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.