CHROME LEATHER MANUFACTURE 143 



chrome and other material, and of time, and also to com- 

 bine with the tannage a pickling effect or a partial alum 

 tannage. Other modifications arise from the precise acid, 

 neutral, or alkaline condition of the pelt, being for example 

 obviously necessary when pickled stock replace neutral 

 pelts. The many two-bath processes which have been 

 found useful have been classified previously by the author 1 

 into three types : (i) The " Schultz type," in which such 

 quantities of dichromate and acid are used that there is 

 no excess of free acid (other than chromic), but an excess 

 of unaltered dichromate; (2) the " Acid type," in which 

 the chromic acid is completely free and the liquor con- 

 tains also some excess of mineral acid also ; and (3) the 

 " Neutral type," in which neither of these main constituents 

 is in excess, just sufficient mineral acid having been used 

 to liberate all the chromic acid from the dichromate. 

 Now : 



K 2 Cr 2 7 +2HC1 =2KC1 +2CrO 3 +H 2 O 

 204 73 



Taking the commercial hydrochloric acid as a 30 per cent, 

 solution, 73 parts will be obtained in about 250 parts of 

 commercial acid. Hence 294 parts dichromate need 250 

 parts commercial hydrochloric acid for the above re- 

 action ; 2 in other words, 5 per cent, dichromate needs 

 4J per cent, commercial acid. Similarly 6 per cent, and 

 4 per cent, of dichromate need 5-1 per cent, and 3-4 per 

 cent, respectively of commercial acid. If therefore such 

 quantities be used we have the so-called " Neutral type" 

 of chroming bath. If less quantities of acid be used we 

 have the " Schultz type," and if greater quantities of acid 

 be used we have the " Acid type." The original Schultz 

 patent used 5 per cent, dichromate and 2| per cent, hydro- 

 chloric acid, and well exemplifies its type, for there is much 

 undecomposed dichromate. The composition of some 



1 "Types of Two-bath Chrome Tannage," Leather, 1909, 227, 259. 



2 Commercial acids of course vary in strength, and the amount needed 

 varies accordingly. 



