Leather: From the Raw Material to the Finished Product

by K. J. Adcock

BARKOMETER

The theory of the vegetable tanning process is not perfectly understood, and the principal trade chemists are not yet agreed as to whether the changes are chemical, physical, or both. Many are inclined to believe that both changes take place, and this view is probably correct, as a pure tannic acid produces a thin, empty leather, while a tanning material which contains a fair proportion of non-tannin matters, yields a full and firm leather. The practical tanner who adapts his work to the theory that tanning is a process of feeding the hides with gradually increasing strengths of tannin, is, in any event, on the right track. The suspender liquors are always weak in tannin, and are pumped from "handler" liquors through which packs of hides have already passed. It is important that these liquors possess a certain degree of acidity, and, if the natural acids of the tan liquor are too weak, or neutralised completely by the lime in a pack of hides that has been treated, a small quantity of acetic, formic, or lactic acid is added. Formic acid has the additional advantage of making the liquor antiseptic. It is absolutely necessary that the liquors have acid properties, otherwise tanning cannot proceed. Acid assists penetration of tannin. Hyposulphite of soda, formaldehyde, or synthetic tannin may be used for the same purpose, while these also help to keep the leather light in colour, owing to their bleaching properties. After passing through three or four suspender pits, in which the tan liquor is gradually strengthened, the hides are often rounded and the butts (Fig. [23]) are transferred to the handlers. The first handler liquor should register about 25 degrees by the barkometer (Fig. [24]), an instrument which registers the density of tan liquors. The handlers consist of a series of six or eight, and the strength of the liquors should be increased gradually. After a day's immersion in the first handler liquor, the hides are hauled out with a sharp two-pronged hook fixed to the end of a long wooden pole, similar to that used in the liming process. After they have been piled up for a few hours, two workmen, one on each side of the pit, place the hides flat in the next pit. The tan liquor is then run off the first handler to the suspender and fresh liquor is pumped into the empty pit, which then becomes the head or strongest liquor of the series. The succeeding packs will, therefore, follow in rotation: the first going to No. 2 pit, the second to No. 3, and so on. In the last two or three handler pits, the hides are sometimes dusted with freshly ground tanning material, such as oak bark, myrobalans, or divi-divi, the object being to strengthen the tan liquor so that it penetrates the hides before they are transferred to the layers. The handler liquors, which are originally sent back from the layers, are also frequently strengthened by the addition of oakwood, chestnut, or quebracho extracts, although some tanners only use extracts in the layers. Gambier is a useful material at this stage, as it keeps the liquors mellow and plumps the leather. In the layers, the last series of tan pits, the hides are treated with very strong extracts, and are also heavily dusted with such natural tanning materials as give firmness and solidity to the leather. For this purpose, nothing is better than valonia, or valonia extract, which deposits a large quantity of bloom, or ellagic acid. Where a mixed tannage of natural materials is used, the density of the liquors may reach 50 degrees barkometer, but if strong extracts be added, the barkometer may exceed 100 degrees. The relation between the layers and handlers in regard to the disposal of the used liquors is similar to that between handlers and suspenders; the first layer used is sent to the last handler.

By the English method, the leather is not, as a rule, transferred immediately from one pit to another, as it is found preferable to lay the hides in pile for a few hours, by which means the great weight helps to force the tannin through them. By draining the hides, they are in good condition for the absorption of the next tan liquor. The English method of handling is too slow for most of the American tanneries, where the greater proportion of the hides are cut straight down the centre to form "sides," as they are technically known. In America, the pieces of leather, whether hides, butts, bellies, or shoulders, are often tied together and transferred from one pit to another by means of a skeleton reel, worked either by hand or mechanically. The latest type of reel is a great improvement on the old reel. The cost of manual labour in many English tanneries is very great, in view of the fact that the hides have to pass through many pits, and the work of transferring the hides from one pit to another is done entirely by hand. It seems that there is plenty of scope for mechanical ingenuity in overcoming this difficulty, and it may yet be found possible to construct a perforated brass platform, connected with a crane, by which the whole of the hides could be lifted out in a second, left to drain for half an hour or more, and then bodily transferred to, and lowered in, the next pit. Naturally, it is easier, and quite within the bounds of practicability, to raise the hides from suspenders by mechanical power, as it would not be difficult to fix a stout cross-beam to the pulley from which the hides are suspended. The leather can be safely treated with very astringent tannin in the last series of pits, and the use of strong extracts is, therefore, a common practice at that stage, not only to give solidity and firmness to the leather, but to increase its weight.

It would be quite fatal to the production of good leather if strong tannins were used in the early stages of the process, because the surface on both sides would tan quickly, the grain of the hide would be badly drawn owing to the sensitiveness of gelatine to astringent solutions, and it would be impossible to make the tannin penetrate the hide. The final product would be a half-tanned leather which would be extremely brittle and poor in quality; in fact, quite unsuitable for sole leather. Despite the well-known axiom that the vegetable tannage (but not the mineral) must proceed slowly and gradually in the early stages of the process if the hides are to be properly tanned, the modern tendency is to speed up the tanning, with the result that, in some cases, the so-called sole leather is really not fit for the purpose for which it is intended. The root of the evil is the desperate attempt made by many boot manufacturers to produce lower-priced boots than those of their competitors.

Taking into account the development of leather trades chemistry, it is not surprising that the problem of reducing the time required to complete the old processes of tanning has received much attention. The results have been successful in some instances and have certainly proved that very good sole leather can be made without leaving the hides in the pits for a year or longer.

It has also been shown that the absorption of tannin can be accelerated by treating the hides, before putting them in the suspenders, with a solution of acid (lactic or butyric for preference), or hyposulphite of soda, or synthetic tannin. The latter penetrates the hides in a few hours without contracting the grain, increases the solubility of the vegetable tannins subsequently used, and helps to keep the leather light and uniform in colour. It is of no use alone for heavy leathers, however, as it lacks the physical properties necessary to make the leather firm and resistant to water. The use of "soluble" oils in the tan liquors is another late innovation which has given good results. The term "soluble" in connection with oils merely means that they are made miscible with water by treating them with acids. Solubilised cod oil might be specially useful, as it has tanning properties and would increase the wearing and waterproof qualities of the leather. The chief objects of using oil in tanning, however, are to increase the weight of the leather and permit the use of strong liquors.

The tanning process outlined is the oak bark tannage, which is now supplanted in many yards by the mixed tannage, in which various tannins, chiefly exotic, are skilfully blended to produce the kind of leather wanted. Each tannin seems to have special characteristics, although the materials are classified into only three groups, namely: pyrogallol, catechol, and mixed (pyrogallol and catechol) tannins. Generally, the best mixed tannages for solid leather consist of a combination of both pyrogallol and catechol tannins. Reviewing the characteristics of some of the principal tanning materials, it may be said that oak bark produces a nice, fawn-coloured leather of strong texture, but tans slowly; valonia makes the leather solid, durable, and waterproof; myrobalans quicken the process and lighten the objectionable colour of other tannins; divi-divi and algarobilla are very rich in tannin, and are, therefore, useful in later stages of the process; gambier mellows the astringency of other liquors and plumps the leather; while most of the tanning extracts on the market penetrate the hides much more quickly, often give a lighter colour than that produced by solutions of the natural materials, and expedite the process considerably.

The most popular tanning materials are: oakwood, quebracho, chestnut, valonia, myrobalan, mangrove, mimosa, hemlock and spruce extracts, oak bark, valonia, gambier and sumach. The following are some of the combinations in use—

For Sole Leather.

1. Oak bark and valonia, or valonia extract.