Leather: From the Raw Material to the Finished Product

by K. J. Adcock

Boot Upper Leathers

Whereas the number of different kinds of boot upper leathers in use less than three decades ago was limited to five or six (waxed calf, calf kid, French kid, mock kid, levant, coloured calf) it is now almost legion. Although the variety is almost bewildering, however, the popular demand is confined to about half-a-dozen sorts, which include real and imitation box calf, box hide, glacé or glazed kid, patent leather, willow calf, dull-finished chrome leather, and waxed kip butts and waxed splits for workmen's boots.

Box leathers and glazed kid are the most popular of all. Generally speaking, glazed kid is more suitable for wear in the summer and autumn; while box leather, being thicker and stronger, is preferable for the winter. Imitation willow calf (i.e., calf skin tanned in vegetable and chrome liquors, either separately or combined) is very suitable for easy-wearing boots, but is not so durable and resistant to water as the pure chrome-tanned article.

The tannage of box and willow leathers may be effected by the one or two-bath process already described (p. 108). Before the tannage, it is advisable to pickle the hides or skins in a solution of alum and salt, with the object of preventing contraction of the tissues of the hide, and providing a mordant for the more rapid absorption and fixation of the chrome salts. The tannage completed, the acid salts in the leathers are neutralised with borax, and after the leather has been washed in warm water it is ready for dyeing. The dyeing of box calf is generally done in the drum tumbler, but there are several other methods. One system used on the Continent is to dip the skins, a pair at a time and placed flesh to flesh, in a very strong dye liquor. This method is slow, and not in accordance with modern ideas; used for producing browns and fancy shades, it is very difficult to avoid irregularity of colour where large parcels are manipulated. Another method is vat dyeing, the process being carried out in a wooden vat (Fig. [25]) and the dye liquor circulated by means of a paddle. The one advantage of this system is that the leather can be easily examined during the process. A useful method of dyeing is carried out by brushing the colouring materials on the grain side of the leather. This process is economical, and it has the further advantage that the finished leather can be used for unlined boots and other purposes where an undyed flesh is necessary.

The original "box" leather was dyed a dark-blue shade in the drum before it was shaved, and, as the dye does not penetrate very far into chrome leather, unless a strong mordant of tannin be previously used, the subsequent shaving left the flesh side of the leather a very pale-bluish tint.

The process of dyeing is least troublesome and most effective when done in the drum tumbler. The leather is run in warm water until the temperature reaches 140°-150° F (60°-66° C.) and the dyeing materials are added gradually in a box at the side of the drum whence it passes through the axle or journal into the tumbler. Although chrome leather is not materially affected by boiling water in contradistinction to vegetable-tanned leather, which cannot be treated with water above 60° C. without injury, it is inadvisable to dye it in boiling solutions, the above-mentioned temperature being the most satisfactory. There are two methods of dyeing blacks in the leather trade: (1) the logwood-ammonia, and (2) the aniline black. The former is the cheaper of the two and quite satisfactory, although many dyers seem to prefer the aniline process. The latter certainly gives a deeper black, but it is not at all necessary to make the flesh side of box calf black, and many buyers prefer the back blue, although, of course, the grain side of the leather must be finished a jet black. The crude logwood is generally subjected to a process of fermentation or "ageing." The new wood has to be placed on stone or cemented flooring in a warm room and frequently turned over until fermentation has ceased, which usually takes about a week. It is then cut into small chips by machinery and packed in bags. This is the form in which it always used to enter the tannery, but it is now generally prepared in paste or crystal extract by makers of tanning and dye-wood extracts, a business which grows in importance every year. The colouring matter of logwood is haematoxylin, which is converted into haematin by oxidation; hence, the extract is often referred to as haematin crystals. The paste may be used where it is desirable to increase the substance of the leather, but most dyers prefer the crystals, which are easily dissolved in hot water. One lb. of crystals, to which is added just enough ammonia to change the brown colour to violet blue, suffices for each 100 lb. of leather dyed. The colouring matter is absorbed by the time the drum has been running half an hour, leaving clear water behind. The next process is known technically as "fat-liquoring," which, as its name implies, consists in lubricating the fibres of the leather with fatty or oily matters. This is a very important process, for, when suitable ingredients are used, the pliability, strength, and waterproof quality of the leather are greatly increased. The making of special fat-liquors for different kinds of leather has become quite an important business, and most makes can be depended upon for the specified purposes. It is a debatable point as to whether fat-liquoring should precede or follow dyeing. Generally speaking, it is better to adopt the latter course, as the fatty ingredients help to fix the colouring matter on the fibres of the leather. The dye is also less liable to fade when fat liquoring follows the colouring.

The number of materials that may be used for the process of fat-liquoring is almost legion, but the principal are soaps, oils, egg-yolk, tallow, flour, Tragasol, Irish moss, china clay, and starch. Unless the special preparations made by leather trade chemical firms be used, much care has to be exercised in selecting the most suitable ingredients for each class of leather. For example, the mixture that would give good results on vegetable-tanned leather might be quite unsuitable for chrome leather. Then, again, the selection of material is based on the style of finish required, a dull finish necessitating heavy fats and greases which would be quite unsuitable for bright leathers, and particularly for those of the chrome-tannage. Among the hundreds of recipes that have been published from time to time, tallow has only found a place in one or two mixtures, yet it is one of the finest materials for strengthening and lubricating the fibres of leather. If its splendid properties were fully known, it would be very largely used for fat-liquoring purposes. Of course, it is well known to the currying trade, having for several centuries formed the basis, together with cod oil, of the dubbin used in stuffing waxed leather and belting butts.

