THE NATURE OF CHROME LEATHERS 129 



process, and are widely used for all classes of chrome 

 leather. A one-bath process which deserves special mention 

 was published in 1897 by Prof. H. R. Procter. In this 

 the tanning liquor was made by reducing potassium 

 dichromate in the presence of a limited amount of hydro- 

 chloric or sulphuric acid by adding glucose. Although a 

 basic chrome salt is the chief tanning agent thus produced, 

 there is little doubt that the organic oxidation products 

 play an essential part in producing the fullness and mellow- 

 ness of the leather thus tanned, but their nature and mode 

 of action has not yet been fully made clear though lyotrope 

 influence is probable. 



More recently Balderston has suggested the suitability of 

 sulphurous acid as reducing agent. A stream of sulphur 

 dioxide gas is passed through a solution of sodium dichromate 

 until reduction is complete. The resulting chrome liquor 

 has been favourably reported upon by some chrome tanners. 

 Bisulphite of soda has also often been used as the reducing 

 agent. Other organic substances are also often used, instead 

 of glucose, to reduce the dichromate. 



Theory of Chrome Tannage. As to the theory of 

 chrome tanning there is still considerable difference of 

 opinion and much room for experiment. Some leather 

 chemists regard the tannage as differing essentially from 

 the vegetable tannages. Mr. J. A. Wilson has even sug- 

 gested that the proteid molecule is in time partly hydro- 

 lyzed with the formation of a chromic salt with the acid 

 groups. The author, however, strongly favours the view 

 that in chrome tanning changes take place which are 

 closely analogous to those which occur in vegetable tannage, 

 the differences being mainly of degree. Thus the hide gel is 

 immersed into a lyophile sol the chrome liquor and there 

 follows lyotrope influence, adsorption, gelation of the tanning 

 sol, as well as diffusion into the gel, and finally also, probably, 

 precipitation of the tanning sol at this interface (see pp. 41-47 

 and 200-219). 



In chrome tannage the lyotrope influence is much more 

 prominent than in vegetable tannage, but the effect is in the 

 E. 9 



