The term "heavy chrome leather" is taken to include chrome sole leather, chrome strap and harness butts, waterproof chrome upper leathers, motor butts and picking band butts. These will be discussed in turn.
Chrome sole leather, as stated in Section I., has made headway in Britain during the European War, the Army authorities having recognized its great advantages in durability and waterproofness. At the time of writing, however, its manufacture has received a set back, and many factories are reducing their output. The primary cause of this is that the Army purchases have largely ceased, whilst the general public have not yet been educated to its value. Men who take chrome uppers for granted talk of chrome sole as a "leather substitute" with an implication that it is of inferior value. It must be recognized, too, that there is some interested opposition to its development. Cobblers and bootmakers complain that it ruins their tools, being so hard to cut. Now, it is manifestly impossible for it to be soft to cut and hard to wear out; the complaint is therefore an excellent testimonial. There is also a stupid fear that an article which lasts twice as long will reduce repairs and retail sales by 50 per cent. Even the manufacturer has sometimes a suspicion that a demand reduced in proportion to durability will not be balanced by an extended export trade. These points of view will become minor considerations when the public realize its relative economy, and when the community as a whole grasp that a durable article is a natural asset. Meanwhile credit is due to those firms who persevere in their pioneering work of educating the public.
The manufacture of chrome sole leather presents many analogies with the vegetable tannages. The soaking and liming should be about identical, but the hides for chrome are generally given more sulphide and the depilation is reduced to about a week. The methods used for deliming differ widely in different factories. Some delime completely with mineral acids, some even pickle in acid and salt, whilst others merely delime the grain with boric acid. The last is really quite sufficient. Again, in tanning one finds similar divergences of method. Drum tanning is practised, but tannage in pits by suspension is more usual, though, as this last involves more dilute liquors, it involves also greater time to tan. In drum tannages a few days only are sufficient. In pit tanning at least a week is given, but sometimes up to a month, according to the strength of the final liquor and the rate of progress of the goods into stronger liquors. Liquors containing over 1 per cent. of chromium may easily be spent out so as to contain only 0.01 per cent. Labour and time are saved in pit tanning by the use of rockers. The press system of avoiding handling, however, so complicates the analytical control that its advantage is doubtful, a better way being to shift the liquors by an air ejector, which may also be used as an agitator of the liquor and thus abolish the need for rockers ("Forsare" patent). Chrome butts are tanned out in suspension. No floats or layers are used. The neutralization need not be so thorough as for light chrome uppers, as dyeing is not practised and trouble does not arise with emulsions made from sulphonated oils. Thorough washing is advisable, and the butts are usually then cut into bends and may be oiled before drying if desired. The bends are dried strained, to obtain flatness and smooth grain, for no machines, such as strikers and rollers, are usually employed. It is necessary to dry very thoroughly, for the bends are waterproofed by dipping the dry leather into molten waxes. The most commonly used wax and the cheapest is paraffin wax with a m.p. of about 127° F. It is rather a brittle wax, however, and as the finished leather consists of up to one-third of the wax, it is better to use at least some proportion of hard fat, Japan wax or ceresin wax, to obtain a stuffing material with less crystalline texture. The use of 10-30 per cent. rosin in the stuffing grease is also usual. This prevents the leather from being so slippery when in wear. The stuffing should take place at temperatures from 150°-195° F., according to the melting-point of the grease employed. The bends are taken out and laid in pile to cool and set in a flat condition, and are then finished.
The chrome tannage of butts for strapping and harness backs, and for motor butts and picking bands may be similar to that for chrome sole, but drum tannages are more common and the two-bath process is often used. In the latter case the acid chroming bath is preferred, using 6 per cent. of dichromate and of acid, with up to 15 per cent. of salt, and reducing with 15 per cent. thiosulphate and acid as needed. This process assists in the production of the light colour which is preferred in the case of some of these leathers.
Strap butts after tanning are very thoroughly washed with cold water in pits, and repeatedly struck out by machine between the washings. They are then oiled with heavy mineral oil, and stretched by powerful machines. They are dried and curried during the stretching. Degras, wool fat and vaseline are greases used, and the drying and stretching finished off at 120° F. They are then fluffed on the flesh, French-chalked and heavily rolled.
Harness backs are neutralized, machine sammed, and lightly fat liquored with 4½ per cent. soap. They are then struck and oiled with heavy mineral oil and dried for stuffing. Hand stuffing, drum stuffing, and "burning in" are all used (see Part I., Section IV.). Stearin, paraffin wax, ceresin wax, wool fat, sod oil and mineral oil are the greases employed. The butts are blacked after stuffing with lamp black and oil, glassed well and buck-tallowed on the grain.
Motor butts are fat liquored lightly, using soap only. They have to be softened, therefore, during the drying by being mechanically worked. A boarding machine is repeatedly used during the drying. They are finished off with French chalk on flesh and grain.
Picking band butts are neutralized by using warm water and then borax solution, and are then sammed by machine and very heavily fat liquored with cod oil and tallow and hard soap, to which degras may also be added. Up to 20 per cent. of greases (on the pelt weight) may be used. They are well drummed in this, struck out, French chalked, and dried out. They are softened finally by machine.
Waterproof chrome upper leathers are manufactured usually from hides tanned by the two-bath process, which is said to give a mellower leather. The neutral type of chroming bath is common. The butts are neutralized, machine sammed and struck, and then fat liquored with 2 per cent. each of neatsfoot oil and soft soap. They are then sammed, shaved and blacked on the grain with logwood and iron, and dried further. They are stuffed then by brushing with an abundant amount of concentrated fat liquor. This gives the waterproofness. They are staked after drying further, and often grained three ways. A further waterproof finish is given consisting of a fat liquor containing beeswax. They are finally brushed and re-oiled with linseed oil, to which some mineral oil may be added. This leather is much the most durable type for a shooting boot, or where waterproof uppers are desirable.
REFERENCES.
Procter, "Principles of Leather Manufacture," p. 198.
Bennett, "Manufacture of Leather," pp. 234, 368.