The following is a useful recipe for a fat-liquor for box calf: 2 lb. Marseilles soap and 3 lb. neatsfoot oil for every 100 lb. of leather. The soap is cut into small pieces and dissolved in hot water, and the oil is added slowly and thoroughly mixed by stirring vigorously. Where possible, emulsification should be done in a machine, in order to atomise the ingredients. The finer they can be brought into a state of division, the better they will be absorbed by the leather. Other suitable fat-liquors for box leathers are the following: (1) Turkey red oil, 2 per cent. of weight of leather and neutral soap 1 per cent.: (2) neatsfoot oil, 2 lb.; tallow, 1 lb.; and Tragasol, 1 lb.: (3) cod oil, 2 lbs.; Marseilles soap, 1 lb.: (4) neatsfoot oil, 2 lbs.; egg-yolk, 1/2 lb., or six fresh yolks. The process is completed in 20-30 minutes.

In the case of fancy colours or browns, the leather should generally be well-drained and warmed up again in the drum in hot water (170° F.) before fat-liquoring, but the precaution is unnecessary for black leathers. After the fat-liquoring process, the leather is removed from the drum, and piled up on the horse to drain, and left for several hours or overnight, to allow it to assimilate the fatty ingredients. It is then submitted to an operation known as sleeking or striking out, with the object of removing surplus water and so facilitating drying, and making the grain side smooth. Machinery is now almost universally employed, especially for chrome leather. A good type of striking out machine is shown in Figure 34.

If done by hand, the skins are placed on a sloping glass or slate table, with the flesh side uppermost, which is then stretched out by strong pressure with a tool known as a sleeker, the strokes being made downward in the direction of the hair growth. The flesh side done, the whole skin is turned over and the grain side is similarly treated. Sometimes only the flesh side is struck out, especially if the grain is soft and tender. The sleeker consists of an iron, steel, or copper blade, about 6 in. square, fitted in a wood handle (Fig. [32]). The skins are hung up in the drying-room immediately they are extended and smoothed by sleeking, and are then either allowed to dry completely, or, as is generally the case with best leather, they are hung up until a great deal of the moisture is evaporated and the skins are left in a slightly moist, or, as it is generally termed, "sammed" condition. The skins are then taken down, damped in dry parts, folded over, laid in piles to equalise the moisture, and finally smoothed out and stretched again by hand or machine. This second operation after the dyeing is known as "setting."

Chrome-tanned skins are usually stretched and nailed on square boards after setting, as, owing to their elasticity, they are liable to shrink considerably in drying. The superficial area can be increased by fully extending the skins. This operation is technically described as "straining." It may be stated here that vegetable-tanned leathers are not generally "strained" on boards, as such leather is better in quality and substance when hung up and dried. Leather of combined tannage (i.e., tanned with vegetable and chrome products) is, however, frequently "strained," as the astringent property of chrome would otherwise cause contraction of the fibres.

When dried on the boards the leather is somewhat stiff, and has, therefore, to be softened before any finishing ingredients are applied. Chrome leather is softened by placing it for a few days in damp sawdust, which, for brown or fancy coloured leather, must be of deal or white pine, and, therefore, free from objectionable colouring matter; but, for black leather, the sawdust of any wood is suitable. The skins must be systematically piled one above the other, and a little damped sawdust scattered over each skin.

When properly "seasoned," the skins are brushed free of the sawdust and "staked," either by hand or machine, usually by the latter means. The staking knife may be fixed in a wood crutch (arm stake), or fixed upright in a vertical wooden stand (knee stake). In the former case, the skins to be staked must be secured in a horizontal wooden groove fixed to two uprights, the workman then pressing heavily in a downward direction with the staking knife, of which the crutch is held under the armpit. In using the upright stake, the leather is moved to and fro over the edge of the fixed knife. Both of these manual operations are arduous and somewhat dangerous, and should be displaced by machinery wherever possible (Fig. [36]).

Fig. 36

STAKING AND GLAZING MACHINE

The thoroughly softened leather is then dried, mordanted with a solution of logwood, or haematin crystals, blacked with levant ink, a specially prepared black dye, or with a suitable aniline dye, and dried again. It is finally seasoned with a glutinous or albuminous mixture (blood albumen, milk, and a little aniline black dye make a good mixture), dried, glazed, seasoned a second time, and reglazed. A light coating of mineral oil turns the finish into a jet black, and the leather is ready for the warehouse. There are several types of glazing machines, but the "grasshopper" (so called owing to its action) is the most popular for box calf. This machine is the same as shown in Figure 36, except that the working tool is replaced with a smooth glass or agate roller.

The term "box" applied to this leather was invented by a prominent American tanner, and had he registered the name and patented his process he would have reaped a very rich harvest, for the leather is used in enormous quantities throughout the world. As it was, the American firm tried to obtain an injunction against English firms manufacturing the leather, but had to withdraw their claim, as they were not the inventors of chrome leather, which was made in Scotland before it was introduced to the States. Moreover, the word "box," as applied to leather, was not registered in the United Kingdom.

Willow calf is exactly the same kind of leather as box calf, except that it is dyed brown instead of black. The seasoning mixture naturally differs and may consist of a mixture of egg albumen, milk, and a little of the same, or similar, dye solution as that used in the dyeing process. In order to get clear colouring, the dyes should be well dissolved and strained through fine muslin.