as the ball is sure eventually to become filled with water. Several traps have been devised in which an open vessel is used as a float, which is always kept empty by the discharge of the water through a pipe dipping into it.
The condensed water from steam-pipes is rarely suitable for use in the tannery, from the dissolved and suspended iron-oxide which it contains, from which it can only be freed by boiling and filtering, or treatment with precipitants ([p. 95]). Its most appropriate use is generally return to the boiler. Systems were formerly in vogue by which it was allowed to run back to the boiler as it condensed, but these could only answer when the pressure in the pipes was equal to that in the boiler, which is rarely the case. It must generally be forced in by the feed-pump or injector.
Hot water has often been advocated in preference to steam for heating, but is more costly, as it requires a separate boiler, and much larger pipe-surface for the same effect. Its only important advantage is that the pipes maintain their heat for some time, even when the fire has gone down, while steam-pipes cool at once if steam is allowed to go down in the boiler. In any considerable tannery, however, this will seldom or never be the case, since if a good pressure of steam is up at night, when the fires are banked up, the boiler will in itself contain a large reserve of heat, and, of course, working pressure will be required before the engines can start in the morning. Hot water systems require careful planning to obtain reliable and uniform circulation.
CHAPTER XXVII.
CONSTRUCTION AND MAINTENANCE OF TANNERIES.
As few architects have specially studied the construction of tanneries, and in most cases much of the arrangement depends on the knowledge of the tanner himself, a short chapter on the subject will not be out of place.
In the selection of a site, a clay or loamy soil is to be preferred to a gravelly or sandy one, as lessening the liability to leakage, and waste of liquor. Perhaps, however, the first consideration of all is the possibility of drainage and disposal of effluent waste liquors and washing waters, since it is now rarely possible to run these, without previous treatment, into a river or stream. Some information is given in [Chapter XXVIII.] on the methods of partial purification which are available to the tanner, but these are always costly and troublesome, and the possibility of running direct into a sewerage system, or a tidal river is of great advantage. Under the Public Health Act, authorities are bound to receive manufacturing effluents into their sewers if the latter are of sufficient capacity, and the effluents not such as either to damage the sewers, or interfere with the processes of purification adopted by the authority. This act is in many districts practically superseded by special legislation, but tanners’ effluents are generally received into sewers if freed from solid matter. When mixed with other sewage, they do not interfere with irrigation or bacterial treatment. In selecting a site within a sewered district, regard must be had to the possibility of causing a nuisance to the neighbourhood by foul smells. Really injurious smells should not be caused by a properly conducted tannery, but it is difficult to avoid odour, and a single badly disposed neighbour may cause infinite trouble and expense.
Another important consideration is the water supply, since for the large quantities used in a tannery, town water is generally very expensive. With regard to quality and impurities of water information may be found in [Chapter X.]; but, as a general rule, the softer and purer the supply the better. It is also of great advantage when the source is at such a level that the water can flow into the tan-yard, or at least into the beam-house, without pumping. Filtration too, when needed, is much facilitated by a sufficient head of water.
Commercial facilities, such as nearness to markets and sources of supply of raw materials, and the availability of rail and water carriage are of an importance at least equal to the points already considered, but hardly come within the scope of this work.
The site chosen, the next question is the arrangement of the buildings. It is very doubtful, where ground is not inordinately expensive, whether it is wise to erect drying-sheds over the pits. In case of fire, very serious damage is done to liquor and leather by the heat and burning timber. If the turret form of drier be decided on, strong foundations are required, and the ground-floor or basement is occupied with heating apparatus; if fan-drying, no lofty buildings are needed, and the drying rooms are conveniently placed over the finishing and currying shops; and, on the other hand, the tan-house may be easily and cheaply covered with slated roofs, with nearly vertical sections of glass, to the north if possible, like a weaving-shed, through which sufficient light for convenient work and cleanliness is admitted. The direct rays of the sun should be avoided, but in the writer’s opinion the balance of advantage is largely in favour of a liberal supply of light. Iron roofs are unsuitable, since the moisture condenses on, and rusts them; and particles of oxide fall into the liquors, and cause iron-stains.
Good ventilation along the ridge of the roof should be provided, wherever there is any steam or hot liquor used; or the condensed moisture soon leads to decay.
In arranging the general plan of the buildings, much depends on local circumstances; but as far as possible, they must be so arranged that the hides and leather work straight forward from one department to another with as little wheeling or carrying as possible; that the buildings where power is used be near to the engine so as to avoid long transmissions, which are very wasteful of power; and that the different buildings be so isolated as to diminish the risk of the whole being destroyed in case of fire.
A chapter on the construction and maintenance of tanneries and leather works would be incomplete if it did not refer to the very important question of Fire Insurance.[185] To an extent this may be regarded as a fixed charge against any business, very much in the same way as local and imperial rates. It is not, however, to be lost sight of, that to some considerable extent the amount of insurance premium is regulated by the insured himself. If a man conducts his business in unsuitable and badly constructed buildings; if attention is not paid to some of the elementary hazards connected with a fire outbreak; he must not blame the insurance companies for the demand of what he considers an excessive premium. If this faulty construction and imperfect equipment of buildings pertain to any considerable extent throughout a given trade where the process is more or less hazardous, it is futile to appeal to insurance companies, which, after all, are merely commercial and not charitable institutions, for a reduction in the rates. The only standard to guide the company is the loss-ratio, and given a high loss-ratio, there must be a corresponding premium paid.
[185] With regard to fire insurance, I am much indebted to Mr. A. W. Bain, of Leeds for valuable information.
There is, however—thanks to modern science—a method available whereby the great bulk of fires may be checked in their inception; an appliance, automatic in its operation, and of proved efficiency. This appliance is known as the sprinkler. A system of water-pipes is fixed under the ceilings of the building to be protected, to which are attached sprinkling jets at suitable intervals, each of which is closed by a valve held in place by a joint of fusible metal, which gives way if the temperature rises beyond a given point. There are two or three recognised patterns approved by the Fire Offices Committee after patient investigation and practical test. These appliances have now been at work for something like fifteen years in this country. One of the first trades to recognise their utility was that of the cotton-spinner. At one time serious fires in the cotton trade were of frequent occurrence. Now—owing to the efficient fire appliances—while fires may be as frequent in their inception as formerly, they are stopped at such a stage as to prevent any considerable loss. The consequence has been that the cotton-spinner, at one time the owner of a highly-rated risk, and one which few companies cared to insure, is now in the position of having his business eagerly sought after, and large discounts offered him off the charges he was once called upon to pay.
More important still is the consideration to him that his business is not so liable to be interfered with or stopped as the result of fire. There are, it is estimated, at the present moment, no less a proportion than 90 per cent of the cotton-spinners whose premises are protected by sprinkler installations.
