68 ANIMAL PROTEINS 



formic acid (M.W. 46) are each monobasic acids ; conse- 

 quently 3 Ibs. formic will neutralize as much lime as 4 Ibs. 

 acetic or 6 Ibs. lactic. Their plumping powers are somewhat 

 influenced by the anion. In determining what quantities 

 to take, the commercial strength of the acids must also be 

 considered. Formic is often 80-90 per cent, pure, acetic 

 60-80 per cent., and lactic 40-60, but may be as low as 25 

 per cent. The blend must be adjusted accordingly. As 

 strap butts do not need the firmness of sole leather, less of 

 these acids may be used than for sole butts. 



The exact nature of the tannage and the strength of the 

 liquors is largely influenced by commercial considerations. 

 If the manufacturer is both tanner and currier, he need not 

 go to such great expense in strong liquors and in time in 

 layers, for he can obtain some of this weight in currying. 

 If, however, the tanner sells the butts rough dried, he 

 must naturally aim at obtaining greater weight in 

 tanning. 



The actual details of the tanning processes are as usual 

 very varied, but may be classified according to type, just 

 as in the case of sole butts. 



Illustrations will now be given. 



Type I, which may be compared with Type i for sole 

 butts, is a tannage of about 5 months. The goods pass 

 through suspenders (8-30) in 2^ weeks, and then pass to 

 the handlers (30-5o), in which they remain a month; they 

 are then put into suspension again and pass through the 

 suspender handlers (40-55), which takes 2\ weeks. In 

 this round much gambier is added, and the goods are 

 frequently handled. Four layers are usually given, viz. 

 first layer 55, one week; second layer 60, two weeks; 

 third layer 65, four weeks ; and fourth layer 75, four 

 weeks. The tannage is thus 20 weeks, of which n weeks 

 (55 P er cent.) are in layers. Extra layers may be given 

 to heavier goods, using stronger liquors made up with extract. 

 All liquors work straight down the yard. 



The tannage consists of 35 per cent, myrabs, 35 per 

 cent, valonia, 10 per cent. Natal bark, and 20 per cent. 



