96 ANIMAL PROTEINS 



durability. Hence sumach is the principal light-leather 

 tanning material, but the tendency is to employ other 

 materials oak bark, myrabs, and chestnut extract to do 

 much of the intermediate tanning, so that the expensive 

 and useful sumach may be used for setting the colour and 

 grain at the commencement, and for brightening, bleaching 

 and mordanting the leather at the end of the tanning process. 

 Weight is generally no consideration, but area is often a 

 definite aim, partly because some goods are sold by area 

 and partly because the striking out, setting out and similar 

 operations improve the quality of the leather by giving 

 evenness of finish. Leather well struck out, moreover, is 

 less liable to go out of shape. As the grain pattern is so 

 important in the finished leather, appropriate care must be 

 taken during tannage. If a smooth or a fine grain finish 

 is wanted, for example, the goods must not be allowed to get 

 wrinkled, creased, doubled or unduly bent to and fro 

 during the tanning. For such goods, suspension, careful 

 handling and even the " bag tannage " may be desirable, 

 whilst for coarser and larger grains paddles or drums may 

 be more extensively used. 



Amongst the finishing processes dyeing holds an im- 

 portant position. The nature of the process has many 

 points of similarity with that of tanning. The great specific 

 surface of pelt is probably more enhanced than otherwise 

 during tannage, at any rate with light leathers, owing to 

 the isolation of fibres, and consequently leather is as liable 

 as pelt to exhibit adsorption. The dyestuffs, on the other 

 hand, are substances very easily adsorbed. Some (like 

 eosin and methylene blue) are crystalloids, some (like 

 fuchsin and methyl violet) are semi-colloids, whilst others 

 (like Congo red and night blue) are undoubted colloids 

 forming sols (usually emulsoid) with water as dispersion 

 medium. The crystalloids and semi-colloids may also be 

 obtained in colloidal solution, sometimes being so changed 

 on the mere addition of salts to the solution. In addition, 

 the pelt has been mordanted with tannin. If, however, 

 leather has been kept long in the rough-tanned or " crust " 