Other hazardous risks such as corn-millers’, woollen and worsted manufacturers’, saw-millers’, engineers’, are adopting these appliances freely, and it is a matter of surprise that so very few tanneries or currying shops—so far as I have been able to learn, not more than twelve—have done the same. The consequence is that the loss-ratio in tannery risks still retains its unenviable notoriety: the rates for fire insurance have risen considerably, and as a result the tanners’ profits are correspondingly less. Considering the extent and importance of many of the tannery risks throughout Great Britain, one can only express surprise that these appliances have been so little adopted.
The construction of a new tannery demands serious attention from an insurance standpoint. The boiler-house should be a detached building; the grinding of bark and myrobalans should be conducted in buildings isolated from the general works; in fact no better advice could be given to a tanner, either in the construction of new premises, or the rearrangement and remodelling of old, than to consult an experienced insurance man, whether official or broker, as to the best means of constructing and arranging to secure the most favourable terms.
Another point which should be provided for, and which is often overlooked, is the feasibility of future extension without serious changes of arrangement. It may be taken as a probability of the future, even if it be not already a fact, that small tanneries cannot be made to pay, and that if a business succeeds, its extension will prove desirable; and in an ill-planned yard this may involve either entire reconstruction of a very expensive and inconvenient sort, or the separation of new departments, so as to involve serious increase of carrying. A good arrangement is that of a long front building serving to connect the whole, behind which the various departments are erected at right angles leaving room for extension backwards as required.
As regards the first of these conditions, if the various soaks, limes, bates, and handlers are well arranged, it is hardly necessary to do more than draw the goods from one pit into the next throughout the whole of the process. To, and from the layers, the goods must generally be carried or wheeled. In the sheds, if it be a sole-leather tannery, the butts should first come into turrets or open sheds for the rough drying; then into a room sheltered from draughts to temper for striking. The striking machines or beams should be in an adjoining room, or immediately below; then a small shed-space for drying before rolling; next the roller room; and then the warm stove for drying off. If two of the latter can be provided to be used alternately, it will allow the goods to be aired off without taking down, and they may then be immediately handed or lowered into the warehouse, without fear of over-drying, which is sometimes difficult to avoid where leather must be taken direct out of the hot drying-room. The same principles are easily applied in yards for lighter leathers.
To lessen loss of power in transmission, the engine should be near the centre of the main range of buildings, with perhaps the grinding machinery on one side, and the leather finishing on the other; but this would be rather liable to increase the fire-risk. A very good plan would be to have the engine-house in the centre as suggested, but separated from the buildings on each side by brick gables; and with the boiler-house behind it, and under a separate roof, say of corrugated iron. If it be impossible to have the engine near its work, it is in most cases better to employ a separate high-pressure engine, which may be within a glass partition, and will work all day with scarcely any attention. The loss of power in carrying steam for moderate distances through sufficiently large and well-clothed pipes is much smaller than that of long lines of shafting. The writer has known cases where fully half the indicated power of the engine was consumed in friction of the engine, shafting and belts. High-pressure engines are as a rule to be preferred to condensing for tannery use, since the waste steam can generally be employed for heating, and both the first cost and that of maintenance are smaller. Where much fuel is used, it is quite worth while to have the cylinders indicated occasionally, both running light, and driving the machinery; much information is gained in this way as to the power spent on the various machines, and very frequently large economy is effected by proper adjustment of the valves. To work economically, an engine should be of ample power for all it has to do; and adjusted to its work, not by lowering the pressure of steam, or by checking it at the throttle-valve, but by setting the slide-valves to cut off as early in the stroke as may be. As to how early this is possible, an indicator-diagram will at once give information. If the whole of the waste steam can be used profitably for heating purposes, economy in the working of the engine is of little consequence, but, otherwise, it is very injudicious, for the sake of a little saving in first cost, to put in an old or inferior engine, which has to be dearly paid for in waste of fuel. In the choice of an engine, the advice of an expert engineer is desirable, since many engines which are mechanically well made, are uneconomical through the faults of a rule-of-thumb design. In this respect the English engine-builder is frequently inferior to his better trained continental competitor.
In place of using small steam engines to distribute power, electric driving deserves consideration. For long drives the loss of power is much less than that of shafting, and by concentrating the whole production of the power in one large and well-constructed engine, the cost per horse-power can be much reduced. While large and well-constructed engines may develop 1 horse-power at a cost in coal of 11⁄2 lb. per hour, it is not uncommon to use 12 lb. for the same output. In tanneries, however, the power used bears a much less proportion to total expenses than it does in the textile and many other trades. The first cost of electric driving is somewhat high. Motors of the “armoured” or iron-cased type must be used in all positions where they are subject to wet or dust. It must be borne in mind that an electric motor will not start against a heavy load, as it only develops its full power at a high speed, and if it receive the full pressure of the current before this is attained, its coils will probably be burnt out, unless saved by the melting of its safety-fuse. A similar danger is incurred, if the motor is brought up by overloading while the current is on. It is therefore generally necessary to connect a motor with its work by a belt which is only brought on to the working pulley when its full speed is attained.
In some cases the use of gas-engines is convenient and economical; for though gas from town-supplies is an expensive fuel, the best gas engines give a higher mechanical efficiency than steam-engines, and they work with very little attention.
In arranging shafting, moderate speeds, say 100-150 revolutions per minute, should be chosen for main lines, and when higher speeds are necessary, they should be got by light and well balanced counter-shafts, with wrought iron or wooden pulleys. (Cp. [p. 452].) In calculating speeds, it must be remembered that they vary inversely as the size of the pulleys. Thus a 3-feet pulley running at 100 revolutions will drive a 2-foot pulley at 150 revolutions, and a 12-inch one at 300. Of course the higher its speed, the more power any shaft will transmit, but increased friction and wear and tear soon limit this advantage. The velocity of a belt in feet per minute is obtained by multiplying the number of revolutions per minute by the girth of the pulley in feet or by its diameter multiplied by 31⁄7, or more accurately, 3·1416.
Pulleys should always be of ample breadth for the power they have to transmit; and it is more economical, both in power and cost, to use broad single belting than the same strength in double. If the pulley will not take a belt broad enough for the work it has to do, a second belt may be made to run on the top of the first, as suggested by Mr. J. Tullis, and will do its share of the work. Belts should be washed occasionally with soap and tepid water, and oiled with castor or neatsfoot oil; but if of sufficient breadth, should not require the use of rosin, or adhesive materials, to make them grip the pulley. Chrome-leather belts should be kept thoroughly oiled. They have a much greater adhesion than vegetable tannages, and this is increased by oiling. Good chrome belting is much stronger than bark-tanned; and is unaffected by damp or steam, but generally stretches somewhat more. Makers of machines often err in constructing their driving pulleys too small both in breadth and diameter.
