CHROME LEATHER MANUFACTURE 145 



in the leather, there is usually small advantage in their 

 employment. The use of salt is common but often un- 

 necessary. It is considered desirable in baths of the acid 

 type to prevent swelling by the excess of acid, and in baths 

 made up from commercial chromic acid to replace corre- 

 spondingly that normally formed from the reaction of 

 dichromate and acid. It is used also in all baths which 

 are intended to treat pickled goods. Like all electrolytes 

 its presence decreases the adsorption of chromic acid. 



All these conceivable modifications will make good 

 leather, and the choice of a process often depends largely 

 upon market prices. On the whole the tendency is to 

 prefer the neutral or acid type, on account of the greater 

 ease and completeness with which the bath may be ex- 

 hausted. Pickled stock may be depickled before tanning, 

 by a bath of salt, mixed with borax, whitening, or basic 

 alum solutions. It may also be placed direct in the chroming 

 bath, but the amount of acid thus added with the goods 

 must be determined and allowed for when making up the 

 bath. No allowance is usually necessary, however, if the 

 " pickle " consist only of alum and salt. 



The chroming operation is carried out usually in drums 

 or paddles. Drums are preferable because more con- 

 centrated baths may be used ; these solutions penetrate 

 quicker and are easier to exhaust economically. They are 

 also preferable for hides and heavy skins. Paddles are 

 preferable where grain is important, and for light skins 

 in which little time is needed. Small variations in the 

 ratio of chrome to pelt, or in concentration of liquor, have 

 little influence upon the resulting leather. 



The analytical investigation and control of chroming 

 baths is usually simple. A suitable volume of liquor is 

 titrated with N/io thiosulphate after acidifying with 

 hydrochloric acid and adding potassium iodide. The 

 operation should be conducted in a stoppered bottle, and 

 the liquor allowed to stand for 10-15 minutes after adding 

 the iodide and before titrating. A little fresh starch 

 infusion should be added towards the end of the reaction. 

 K. 10 



