In the "sun bleach" or "grass bleach" the goods are soaked in a 1½ 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 ¾ per cent. solution of soda crystals and then drumming for 30 minutes in water at 95° F. They are then paddled in a ⅛ 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.
SECTION IV.—FORMALDEHYDE TANNAGE
The use of formalin for hardening gelatin has long been known, but it was left for Payne and Pullman to devise a commercial process for tanning pelt into leather by means of formaldehyde (H·CHO) solutions. Their process, which was patented, specified the use of alkalies in conjunction with formaldehyde or other aldehydes. The function of the alkalies is not very obvious, for it has been shown that formaldehyde will tan also in neutral and in acid solution. The precise action of the aldehydes is also as yet somewhat obscure, but it is noteworthy that very small proportions of formalin will give a complete tannage. It is probable that the action of formaldehyde is not perfectly analogous with that of its homologues, for it is a most reactive substance, and will certainly with proteids undergo reactions which are not analogous to those with other aldehydes. The leather obtained by tanning with formalin is quite white and resembles buff leather, but has advantages over the latter in that no bleaching is necessary.
According to the patent specifications the pelt should be drummed in water and the tanning liquor—a solution of formalin and sodium carbonate—added gradually at 15-minute intervals. Up to 6 hours for light skins, and up to 48 hours for heavy hides, are required for complete tannage. The temperature is raised during the process from 100° to 118° F. The tanning liquor may be made from 16 lbs. of commercial formalin (36 per cent. formaldehyde) and 32 lbs. soda (80 per cent. Na2CO3) in 10-15 gallons of water. This should be added, one gallon at a time, to 4 cwt. pelt in 100-120 gallons of water. After tannage is complete the goods should be paddled with a 1½ per cent. solution of ammonium sulphate to remove the soda, and "nourished" in a solution of soft soap and salt, about 2¼ per cent. of each on the weight of pelt. The goods are then dried out, and may be finished like chamois, buff, and buck leathers (Section III.).