The horse-power which a belt is capable of transmitting obviously varies extremely with circumstances, but may be approximately calculated by the formula a . v66000, where a is the area of contact of the belt with the smallest pulley, and v its velocity in feet per minute. Another rule is, that at a velocity of 1000 feet per minute, each inch of breadth of belt should transmit 21⁄2 horse-power on metal pulleys, or 5 on wooden ones, on which the adhesion is greater. Adhesion may also be increased by covering the pulleys with leather or indiarubber. Both rules assume that the belt is of ample strength. One horse-power would be transmitted by a belt running 1000 feet per minute with a pull of 33 lb. A good single belt should not break with a much less stress than 1000 lb. per inch of breadth, and should stand about 1⁄10 as much as a working stress.
The following table gives the experimental breaking stresses and extensions of some leathers. It may be noted that 1 square inch sectional area is equal to a belt 4 inches wide × 1⁄4 inch thick; and that kilos per cm2 × 14·22 = lb. per inch2.
Breaking Stresses of Leather.[186]
| — | Kilo per sq. centi- metre. | Lb. per sq. inch. | Stretch per cent. | ||
|---|---|---|---|---|---|
| Belting | leather, | layer system | 283 | 4,030 | 25·4 |
| „ | „ | Durio system | 298 | 4,240 | 21 |
| Well-tanned chrome leather | 740 | 10,500 | 32·5 | ||
| Over-tanned chrome leather | 234 | 3,330 | 23 | ||
| Stuffed alumed leather | 835 | 11,900 | 38·3 | ||
| Alumed “rawhide” | 921 | 13,100 | 31·4 | ||
[186] ‘Gerber,’ 1900, p. 73.
Good English tanned belting leather breaks at from 4500 to 5500 lb. per sq. inch sectional area.
Over-tanned leathers are less tough, whether of vegetable or mineral tannage, than those somewhat lightly tanned, and the tensile strength of leather varies considerably with the part of the hide from which it is taken, that from approximately over the kidneys being the strongest. Even thick and tough leather is easily torn if a cut or nick is once started, and all holes used in jointing belts should be carefully rounded. Glucose, and the use of acid in bleaching both lessen the toughness of belts, and they may also be rendered tender by the heat evolved in slipping on a pulley.
Countershafting and high-speed machinery, such as disintegrators, striking machines of the Priestman type, etc., should run without material jar or vibration. If this occurs, it is generally a sign that the running part is not equally balanced. In this case the shaft or spindle must be taken out of its bearings, and supported on two exactly horizontal straight-edges, on which it will roll till the heaviest part is downwards; and weight must then be taken off or added till it will lie in any position. In this way the writer has had to add fully 2 lb. of iron to balance the drum of a striking machine before equilibrium was secured, and a most troublesome vibration prevented. Of course all machinery should be supported as solidly as possible; and if circumstances permit, most machines are better on a ground floor. In placing bark mills, however, it is frequently convenient to fix them at a higher level, so that the ground material may be sent down shoots by its own weight to the required places. An alternative plan is to set the mill on the ground over a pit, and to raise the ground material with a bucket-elevator. This may be done successfully by letting the material fall directly from the mill into the buckets; but otherwise it must be thrown in with a shovel, as buckets will not pick up ground bark, even from a hopper; and in any case such elevators are apt to be troublesome. In a grinding plant designed by the writer, the unground material was filled on the basement floor into an iron barrow, which was wheeled into an iron sling working between upright guide-rails like a hoist. On pulling a brake line, the barrow was raised to the top of the building, and its contents were tipped into a large hopper, after which the barrow righted itself, and descended for another load. In the bottom of the hopper was a sliding shover, which forced the material on to vibrating screens, by which it was guided either into a disintegrator, or crusher-rolls, at pleasure. Both these discharged through iron spouts into large hoppers on the outside of a brick gable, from which powdery materials like myrobalans and valonia could be run direct into barrows or trucks. It is very desirable that such hoppers should be separated from the main building by a fireproof partition. Fires may occur from hard substances getting into disintegrators along with the bark, etc. and if this occur with a dry and dusty tanning material, it is not unlikely that it may result in an explosion such as sometimes happens in flour mills, in which the fire is rapidly conveyed along spouts, and into chambers filled with dusty air. Insurance companies generally charge an extra rate for disintegrators, and it is very desirable to keep the mill-house structurally apart from other buildings, either by actual separation or by the introduction of brick gables dividing the roofs. On the whole, however, mills of the coffee-mill type are probably quite as dangerous as disintegrators; since if they become partially choked, the heat caused by friction is very great.
In America, the fire-risk from mills is often lessened or prevented by the introduction of a jet of steam into the chamber or spout by which the mill discharges, but this is only permissible if the tanning material is conveyed at once to the leaches or yard.
The use of chain-conveyors for handling tanning material both wet and dry is practically universal in America, though comparatively rare in England. Various forms are used, the most common consisting of a chain of square links of malleable cast iron which hook into each other, so that a broken link can be immediately replaced (see [p. 325]). At intervals special links are inserted, which can be had of various patterns, for the attachment of scrapers or buckets. The endless chain runs in a trough of rectangular or V-shaped section, and is driven by a toothed wheel, over which it runs like a belt. In some cases the returning half of the chain can be utilised to bring back the spent tan on its way to the boiler house. For dry materials, cotton or leather belts with short wooden cross-laths attached, may often be used satisfactorily in place of the chain.
For lubricating purposes, mineral oils of high density are not more dangerous than animal or vegetable, but rather the reverse; as, though they are possibly more inflammable, and make more smoke, their mixture with cotton-waste and other porous materials is not spontaneously combustible, as those of vegetable and animal oils occasionally are. The danger of spontaneous combustion is very considerable when heaps of leather shavings or cuttings containing fish-oils are allowed to accumulate in warm workshops, and, especially near steam-pipes. Heavy mineral oils should always be used as cylinder-oils in high-pressure engines, in preference to other oils or tallow, since they are not decomposed by steam, and do no harm if blown into the feed-water, but serve to loosen and prevent scale and deposit. Ordinary oils and tallow, on the other hand, when submitted to the action of high-pressure steam, are separated into glycerin and fatty acids (see [p. 351]), and the latter corrode the valve faces and seatings, and are liable with “temporary hard” waters to form a very dangerous porous deposit in the boilers, which often leads to overheating of the tubes.
Next to the machinery, the pits demand special consideration. The chapter on the subject in the late Mr. Jackson Schultz’s book on ‘Leather Manufacture,’ is well worth attentive study as giving American practice on the subject.
