The Principles of Leather Manufacture

by H. R. Procter

No satisfactory leather can be produced with a solution of alum or aluminium sulphate alone, the skin drying horny, and incapable of softening by stretching. In practice, salt is always used in addition, the proportion being very variable, but averaging about half the weight of alum, or two-thirds the weight of sulphate of alumina employed. The mode of action of the salt has long puzzled chemists, and it has been supposed that its use was to convert the aluminium sulphate into chloride, a reaction which takes place to some extent, but which fails to explain the production of a soft leather, since aluminium chloride, though freely taken up by the skin, produces no more satisfactory leather than aluminium sulphate. The real explanation is found in [Chapter IX]. Alumina is a weak base, which readily gives up its acid to the pelt, becoming converted into a basic salt (see [p. 187]). The acid not only swells the pelt, and renders it incapable of producing a soft leather, but the swollen pelt is less ready to absorb the alumina salt, and so remains undertanned. The addition of salt prevents the swelling effect of the acid, and produces a partial pickling of the skin ([p. 89]), which, in conjunction with the tanning effect of the basic alumina salt formed, yields a satisfactory leather, though one which is readily affected by washing. If instead of adding common salt to the alum solution, an alkali such as soda is added, it combines with a portion of the acid, forming sodium sulphate, while the alumina remains in solution as a “basic salt.” As the term “basic salt” must be frequently employed in connection with mineral tannage, it may here be explained. Basic salts are compounds intermediate between the normal salt, in which the whole of the base is combined with acid, and the hydrated oxide in which the whole is combined with OH groups. Thus aluminium chloride, Al₂Cl₆, is a normal salt, in which the whole of the combining powers of the aluminium are saturated with chlorine: aluminium hydrate, Al₂(OH)₆, is the hydrated oxide, and Al₂Cl₅OH, Al₂Cl₄(OH)₂, and so on are basic salts in which successively more of the Cl is substituted by OH. Generally, as a salt becomes more basic, its solution in water becomes more unstable, and very basic salts are either insoluble, or are precipitated from their solutions by very trifling causes, such as boiling, dilution, or the attraction of animal or vegetable fibres; separating into free acid and either hydrate or a still more basic and insoluble salt. On this property depends their importance in tanning and dyeing, many of the metallic mordants being solutions of basic salts. Basic salt solutions are formed in various ways, the most common being the direct solution of a hydrated oxide in a solution of the normal salt, or the neutralisation of a part of the acid of the normal salt by the addition of a stronger base. This is what takes place on the addition of soda to an alum solution. If the soda is added in excess, the whole of the alumina is precipitated as hydrate, or as an insoluble basic salt, but if a proportion not exceeding about four parts of crystallised sodium carbonate be dissolved separately, and added slowly with constant stirring to the ten parts of alum dissolved in water, no precipitation will take place. In this solution leather can be tanned, either with or without addition of salt, the alumina is taken up more freely than from the normal alum, and the leather is more easily softened, and more resistant to water. In fact such leather bears a strong resemblance to the chrome tannages, standing a great deal of washing, and considerable temperatures without returning to a pelty condition. The more basic the solution that is used, the fuller and softer is the leather produced. The alumina-salt taken up by the skin from such basic solutions is always basic, while that absorbed from alum or alumina sulphate is apparently the normal aluminium sulphate. It is probable however that the actual tanning salt is in both cases basic, and that the acid is fixed as free acid, as in the pickling process, as the proportions of acid and base found in the residual liquor are somewhat variable.

Basic alumina solutions have hardly taken the place in practice which they deserve, though they were described by Knapp in 1858[112] and have since been patented by Hunt, but the patent (probably invalid) has been allowed to lapse. A good stock solution for practical use is made by dissolving 10 lb. of sulphate of alumina in 10 gallons of water, and 4 lb. of washing soda in 4 gallons, and gradually mixing the latter with the former. Salt can be used in addition if desired, and flour and egg-yolk may also be added.

