CHROME LEATHER MANUFACTURE 147 



acid ; (2) the formation of a basic chromic salt owing to 

 the excess of thiosulphate ; (3) the reaction of the added 

 acid and thiosulphate to give free sulphur, which is 

 deposited in and on the leather. The relative intensity 

 of these effects is variable, according to the conditions 

 of operation, e.g. the amounts of chemicals used, their 

 concentration, the nature and condition of the goods, 

 the time of application, the manner of application, etc. 

 In practice the most favourable conditions are usually 

 discovered empirically, but, broadly speaking, the goods 

 are usually added soon after the thiosulphate and acid 

 are well mixed. There is some evidence that the reduction 

 is in steps, intermediate products such as sodium tetra- 

 thionate and chromium dioxide are known to be formed. 

 The goods change from yellow to dark brown, then to 

 green, and finally to the familiar blue. The sulphur 

 makes the final colour a lighter blue than in the case of 

 a one-bath tannage, hence the two-bath process is often 

 preferred for " colours." 



On account of the empirical character of this " hypo 

 bath," it is impossible to fix any exact relation between 

 the quantities of material used in the chroming bath, and 

 the quantities of " hypo " and acid used in the reducing 

 bath. The following rules, therefore, must be understood 

 as rough approximations for practical use, and though 

 they have been empirically discovered their theoretical 

 significance is often fairly obvious. 



i. The amount of hypo necessary is almost directly 

 proportional to the amount of dichromate used. In 

 chroming with baths of the acid or neutral type, the per- 

 centage of hypo should be about three times the percentage 

 of dichromate used. Thus 4 per cent, dichromate needs 

 12 per cent, hypo ; and 6 per cent, dichromate needs 18 

 per cent, hypo on the pelt weight. In baths of the Schultz 

 type a less proportion of hypo may suffice, but the 10 per 

 cent, hypo for 5 per cent, dichromate, recommended 

 by the Schultz patent, is generally considered rather 

 insufficient. 