The old-fashioned method of sinking pits is to make them of wood, and carefully puddle them round with clay, which should be well worked up before use. It is of no use to throw it in in lumps and attempt to puddle it between the pits, which will not be made tight, but probably displaced by the pressure. Such pits, if made of good pine and kept in constant use, are very durable, some of the original pits at Lowlights Tannery, constructed in 1765, having been in use till 1889. Loam mixed with water to the consistence of thin mortar may also be employed, the pits being filled up with water, to keep them steady, at the same rate as the loam is run in. Probably the best materials for pit-sides are the large Yorkshire flagstones. Where these are not attainable, very durable pits may be made of brick, either built with Lias lime, and pointed with Portland cement, or built entirely with the latter. Common lime cannot be used, as it spoils both liquors and leather; and even cements with too large a percentage of lime are unsatisfactory. Brick and common mortar are, however, suitable for lime-pits, and for these Mr. C. E. Parker’s plan of constructing the bottom of cement, the ends and sloping hearth of brick, and the sides of 3-inch planks bolted together is also very satisfactory ([Fig. 96]).
The writer has constructed wooden pits in two ways. In the one case, after making the excavation, beams were laid in a well-puddled bed of clay; on these a floor of strong tongued and grooved deals was laid, and on this the pits were constructed of similar wood to the floor, and puddled round with clay. In the second case the pits were built like large boxes above ground, and when finished, lowered on to a bed of clay prepared for them, and then puddled both around and between. It may have been due to defective workmanship in the first case, but those made on the last-named plan, which is that adopted from very early times, certainly proved the tightest and most satisfactory. Mr. Schultz describes a plan as the Buffalo method, in which a floor is laid as just described, and grooves cut with a plane for the reception of the sides, which are formed of perpendicular planks, each end and side being finally tightened up by the insertion of a “wedge plank.” Owing to the perpendicular position of the side-planks such pits would be difficult to repair in the common case of decay at the top.
Fig. 96.—Mr. C. E. Parker’s construction of Lime-Pits.
If bricks be used, great care must be taken that the cement is not merely laid so as to fill the joints towards the two surfaces of the wall, as is the habit of modern bricklayers, but actually floated into all the joints so as to make the wall a solid mass; or leaks can hardly be avoided. Hard pressed bricks are best, and should be tested as to whether they discolour liquor. Cement-pits are very good, and, though not particularly cheap in material, which must be of the best, are readily made by intelligent labourers under good supervision. The first step is to lay a level floor of good concrete, in which glazed pipes for emptying the pits may be embedded; care being also taken that all joints in these are thoroughly tight, since future repairs are impossible. The next step is to make frames, the exact length and breadth of the pits required, and perhaps 15 inches deep. These are arranged on the floor where the pits are to be, and the intervening spaces are filled with concrete of perhaps 1 of cement to 3 or 4 of crushed stone or brick. Rough stones and bricks may also be bedded in the concrete as the work goes on, to help to fill up. After the first layer has set, the frames may be raised and a second added, and so on. The work is generally finished by floating over it, while still damp, a little pure cement, to give a smooth surface. Before using, the cement should be tried on a small scale, to be sure that it does not discolour leather or liquors, and the pits should always be seasoned with old or cheap liquor before actual use.
Fig. 97.—Cleaning Rod Joint.
If possible, both leaches and handler-pits should be provided with plugs and underground pipes, communicating with a liquor-well some feet below their levels. Glazed fire-clay is very suitable both for pipes and plug-holes, which should be in the pit corners. If fire-clay blocks for plug-holes cannot be obtained, they may be cast in good cement, the wooden mould being soaked with hot paraffin wax to prevent adhesion. Means must be provided for the ready clearing of the pipes when choked with tanning materials. A good plan is to let each line of pipes end in a liquor-well large enough for a man to go down. As it is almost impossible to make plugs fit without occasional leakage, it is not well to run pits with very different strengths of liquors to one well, but the layers, handlers, and different sets of leaches should each have their own, so as to avoid mixture. A good means of clearing pipes consists in a series of iron rods 3-4 feet long, connected by hooks fitting into double eyes, as shown in [Fig. 97]. It is obvious that in a narrow pipe or drain, these cannot become disconnected. Pipes may often be forced out by fitting a strong delivery-hose of a steam-pump into one of the plug-holes.
It is, as Schultz points out, of questionable advantage to lay wooden troughs under the alleys for supplying liquor to each pit, since it is almost impossible to preserve them from decay; but the same objection would not apply to glazed pipes, jointed with pitch or cemented. A good and cheap plan in practice, is to let the liquor-pump, or a raised liquor-cistern, discharge into a large and quite horizontal trough raised 6 or 7 feet above the level of the yard, and provided with plug-holes at intervals, from which the liquor can be run into the various pits by short spouts or sailcloth hose. In place of plugs in the raised trough, a simple and convenient valve devised by the writer may be advantageously employed. A lead weight is made by casting in a hemispherical tin basin of about 5 inches diameter and 2 inches deep in the centre, a loop of strong brass wire with turned up lower ends, being suspended in the middle, so as to become fixed in the lead. To prevent adhesion, the tin must be previously burned off, and the basin well blackleaded. This weight forms the valve, which rests in use on a 6-inch washer of good indiarubber with a 4-inch hole, which is held by a wood block against the bottom of the trough, through which a 5-inch hole is cut. The valve is raised by a lever or cord, and is absolutely water-tight in use. It is shown in section in [Fig. 79], [p. 333].
It is very advantageous in practice, instead of pumping direct into the pits, to have one or more tanks, into which liquor can be delivered by the pump, and which are sufficiently raised to allow it to be run from them into the horizontal distributing troughs which have been mentioned. This is specially important with regard to liquors for leaches and suspenders which are worked on a circulating system, since they do not run very quickly, and much time is lost in pumping out pits, if the speed of the pump has to be regulated by the rate at which the liquor will circulate. It also enables liquors to be run through suspender- and rocker-pits during the night or at meal-times while the machinery is standing; and it is often useful on beginning work in the morning, to have an empty tank into which the first liquor can be pumped.
Direct-acting steam-pumps without fly-wheels are very unsatisfactory for tan-yards, since they are usually uncertain in their action, difficult to run slowly, and apt to “hammer”; and they are also costly in steam, which cannot be used expansively. Steam-pumps with fly-wheels, operating the steam-valve by an eccentric, are free from these defects, and though more costly at the outset, soon save the difference in lessened repairs and consumption of steam. Pumps with a capacity of 8000 gallons per hour are very suitable, and can be used with a 3-inch hose pipe; smaller sizes are decidedly more liable to choke with tanning material. Rubber mitre-valves work satisfactorily, and do not choke frequently, but are costly, and easily damaged by hot liquors. On the whole brass clack-valves are the most satisfactory, but the hinge-pins, instead of fitting neatly in circular sockets, should be held in slots, allowing the back of the valve to rise half an inch, when it will clear itself of small hard myrobalan stones and suchlike things, which getting under a more tight-fitting hinge would prevent the valve closing, and so stop the pump. Whatever valves are employed, means should be provided for easy access without unscrewing too many bolts. If the several valve-chambers of the pump are closed by a single cover with an indiarubber washer, the spaces between them which make the joint should be faced with brass or gun-metal, as, if the least leakage takes place over an iron surface, the friction and solvent power of the liquors soon eat away the metal and render a good joint impossible. Where colour is of first importance, it is well to have the whole pump of gun-metal, but in any case the working cylinder should be brass-lined, and the piston and rod, and the valves and seatings should be of brass or gun-metal. Spring-rings are far better than pump-leather and are unaffected by hot liquors; chrome leather, however, will stand a good deal of heat. Double-acting force-pumps have practically superseded the older single-acting double or triple pumps. Instead of direct driving with a steam cylinder, it is sometimes advantageous to drive by belt, but at least one steam pump should be provided, so that pumping can be done when the main engine is not running, and the speed of the pump can be regulated to the work, which is impossible in a belt-driven pump. Steam pumps are sometimes very useful as fire engines.