[112] ‘Natur und Wesen der Gerberei,’ Braunschweig, 1858.

In curing small skins, where it is not desirable for the fur to come in contact with the liquid, or in the tawing of wool rugs, it is often convenient, after freeing the skin as much as possible from blood and dirt, and adhering flesh, to stretch it on a frame, or nail it out on a board, and apply a strong alum-and-salt solution, as hot as the hand will bear, with a sponge, repeating the operation till the skin is struck through. About 1 lb. of alum and ¹⁄₂ lb. of salt per gallon is a suitable strength. In place of applying the solution, powdered alum and salt is sometimes rubbed into the wet skin. Alumed goods should generally be dried out rapidly, and finally at a good temperature, as this tends to fix the tannage, which is also made more permanent and resistant to water by keeping the skins for a month or more in the alumed condition, an operation known as “ageing.” When first dried, alumed goods are invariably stiff and horny, and, to give them softness, must first be damped back to a half-dry condition, and then gradually softened by mechanical means. “Staking,” and “perching” are the usual methods, the first consisting in drawing the goods vigorously over a bluntish blade fixed on the top of a post, and the second in fixing the skins on a horizontal pole (the “perch”), and working them with the “crutch stake,” a tool formed somewhat like a small shovel with a semicircular blade, in place of which a “moon-knife” (a round blade somewhat like a broad thin quoit) is often fixed in a wooden crutch. The tools, and mode of using them are shown in [Figs. 36] and [37].[113] Machines, described on [p. 192], are now generally used for these operations. After the first staking or softening, the skins are allowed to become nearly dry, and are then staked a second time. Some judgment is required as to the precise degree of moisture in each case: in the first instance the skins must be sufficiently damp to yield without injury to the mechanical stretching, but in this state they retain sufficient moisture to enable the fibres again to adhere on drying; and at the second staking or perching, they must be damp enough to allow these fibres to be again loosened without violence, and dry enough to prevent their again adhering.

[113] The process shown in [Fig. 37] is not actually “perching,” but “grounding,” in which a moon-knife with a sharp turned edge is used to reduce the thickness of the skin on the perch, at the same time as it stretches and softens it.

Fig. 36.—Staking White Leather.

Fig. 37.—Grounding with the Moon-knife.

