8 W. H. EMIG 



or copper. Frequently, tin salts are employed with Cochineal. 

 For light colors, an aluminum salt is preferred because it does not 

 alter the tint. Chromium and iron compounds, which have a 

 deadening effect on bright tints, are combined with dark colors 

 such as Haematoxylin. 



Vat dyes on cotton cloth are very fast to light. They are 

 worked into a paste with a strong alkaline liquor and are reduced 

 to a soluble form with sodium hydrosulfite; this caustic solution is 

 called a vat. Basic dyes provide brilliant colors either on silk, in 

 a weak acid solution, or on cotton with the aid of tannic acid. The 

 latter method of operation suggested the term tannin dyes. 