Centrifugal pumps are very suitable for tannery work, where the liquor is drawn from a well, but are not well adapted for use with suction-pipes. If the form with vertical spindle is adopted, which is sunk below the liquor in the well, the pump fills itself, and needs no foot-valve, but unless the well is very large, or some convenient means is devised of withdrawing the pump, repair or cleaning is difficult. If the horizontal pattern is used, which is above the ground, repair, cleaning, and driving is much easier, but a foot-valve is necessary, which may itself give trouble, and some convenient means, such as a pipe from a raised tank, should be provided for filling the pump with liquor, as, unlike suction pumps, centrifugals will not start unless full, although they raise very large quantities when running, and from their steady flow, will deliver much more through a given pipe than an ordinary reciprocating pump with the same power. In selecting the pump, care should be taken that the pattern allows ready access, not only to the foot-valve, but to the body of the pump.
It is seldom satisfactory to use windbores or strainers to prevent tanning material getting into a pump, as they speedily become choked; and it will be found better, after taking such precautions as are possible, to have the pump and valve of ample size and suitable construction to pass what comes with the liquor. The writer has known a mop-head pumped and delivered through a 3-inch hose without stoppage, by a Tangye fly-wheel steam-pump with brass clack-valves such as have been alluded to.
Pulsometers have not, in the experience of the writer, proved satisfactory in tanneries, warming and diluting the liquor, consuming much more steam than a pump of the same power, and becoming easily choked. For the same reasons, steam-jet water-raisers are not to be recommended except where raising is to be combined with heating, as in some leaching devices ([p. 334]).
CHAPTER XXVIII.
WASTE PRODUCTS AND THEIR DISPOSAL.
The products which are of no direct value to the tanner and currier in the manufacture of leather, and which are nevertheless obtained in fairly large quantities, are of very varying characters. In the present chapter, the most important of them will be described, and some of their uses mentioned.
Hair is removed from the skin of the animal in the process of depilation ([p. 143]) in the form of a wet sodden mass, containing a considerable amount of lime when the skin has been through the lime-pits.
As white hair is the more valuable, care should be taken in the unhairing to keep it separate from the coloured. It is washed first in plain water to get rid of as much of the lime as possible, and then in water containing a little acid. Hydrochloric acid is often used for this purpose, but sulphurous acid ([p. 25]) is preferable as it has a slight bleaching action on the hair. The acid neutralises and renders soluble the lime which still remains in the hair, so that it can be easily removed by washing with water. In many tanneries, hair-washing machines are used. The washed hair is dried by laying it out on frames; or preferably, the greater part of the water is first removed by a centrifugal drier, or by pressing, and the drying is completed in a drying room, the temperature of which is a few degrees higher than that of the outside air, and which is provided with a fan or some other appliance for mechanical ventilation. Tables of wire gauze on which the hair is spread, and through which the warm air of the room is drawn by a centrifugal fan, are the most effective.
Coloured hair is sometimes washed and treated like the white hair, but is usually sold direct to plasterers, in which case there is no necessity to remove all the lime and other impurities which the hair contains. A considerable amount of hair is also sold to iron founders, who use it in preparing cores and in loam-casting. The loose lime may be effectively beaten from dried hair by passing it through a disintegrator with one of the grates removed.
Fleshings and Glue-stuff.—The various scraps of fat and flesh, more or less free from actual hide substance, are usually worked up for glue, though if they cannot be sold for a fair price it will pay to boil them in order to recover the fat they contain. If this is to be done, the fatty portions may be thrown out at the beam and not mixed with the fleshings as in the ordinary way. Before boiling, the fat is treated with sulphurous, sulphuric or hydrochloric acid, sufficient to neutralise the lime present. The boiling should be carried on very gently, so as to allow the fat to rise without emulsifying with the gelatinous matter. For boiling, open steam may be used, but in this case the size formed will have little value; on the other hand, if sulphurous acid has been used and a wooden vat with a copper steam-coil be employed, really good glue may be obtained, and the slight trace of bisulphite which it may contain will prevent its putrefaction. Except under special conditions it will not pay to make glue on a small scale in England, as its value depends much on its appearance, and the necessary plant is somewhat expensive. In some places, however, size can be sold to advantage. [Fig. 98] shows a glue-boiling plant.
After separation of the fat by skimming, the clear size is run off from the residual matter into wooden cooling troughs about 5 feet long by 9 inches deep and 15 inches wide, in which it is allowed to set ([Fig. 92], [p. 425]). Great care is required that both size and coolers are quite sweet and free from putrefaction, the coolers being frequently washed with sulphurous acid solution or fresh milk of lime. The jelly is cut out in blocks, and sliced into cakes of appropriate thickness by means of a series of frames like slate-frames which fit over the block of glue, and between which a wire or thin blade stretched on a saw-frame is inserted to cut the glue into sheets. In some factories a machine is used, with a series of parallel blades against which the glue-block is pushed. The sheets are afterwards separated by girls and laid to dry on nets, on which they are frequently turned. When dry, the cakes may be washed with warm water to remove any adhering dirt, but this causes some loss of weight, and in many cases it pays better to dry in a stove until quite hard, then grind in a disintegrator and sell as “size-powder,” in which appearance counts for little if the colour and strength of the size are good.
Fig. 98.—Glue Boiling.
Fat.—The fat, whether obtained in the manufacture of glue, or by boiling the fleshings and shavings for its recovery alone, is skimmed from the surface of the heated liquor, and should afterwards be freed from gelatinous matter by washing it with hot water in a tub and running off the upper layer after allowing the water to settle out. The fat thus obtained is a light-coloured grease of buttery consistence.
