The natural dyes are further developed with "strikers," which mainly consist of metallic salts. Iron, copper, and zinc sulphates, nitrate and acetate of iron, bichromate of potash, and titanium salts (titanium lactate, titanium potassium oxalate, and tanno-titanium oxalate) are the most important. The lactate, sold commercially under the name of "corichrome," is especially suitable, as, unlike the mineral acid salts, especially the sulphates, it has no destructive effect on the fibres of the leather.
Dye-woods are now concentrated in the form of a paste, or dry extract, the latter being the more reliable. They are also very convenient to use and dissolve, while mixtures are, of course, easily prepared. A good tan shade on Cape sheep can be obtained by mordanting the leather with a solution of 1 lb. of bichromate of potash for every 100 lb. of leather, drumming it in 8-9 lb. of pure gambier, and then with a mixture of cuba wood extract, 1 lb.; fustic extract, 3/4 lb.; Brazil wood extract, 1/2 lb.; and logwood extract, 1/2 oz. After drumming the leather in this dye liquor for about an hour, the colour is developed with corichrome. If a darker shade be required, the leather can be treated with a suitable basic dye. After dyeing the leather, some dressers only fat-liquor it with egg-yolk and a small quantity of olive oil, while others prefer to re-dress it with a similar mixture to that used for tawing, namely, alum, salt, egg-yolk, and flour; but, where titanium salts are used, the latter method is not essential, because titanium has tanning properties. When dry, the leather is ready for finishing, but it is advisable to keep it in store for a few days before packing it in damp sawdust or sprinkling it with water to prepare it for the operation of staking. Anything more unlike leather would be difficult to imagine at this stage, but, after stretching the skin in the staking machine, or by drawing it over the upright stake, the dry, stiff, and shrivelled leather is reduced to a very supple condition. The flesh side of the leather is then pared with the moon-knife, or in the shaving machine, to equalise the thickness. In some works, a special tool which pares the leather on a flat table is preferred; this particular operation is called "doling." The flesh side is finished by fluffing it on the emery or carborundum wheel (Fig. [33]). Finally, the grain is brushed and polished with the glass sleeker, or ironed.
Chamois leather has been largely used for gloves of late years, but this leather has the defect, in common with suède leathers, of getting soiled much more quickly than grain leathers, such as kid, lamb, or Cape sheep. Nevertheless, suède and chamois gloves are likely to remain fashionable to a more or less extent. The manufacture of chamois leather is described on page [144]. Sun-bleached skins are the best for dyeing, especially if delicate shades are wanted. The frontispiece shows a field covered with skins bleaching in the sun. Chemically bleached leather is likely to become discoloured after dyeing. Defective skins are often dyed with pigments (dust colours), and this system is also applied to skins which have to be dyed such delicate shades as cannot be produced by wood or aniline colours. Although it gives attractive results to the eye, and certainly covers up any defects of the grain, this method of dyeing is not altogether satisfactory, as the leather remains unpleasantly dusty in wear for quite a long time.
The dyeing of chamois with wood-dyes or coal-tar colours is by no means easy, but this method gives the best results when successful. The grease must first be removed from the leather with a solution of 5 lb. of borax or 3-1/2 lb. of soda for every 100 lb. of leather. If the leather is still greasy on the surface, a further quantity of soda or borax is given, after which the leather is well washed in warm water, sumached, rinsed to remove the particles of sumach, and mordanted with titanium salts. The dyeing is then done with anilines or wood-dyes, or a combination of both, and this is followed by fat-liquoring with egg-yolk and a sulphonated oil. The finishing operations are staking and fluffing.
To get a good, fast black on chamois and suède leathers is one of the difficult processes in the leather trade, although it is easier to get a good black on alumed or chromed leather than on vegetable-tanned. Alumed leather is washed in a solution of borax or carbonate of ammonia to remove uncombined dressing in order to prepare it for dyeing. Chrome-tanned suède leather does not need this preparation. The leather is first mordanted with dye-wood extract, of which a suitable mixture is logwood and fustic, or logwood and quercitron, in the proportion of 4 lb. and 2 lb. to every 100 lb. of leather. After drumming the leather in this solution for about an hour, a weak solution of copperas (ferrous sulphate) and bluestone (copper sulphate) is added, and the milling is continued for twenty minutes, when the leather is well prepared to receive the black dye. Instead of the iron and copper salts, corichrome is often preferred, as it is quite safe to use, whereas iron salts have a destructive action on the fibres of the leather, unless the precaution be taken to mordant the skins with a good quantity of dye-wood extract. Following the application of the iron or corichrome striker, the leather is dyed with suitable aniline black (leather black, or corvoline) and finally fat-liquored to nourish the leather, and to fix and intensify the black. This recipe also gives good results where the skins are dyed only on the flesh side, the solutions being applied with a brush.
White Washable Leather
Among the new kinds of leather for gloves, none is more remarkable or more useful than the washable sheep or goat skins. The great advantage of this leather is that it can be washed in warm water and soap any number of times without injury, whereas gloves of ordinary tawed kid and lamb skins have to be dry cleaned and cannot be renovated many times. An additional advantage of washable leather is its warmth. After being dehaired, puered and drenched, the skins are drummed in a solution of formaldehyde and soda. In two or three hours, the skins are tanned, and are then treated with a solution of sulphate of ammonia. The quantities required are about 3 lb. of formaldehyde (40 per cent.) and 8 lb. of sodium carbonate (80 per cent.), and 1 lb. of sulphate for 100 lb. of pelts, using sufficient water to cover the skins well in the drum. This tannage produces a white but somewhat thin and empty leather, and the fat-liquoring must, therefore, be filling and softening. An emulsion of white curd soap and olive oil, or of egg-yolk and neatsfoot oil, is suitable. "Crestanol," a special preparation, also gives satisfactory results, since it is adapted for giving nourishment and resiliency to thin, empty leather.
Fancy Leathers
The best known of the fancy leathers is "morocco." This variety has been made for ages, and the name probably originated from the fact that very fine leathers of this kind were manufactured in Morocco a few centuries ago. History records that a similar leather, dyed red, was made in the ninth century before the Christian era.
The best morocco leather is made from Continental goat skins, which are mostly obtained from Central Europe and Spain. The Norwegian goat skins are also said to be of good quality for the morocco finish. An inferior morocco leather, which is produced in large quantities, is manufactured from East India goat skins, while a cheaper grade still can be produced from certain classes of East India sheep skins. The real moroccos are tanned in sumach, but the cheaper sorts are tanned in India with babool or turwar bark and re-tanned in sumach in the countries to which they are exported, chiefly Great Britain, Germany, France, and America.