Chapter XXXII: Section II: and the oil tannages now to be dealt with, but there is

nevertheless a distinct departure in principle involved. In the oil tannages the mechanical treatment is generally more vigorous, and the "drying" process is conducted at a much higher temperature, with the result that there is a vigorous oxidation of the oil. This results in the formation of insoluble oxidation products which coat the fibre and play an essential part in the production of a permanent leather. Pungent vapours are evolved in the drying operations, amongst which is acrolein and probably also other aldehydes, and it is thought by Procter that these aldehydes also are essential tanning agents and typical of the process (cf. Section IV.). Fahrion considers that the tanning action is due solely to unsaturated fatty acids with more than one double linkage. Garelli and Apostolo, however, believe that the tannage is due to a coating of fatty acid whether saturated or not. These observers made leather with stearic and palmatic acids in colloidal aqueous solution.

The manufacture of chamois leather from the flesh splits of sheepskins comprises the largest and most typical branch of the oil tannages. The sheep pelts are split in the limed state, and the fleshes are given another sharp liming which may last up to a fortnight. They are next "frized," _i.e._ scraped over the beam with a sharp two-handled knife, to remove roughness and loose fat. The goods are next thoroughly washed in running water and drenched. A paddle drench is often preferred, and if not used the handling should be frequent. Paddling drenching reduces the time required from about 16 hours to about 6 hours. An hour or more in a hydraulic press removes superfluous liquor and some more grease. The fleshes are separated, cooled and then stocked for 30 minutes to equalize the moisture in them. After removing from the stocks they are sprinkled on both sides with cod oil and thrown back into the stocks for a few hours. They are then dried cold for a day or two. The stocks used are similar to those once popular for softening dried hides during soaking, and consist of two heavy hammers which fall alternately upon the goods which are contained in a curved box below. The result is a mechanical kneading action. The fleshes are again sprinkled with cod oil, restocked for a few hours and dried again, this time at 100° F. They are then repeatedly sprinkled, stocked and dried, the last operation being conducted always at an increasing temperature until finally the final "heater" is even up to 160° F. As the operation proceeds it is advantageous to hang the splits also nearer one another, and in the final "heater" they are quite close. The next stage is to pack the goods quickly into suitable boxes and allow them to "heat," _i.e._ to oxidize further. This is a rather critical stage in the process, and to prevent overheating ("burns") it is often necessary to open out and repack into another box, with possibly some little intermediate cooling. They are turned over thus repeatedly until the oxidation is complete, and then spread out to cool.

The fleshes are now a dark brown colour, and are next treated to remove excess of oxidized oil products. The goods are dipped through water at 110° F. and then subjected to hydraulic pressure. The grease and water which exude are allowed to separate by settling, and the thick yellow oil so obtained, known as "degras," forms a valuable material for leather dressing, as it more readily emulsifies with water than many oils, and impart this quality to other greases mixed with it. A further quantity of a similar oil is obtained by paddling the goods with a weak soda solution. The liquor obtained is treated with sulphuric acid to neutralize the alkali, and the grease recovered is known as "sod oil." The fleshes are now well washed with hot water (140° F.), fat liquored with cod oil and soft soap, machine sammed, either by a wringer or a centrifuge, and then dried out.

Much chamois leather is also made in France by closely similar methods. The skins are usually oiled on tables and folded up before stocking. Other marine oils (seal, whale, etc.) replace cod oil. Generally speaking the oxidation is more moderate, and the grease from the hydraulic press (_moellon_) is mixed with other fish oils to form commercial degras. An inferior quality of degras is obtained by subsequent treatment with soda.

The crust chamois obtained as above has only to be thoroughly staked to soften, "grounded" and "fluffed" to raise the nap, and then trimmed, and the ordinary wash-leather is obtained.

If intended for glove leathers superior skins are selected. These are fluffed carefully upon emery wheels, using first a coarse surface and eventually a fine surface so that a fine velvet effect is attained. The skins are next bleached.

In the "sun bleach" or "grass bleach" the goods are soaked in a 1-1/2 per cent. soft soap solution and exposed to sunlight after being wrung. They are bleached in about 3 days in summer, but nearly a fortnight may be necessary in winter.

In the permanganate bleach, which is less tedious, the skins are first degreased by soaking in a warm 3/4 per cent. solution of soda crystals and then drumming for 30 minutes in water at 95° F. They are then paddled in a 1/8 per cent. solution of commercial permanganate for an hour at the same temperature, rinsed through water, and the brown manganese dioxide is then removed by paddling or drumming the goods in a 3 per cent. solution of sodium bisulphite to which hydrochloric acid is added as required. The goods are well washed in warm water, and are then "tucked," _i.e._ placed in a vat of boiling water containing a little soft soap, just for a few seconds. The goods shrink and curl up, and they are then dried out at 120°-140° F. to fix the tuck. They are then staked, fluffed, and dyed.

In dyeing with coal tar colours the alizarin colours may be used after mordanting with chrome alum. Direct dyes, natural dyestuffs and pigment dyes are also used. The goods are struck out after dyeing, lightly fat liquored with commercial egg yolk, dried out at 110° to 120° F., staked and fluffed on the face side.

Buff leather is a similar leather made from hides. They are limed mellow for a fortnight, unhaired, fleshed, and then limed again for another week in sharp limes. The grain is then split off, and the goods rinsed and scudded, slightly delimed and hung up to dry. They are then treated in much the same way as fleshes for chamois, but lime is often added to the cod oil used in stocking.

Buck leather is a similar product obtained from deerskins, but much mock buck is made from cheaper raw material.

REFERENCES.

Bennett, "Manufacture of Leather," pp. 247-250 and 376-379.

Procter, "Principles of Leather Manufacture," p. 378.