There are various other sources of waste fats which may be considered here. If glue is made from dried glue-stuff without previous treatment with acid, the fat skimmed off the pans, though dark in colour, will be neutral or alkaline, and a considerable additional quantity of fat and free fatty acids may be obtained by reboiling the “scutch” or refuse with open steam in lead pans with the addition of water and enough sulphuric acid to render the contents of the pan distinctly acid. This grease will be dark and of unpleasant smell from volatile fatty acids, but its odour may be to a considerable extent improved by blowing air and steam through it, and washing with water, or by heating to a temperature somewhat above the boiling-point of water for a considerable time. The same sort of treatment may be applied to the fat pressed out of sheepskins, and to that obtained by boiling currier’s shavings with water and a little acid.
Recovered fats may be separated into a tolerably firm grease suitable for use instead of tallow in currying, and an oil not unlike neatsfoot oil, by melting, allowing to cool slowly to a soupy consistency to promote the crystallisation of the harder fats, and forcing the mixture through flannel cloths in a filter press. The temperature at which the filtration should take place is generally 20-25° C. The oil is, of course, “tender,” or liable to solidify in cold weather; and the more so the higher the temperature at which filtration takes place. The tallow is obtained in cakes. If from fresh fleshings, it will be white and with little odour, but that from dried glue-stuff is usually brown and of unpleasant smell, while recovered grease from curriers’ shavings or “moisings” is always dark in colour.
If the fleshings are to be sold wet, they should be preserved in a sweet lime liquor; if to be dried, they are washed carefully in a fresh lime, spread on frames, and frequently turned over so that they may dry evenly and rapidly. Heat, if employed at all, is in most cases only used at the end of the drying operation, but some tanners dry from the first in a room the temperature of which is a few degrees higher than the normal, and which is provided with good ventilation. For the purposes of the glue manufacturer, the roundings and larger pieces are more valuable than the fleshings, and should be treated with correspondingly greater care by the beamsman and his assistants.
Bate-Shavings are very valuable as sizing materials. They should be well washed in water, or with a very dilute solution of sulphurous acid, and are then laid out in thin layers to dry. They may also be partially dried by pressing between latticed boards in a screw or hydraulic press, and are then best finished as cakes. On the manufacture of sulphurous acid compare [p. 25].
Horns are usually kept until the “slough,” “pith,” or internal bone can be knocked out, having become loosened through drying and putrefaction. If kept dry, practically no longer time is required, and the smell and other annoyances incidental to storing in a damp place are avoided. The sloughs may be removed by steaming, but the horns are somewhat damaged by this treatment. The sloughs are principally ground for “bone-meal,” but some are boiled for glue, either without preparation, or after decalcifying with dilute hydrochloric acid.
The actual horn itself, which is quite incapable of making glue, is used chiefly in the manufacture of combs, buttons, and similar articles. The value of horns is to a considerable extent dependent on their size, small horns being unprofitable to work up for the articles above mentioned.
Spent Tan.—The tan as it is obtained from the leaches after extraction has, naturally, no value for the tanner except as a fuel. Spent tan cannot be profitably sold as manure, as its worth in this respect is extremely small. In those places where white lead is still made by the Dutch process, oak-bark is used to cover up the earthen pots, and commands a good price. It is, however, essential that oak-bark only should be used, as many other tanning materials give off products which injure the colour of the white lead. The quantities of tan used for hot-beds, and for deadening the noise of traffic in the streets, are so small that they are of no practical account in the disposal of this product. Spent tan is not nearly so good as wood for the manufacture of paper, and an attempt to distil it and thereby obtain pyroligneous acid and wood-spirit did not result in any commercial success. On the Continent, fine-ground tan is usually pressed into briquettes for use as domestic fuel, but it would be hard to obtain a market for these in England.
On the whole, in spite of its low heating value, spent tan is best utilised as a fuel. For this purpose specially constructed furnaces are necessary on account of the dampness of the tan, and its low calorific value, which varies, however, with the particular materials: thus while oak-bark and valonia are only poor fuels, hemlock and myrobalans are much better on account of the resin and lignine they contain.
The first successful furnaces for raising steam with wet tan were introduced in the United States, and consisted of large arched combustion chamber with abundant grate-area, and with four or six feed-holes in the fire-brick top which formed a floor on which the spent tan was laid, and where to some extent it was dried by the waste heat. The flames and furnace gases were conducted under the boilers, the flue being very large and deep so as to collect the light ash which was drawn in great quantities from the furnace, and the gases then returned through the tubes of the boiler, afterwards passing down the sides and going to the chimney. The wet fuel was fed in through the firing holes alternately, so that only a part of the grate-space was covered at once with wet fuel; which was speedily ignited by the heat from other parts of the furnace, and especially from the vaulted arch.[187] The large grate-area was a necessity not only on this account, but because of the light weight of the fuel and its low calorific power, which involved the need of burning a large volume. [Fig. 99] represents a furnace of similar principle constructed by Messrs. Huxham and Browns. Furnaces of this type are, the author believes, still largely in use in the United States, but in Germany “step-grates” sloping from the furnace-doors towards the back, are now preferred. In these the combustible material rests upon the flat surfaces of the grate, while the air enters by the spaces between the steps without the fuel being able to fall through. [Fig. 100] represents the furnace on this principle constructed by the Moenus Co. of Frankfort.
[187] Detailed drawings and particulars are given in Jackson Schultz’s ‘Leather Manufacture in the United States,’ New York, 1876.
Fig. 99.—Huxham and Browns’ Furnace.
The essential conditions which are to be observed in the proper burning of the tan are a sufficiently large grate-area, a correct and sufficient supply of air, and a combustion-chamber of very high temperature. It is consequently not possible to burn tan very successfully in an ordinary Lancashire or Cornish boiler, since not only the grate-space is too limited, but the water of the boiler prevents the upper part of the furnace from attaining a high temperature; and it is therefore difficult to get the damp tan rapidly into vigorous combustion. The difficulty may to some extent be overcome by mixing the tan with a proportion of coal, and by closing the ash-pit and employing a forced draught unless the chimney is a very powerful one. In this way large quantities of tan may be burnt, but without effecting any great saving of coal. The heating power of the tan is improved by the partial removal of its water by pressing, and this is almost essential where a special furnace is not employed.
Fig. 100.—Moenus Step-grate Furnace.
The answer to the question as to whether tan should be used as fuel in the wet state in which it is obtained from the leaches, or whether it should be previously pressed, depends upon the nature and quantity of the tan. Where abundant quantities of a fairly good material such as hemlock bark are to be disposed of, the cost of pressing is an unnecessary expenditure; but if it is desirable to obtain the highest value from the fuel, or if the furnaces are not well constructed for burning very wet fuels, it will be profitable to press the tan. Hydraulic presses have been used for this purpose, but those now commonly employed consist of powerful rollers arranged in the same way as those of the valonia-crusher ([p. 322]). The pressure is given by levers loaded with weights or fitted with powerful springs. The liquid which runs from these presses is of little value, as it contains such large quantities of finely divided material that it is almost impossible to filter it, and if run upon the leaches it chokes them and prevents their proper circulation. Much of the cost of pressing is caused by the labour of feeding it to the press, and this may be greatly reduced by the use of mechanical conveyors ([p. 325]) from the leaches. A tan press is shown in [Fig. 101].