The following slight sketch of the manufacture of calf-kid will serve to illustrate the practical manufacture of the finer alumed or “white” leathers. The raw material is in England mostly large market-calf, though salted and dried skins are sometimes employed. After sufficient soaking or washing in water, they are limed without arsenic or other sulphides, in limes which must not be allowed to grow stale or putrid, until the hair can be easily removed. After unhairing and fleshing in the usual way, they receive a few days in a pretty fresh lime, in order to plump them, and are then freed from lime gradually but as completely as possible, by successive steepings and washings in water softened by a mixture of that already used on other goods and by working on the beam. This acts as a partial substitute for puering with dung, which is now no longer used on calf-kid. The goods are next drenched in the ordinary way, 3-4 % of bran being used, and the goods allowed to rise two or three times in the drench, which should be conducted with the usual precautions ([p. 167]) to avoid the danger of butyric fermentation in hot weather. The goods should come out of the drench free from lime, and unswollen by acid, but full, white, and soft. The tanning (or “tawing” as it is usually called in the case of alumed goods) is done in a rotating drum with a mixture of alum or sulphate of alumina, salt, flour, egg-yolk, and olive oil. About 5 per cent. of flour, 2·5 per cent. of alum, 1 per cent. of salt, the yolks of 25 eggs, or 1¹⁄₂ lb. of preserved egg-yolk, 2 oz. of olive oil, and 1¹⁄₄-1¹⁄₂ gallon (12-15 lb.) of water are required per 100 lb. of wet pelt. The flour is first made into a smooth paste with a little water, the egg-yolk, somewhat diluted with warm water and strained, is mixed in together with the oil, and finally the alum and salt solution is added at such a temperature as to bring the whole mixture to blood-heat (38° C.). The length of drumming depends on the thickness of the skins, several hours being required for very thick ones, but care must be taken to stop and ventilate the drum at frequent intervals, so as to prevent the skins becoming hot by friction. This part of the process was formerly accomplished by treading with bare feet in a tub. After tawing, the goods are allowed to lie in piles over-night, or are sometimes laid in tanks for a day or so with any that remains of the tawing paste, to complete the absorption of the salt and alum, and are then frequently split with the band-knife machine, though it would be better, as is often done on the Continent, to split them before tawing, the materials of which are not only costly, but unfit the splits for many purposes for which they might be employed. The drying should be rapid, but is best done first at a moderate temperature, or in the open air, and then in a rather hot stove. They may now be allowed to “age” from one to three months, but it is usually better before ageing to do the first part of the finishing process, consisting of damping back, staking, and if necessary, shaving. Machines are now almost invariably used for the staking, the principle of which may be described as that of a pair of tongs, carrying one or generally two staking blades on one limb, and a roller on the other which closes on the skin, and presses it against and between the blades, while the tongs are drawn backwards, allowing it to slip through. [Fig. 38] illustrates the Slocomb, one of the most popular machines of this type. After staking and ageing, the skin is soaked in water till thoroughly wet in all parts. This not only softens the skin, and prepares it for dyeing, but takes out the superfluous alum and salt, and at the same time a good deal of flour and egg. To replace these, “re-egging” is necessary, and while some manufacturers give egg-yolk, or egg-yolk and flour only, many add a proportion of salt, and sometimes also of alum. This is done before dyeing, if the skins are to be blacked on the table, but as tray-dyeing (see [p. 406]) would again wash out the egg, the re-egging is deferred till after dyeing if this process is resorted to. Before dyeing, the skins receive an alkaline mordant to overcome greasiness, and enable them better to take the colour. In former times this was usually stale urine, but this has mostly been superseded by solutions of “hydroleine” (a washing powder), or of soap rendered more or less alkaline with ammonia. Eitner gives the following recipe, viz. ¹⁄₂ lb. Marseilles soap dissolved in boiling water, 5 or 6 egg-yolks added, and the whole made up to 4 gallons with water and ¹⁄₄ lb. potash bichromate. The colour used is infusion of logwood or its extract, or two-thirds logwood and one-third fustic, which is best extracted without alkali, a small quantity of soda or ammonia being afterwards added. It is fixed and darkened by a wash of iron-liquor or a solution of 1 of ferrous sulphate in 75 of cold water. After being again dried, the skins are sometimes grounded with the moon-knife, softened again by staking or perching, for which a machine with inclined or spiral blades attached to a drum and working on a sort of leather apron is often preferred to machines of the Slocomb type, and rubbed over on the grain with a composition containing oil, wax, etc., and are finally ironed with a heavy flat-iron, to give them a fine and smooth surface. Eitner gives a recipe for the gloss:—1 kilo gum arabic, ¹⁄₂ kilo yellow wax, ¹⁄₂ kilo beef-tallow, ³⁄₄ kilo Marseilles soap, 1 liter strong logwood infusion, and 5 liters water. The water is brought to a boil in an earthen pot, and then the soap, wax, gum, and tallow are added successively, each being stirred till dissolved before adding the next, and lastly the logwood. After boiling for an hour, it is allowed to completely cool, being incessantly stirred during the whole process. After ironing the goods are rubbed over with a final gloss, for which Eitner gives the following recipe:—8 liters olive oil, 500 grm. tallow, 500 grm. yellow wax, 500 grm. rosin, 500 grm. gum arabic. (No water is given in the recipe, but the gum arabic is presumably softened in water.) The mixture is cooked for two hours in an earthen pot till the water is evaporated, and allowed to cool with constant stirring. The skins are then rubbed with a flannel with a very small sprinkling of French chalk, and are ready for sale.