The same tendencies for reducing the time taken to tan, employing stronger liquors, and securing economy of labour in handling, have been evidenced in the tannage of strap butts as in sole butts. It is nevertheless true that, broadly speaking, strap butts receive rather more handling and rather weaker liquors than sole butts. A greater amount of mechanical assistance is also employed with early stages (paddling, drumming, rocking). This is less objectionable for curried leather than for sole butts. The handling is more usually in suspension. The liquors are usually worked straight down the yard as a greater mellowness is needed in the early liquors than for sole butts. The offal is given a separate tannage and often used for different purposes, _e.g._ the shoulders for welting and the bellies for fancy goods. Plumping with sulphuric acid is generally considered inadmissible for strap butts. It has been shown that leather containing sulphuric acid tends to perish after the lapse of a number of years. Sole leather will be worn up before this effect is observed, but belting is an article which is intended to last much longer, and the use of sulphuric acid is consequently inadvisable. Plumping must be obtained, to a considerable extent, but must be achieved by the organic acids (lactic, acetic, formic and butyric acids). A few gallons of such acids are consequently added to the handlers, especially in the winter and spring. Less may be used in the autumn, when the layer liquors which fermented in the summer months have worked down to the suspenders. A mixture of these acids is usually better than any one alone, for they not only differ very considerably in price, but also have different powers of neutralizing lime and plumping the goods. Lactic acid (M.W. 90), Acetic acid (M.W. 60), and formic acid (M.W. 46) are each monobasic acids; consequently 3 lbs. formic will neutralize as much lime as 4 lbs. acetic or 6 lbs. 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.