Fig. 101.—Tan Press.
Sewage and other Waste Liquids.—The waste liquors from the different liming, bateing, puering, tanning, washing and other soaking processes are, without any doubt, the most troublesome of any of the side-products which are obtained in the manufacture of leather. In former times they were simply run into the nearest stream, but nowaday the various sanitary authorities and other similar bodies will only permit comparatively pure waters to be turned into a public stream or watercourse.
Various methods of effecting the necessary purification of the waste liquors from tanneries have been proposed at different times, and have been used with varying degrees of success. These methods may be divided into three heads: precipitation, followed by filtration or sedimentation land-treatment; and bacterial purification.
The first of these depends on the power of certain substances, such as alumina and oxide of iron, to carry down organic matter with them if precipitated in solutions containing it. The method usually consists in adding a sufficient quantity of lime to render the waste liquid slightly alkaline, and then treating it with some crude salt of aluminium or of iron. By this means a precipitate of aluminium or iron hydrate is formed, which encloses within itself a considerable proportion of the organic matter of the liquid, and after settling to the bottom of the precipitation-tank is drawn off as “sludge.” Various chemicals are sold under fancy names, such as “alumino-ferric,” “ferrozone,” etc., and have a composition not very dissimilar to that of crude sulphate of iron or alumina. In some cases by-products, such as the acid liquors used in preparing iron articles for “galvanizing,” can be used with advantage.
In the case of the waste liquors from a tannery, the use of these chemicals may often be avoided if sufficient care be taken in regulating the proportion of the various liquids which are to be mixed together and run into the settling tank. As tanning matter combines with lime and dissolved hide-substance to form a heavy brown insoluble precipitate, it is clear that if care be taken to have rather more waste lime-liquor mixed with the waste tan-liquors than is necessary to throw all the tan out of solution, a very considerable amount of purification of the effluent will have taken place without any cost whatever to the tanner. Hence, if the proportion of waste lime is small in comparison to that of the tanning liquors, an extra addition of lime may be necessary in order to precipitate the tannin.
The precipitation- or settling-tanks are usually square or rectangular vessels or pits, the size of which varies with the quantity of liquid to be treated, but the depth of which rarely exceeds six feet. They may be divided into two classes—the “intermittent,” and the “continuous.” In the former class the tank is filled with the mixed waste liquids, taking care that such a sufficiency of lime is present that the mixture is faintly alkaline to phenolphthalein paper, and is then allowed to rest until the suspended matter has settled down to the bottom of the tank, when the clear, or almost clear upper liquid is drawn off, the remainder being the “sludge”; some means must also be employed to prevent the passage of scum and floating matters. In the case of the intermittent process it is advisable to have two tanks, one of which is being filled while the other one is settling or being emptied. With the continuous process the liquids are run into the tank in the proportions calculated to give a maximum amount of purification, as described above, but as they enter very slowly the undissolved matter soon settles, and consequently the liquid may be continuously run out at the further end of the tank. This plan, though it does not yield such good results in the hands of unskilled workmen, is yet useful in many cases, as only one tank is absolutely necessary. It is desirable that in running off the tanks, the effluent should be taken as near the surface as possible, by means of a hinged pipe attached to a float, or some equivalent device; and care is required, as the tank gets low, to avoid the escape of any of the sludge.
For continuous settling the tanks are usually long and somewhat shallow rectangular ponds, into which the previously well-mixed precipitating liquid flows through a wooden trough fixed across one end and as long as the breadth of the tank, and perforated with holes to allow the uniform and quiet influx of the liquid, which finally escapes by a similar trough crossing the opposite end of the tank. In front of the exit-trough a “scum board” must be placed, which is a simple plank dipping slightly below the surface of the liquid, so as to prevent any oil, scum or other floating matter from passing out of the tank along with the clear effluent. Whether the intermittent or continuous system is employed, the effluent should in most cases be afterwards passed through a bacterial filter-bed, or treated by land filtration before it is allowed to flow into a stream or river. Tannery effluents are usually received into sewers without further treatment than mixing and settling to remove solid matter, and many authorities are satisfied with the removal of merely such coarse suspended matters as might choke the sewers. Where continuous precipitation-tanks are used, they must be emptied at frequent intervals, and the sludge run on to cinder-filters, to part with most of its water. These filters are conveniently placed at a lower level than the settling tanks, and it is generally necessary to return the effluent from them for further precipitation and settling. Several types of continuous settling tank with upward flow have been devised by Mr. Candy and others, which are very suitable for use where space is limited; but otherwise less costly constructions are often sufficient. Apart from the question of obtaining an effluent sufficiently good to satisfy the sanitary authority, the treatment of the sludge is one of the greatest difficulties in the purification of effluents. It is usually very bulky, easily putrescible, and therefore difficult to dry; it is of little value for manure; and if allowed to remain long wet, its smell is very offensive.
It has been mentioned that in most cases the liquid, and in every case the sludge, must be freed from solid undissolved matter by filtration. This may take place through open filters or through filter-presses. The open filters generally consist of a pit with an exit at the bottom for the filtered liquid. This pit is filled with either stones and sand, with clinker, ashes or coke. Most tanners use clinker and ashes, as they do not cost anything; and the material should be so arranged that while the lowest layers are very coarse, the surface of the filter-bed should be of the finest material. As soon as this has become covered with so thick a layer of solid matter that the filtration proceeds too slowly, the top surface of the filter may be removed with a rake (taking care to remove as little of the ashes or sand as possible), and burnt, or dried and used as manure. In some cases, filter-presses are used which are composed of grooved or perforated plates with cloths between them through which the liquid is forced by pressure. The solid matter remains behind in the form of a comparatively dry “cake.” The filter-cake, dried if desired, is sold as manure, for which it is in many ways very suitable. Although they work much more rapidly than do the open filters, the cloths so soon become rotten and have to be replaced, that the open ash-filter is on the whole the most convenient for the tanner’s use. It will be readily understood that apparatus of this kind, though very efficient on a small scale, is quite out of the question when many thousand gallons of liquid have to be filtered daily, and so can only be effectively applied to “sludge.”
No system of chemical precipitation has as yet proved entirely satisfactory. Undoubtedly a great deal of purification is effected by this means, but in most cases the “purified” liquid is still too impure to be turned into a stream, though for various reasons this is often permitted by the authorities.
