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.

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.

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.

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.

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.)

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.