CHROME LEATHER MANUFACTURE 153 



reduction should be careful and regular, or the oxidation 

 products will be irregular and have a varying effect upon 

 the tanning. Molasses can be substituted for glucose, in 

 amounts varying with its strength. 



Of the third type the most common is that in which 

 the dichromate is reduced by sulphuric acid and sodium 

 bisulphite. Solid bisulphite may be used, but it is usually 

 dear, and solutions are more commonly employed. Into 

 this type fall also the liquors formed by passing sulphur 

 dioxide gas into dichromate solution. Stock liquors of 

 this type have the advantage that strong solutions may 

 be made (up to 18 per cent. Cr 2 O 3 ) ; they have the dis- 

 advantage that they are liable to contain excess of free 

 sulphurous acid. 



The method of application of chrome liquors is usually 

 by paddling or drumming the goods in solutions of appro- 

 priate strength broadly speaking, paddles used for lighter 

 goods and plain finishes, and slowly revolving drums for 

 heavier hides and grained finishes. Heavy chrome leather 

 is often tanned in pits by suspension just as in vegetable 

 tanning. In such instances rockers may be usefully 

 employed. 



In any case, the goods are successively brought into 

 contact with liquors of increasing strength, as in vegetable 

 tannage, and the liquors are thus most conveniently 

 exhausted economically. The green goods thus receive 

 first nearly spent liquor and finish out of fresh strong 

 liquor. The goods may be, of course, handled from drum 

 to drum, or from pit to pit, but the modern tendency is 

 to save labour by moving the liquors instead. Thus in 

 drum tanning the liquor is run out and pumped into the 

 next drum. In pits air ejectors have proved suitable, 

 not only as lift pumps, but also as agitators of the liquor 

 in which goods are suspended. The press system is also used. 



Finishing Operations. In nearly all cases the chrome 

 leather has to be " neutralized " after tanning. This 

 consists in removing the acid " reversibly adsorbed." This 

 removal is necessary to the finishing processes, as well 