A great advance was made in the purification of effluents when manufacturers were compelled by law to allow the effluent from the precipitation-tank to filter through land set apart for that purpose. In this case certain hardy cereals were sown on the land, which was watered as often as possible with the effluent. This latter, after soaking through the land, was drained off into the nearest stream. Although in many ways this treatment was satisfactory, it had the disadvantage of being very expensive, especially in the neighbourhood of large towns where the price of land is high, and, in addition to this, the conditions necessary for success were far from being correctly understood, so that the land often became “sewage-sick” or waterlogged, and ceased to purify the effluent. It was not until the researches of bacteriologists proved that the purification by land-filtration was mainly due to the bacteria in the soil, that any really satisfactory solution of the problem could be found, but the question has now been to a considerable extent simplified by the introduction of “bacterial treatment.”
Bacteria, considered from the point of view of their action on organic matter, are often classified as “anaerobic” and “aerobic,” though many species are capable of existing under both conditions (Cp. L.I.L.B., Section XXIV.). The anaerobic bacteria thrive only in the absence of air, and their chemical action consists in breaking down the organic matter on which they feed into simpler, and generally more soluble forms, by processes which do not involve oxidation. The aerobic bacteria, on the other hand, require air or oxygen for their existence, and produce changes which are generally of a less complex character, but result in the complete oxidation and conversion of the organic matter to simple compounds, such as nitrates and carbonic acid, which are perfectly harmless and inoffensive. The two classes therefore are to a large extent complementary to each other, the anaerobic bacteria converting the animal or vegetable substances into more soluble and simple compounds which are adapted to the needs of the aerobic, which complete the destruction of the organic matter.
In harmony with what has just been said, bacterial treatment of sewage is of two kinds, each of which may be used alone, or in conjunction with a preliminary precipitation-process, but which are generally best used successively. The oldest form of bacterial purification depends mainly on the action of anaerobic bacteria, and is known as the “septic tank.” This originally consisted of a tank sometimes filled with small pieces of coke, but generally containing the liquid only, and which was tightly closed to prevent access of air and escape of foul gases. It has, however, been found that if deep tanks (6 to 10 feet) are employed, they soon become in continuous use so covered with scum and floating matter as effectually to prevent access of air and light, or any serious escape of smell. The liquid to be purified is allowed to flow very slowly through a tank or series of tanks of this description, entering about a foot below the surface through a distributing trough at one end, and flowing out similarly at the other, at such a rate as to change the contents of the tank about once in twenty-four hours; and when the tank is in working order, the liquid is much purified by the process, and most of the solid organic matter has become liquefied and disappears. It not unfrequently happens, especially where the septic tank treatment is not very prolonged, that the liquid which escapes has a stronger and more offensive odour than it had on entering the tank. It is nevertheless really purer than before, the increased smell being due to the volatile products of the partially decomposed organic matter; and, by passing the liquid through an open coke-filter, the smell will be effectually removed. In all cases it must be borne in mind that as septic tanks and bacterial filters depend for their efficiency on the organisms they contain, time must be allowed for these to develop and accumulate before good results are obtained; and for this about six weeks’ use is generally necessary, after which they will continue to act for an indefinite period until they become choked by sand and inorganic matter.
It must not be supposed that the action in the septic tank is wholly anaerobic; and with weak sewage, most of the organic matter may under favourable circumstances be converted into nitrates and carbonic acid by this means only; but generally a much more complete purification is effected by the subsequent use of “bacterial filters.” These in their simplest form consist of tanks of about 4 feet deep, filled with coke, broken bricks, or clinkers, and fitted with drain pipes at the bottom, by which they can be easily emptied. These tanks, often known as “contact-beds,” are filled with the sewage or septic tank effluent, which is allowed to remain on them two hours, and the tank is then emptied, and allowed a rest of six hours for oxidation and aeration. In most cases the sewage requires two such treatments, the last often through a bed with finer coke, in order to be completely freed from putrescible matter. In place of the intermittent process, as applied on the contact-beds, continuous aerobic filtration is often employed, the bed being so constructed as to allow of free admission of air at the bottom and sides, and the liquid to be purified being distributed on the surface by a sprinkler, or some similar device, and allowed to trickle through the bed. The continuous process seems likely to supersede the intermittent one, as the beds are not only capable of treating a much larger quantity of sewage in proportion to their area, but are also less liable to choke. About six weeks is required, with either contact-beds or continuous filters, before the material they contain becomes coated with the necessary bacterial layer and they get into full working order. The results as regards the effluent are perfectly satisfactory, and the great difficulty and cost consists in the slow but inevitable choking of the beds, which involves the replacement of the porous material. This is considerably delayed by the use of a settled or precipitated sewage, and in this respect, beside its bacteriological function, the septic tank serves a useful purpose in settling insoluble matter, which is much more cheaply removed from it than from the filter-beds. It will be obvious that ordinary settling-tanks, if deep, fulfil many of the functions of the septic-tank, and both lead to the production of a much more uniform liquid from the different effluents which the tanner produces, which is important in the subsequent bacterial purification. A good deal of interesting information on these subjects will be found in a paper by Mr. W. H. Harrison on the ‘Bacteriological Treatment of Sewage.’[188]
[188] Journ. Soc. Chem. Ind., 1900, p. 511.
There are a good many patents in connection with the various methods of sewage purification, and some caution is necessary to avoid their infringement, though of course the general principles of settling and filtration, and the destruction of organic matter by bacterial action, are open to all.
As a general rule the waste-liquors from a tan-yard or leather dye-works are exceedingly impure. They contain the organic matter (in a state of great putrefaction) from the soaks, bates and puers; other organic matter, also more or less putrefied, from the tan-pits; the lime liquors, with their large proportion of lime and of dissolved hide-substance, and in addition the various dyes and other chemicals which may have been used in the conversion of the raw hide into the finished leather; and hence their efficient purification has presented difficulties which do not occur in most other trades.
The different waste liquids are best run into a capacious tank, and, after being thoroughly mixed up together, are allowed to settle for some hours. By this means the greater part of the tanning matter will combine with the lime also present to form a heavy, brown insoluble substance; some of the dye and other organic matter will become entangled in this, and thus be removed from the liquid. The clear liquid is next run off into a bacterial filter (preferably a septic tank, followed by an open coke filter), and then into the nearest stream. If the tannery is near to a town, and the corporation sewers can be utilised, it is probable that a filter made of spent tan may be substituted, as this material will not only remove all excess of lime from the liquid but will also fix much of the colouring matter as well (Koenig). The tan, after being used for this purpose, contains so much lime in its pores that it is said to be useful as manure.
In tanneries where large quantities of disinfectants such as mercuric chloride, carbolic acid, etc., are used, it is necessary that the mixed liquids shall contain so much lime as to make them distinctly alkaline. In this way most of the disinfectants will be either precipitated or rendered inactive. Where arsenic is used in the limes it may be advisable to add a little ferrous sulphate (green vitriol or copperas), in order that the arsenic may form an insoluble compound with the iron, and so be removed along with the sludge. The ink produced by the action of the iron salt on the tan liquors will be completely removed by the bacterial filter.