20S DYEING 



protein fraction of the crude dye, and also combines with at least 

 part of the true dyestuff. 



In a valuable work on dyeing published in 1813/^ it was sug- 

 gested that dyes used with mordants should be called 'adjective' 

 dyes; those used without mordants, 'substantive'. These words are 

 still used, but it must be remembered that there are not really two 

 classes of dyes. It is true that many dyes cannot be used adjec- 

 tively; but dyes that can be so used wdll also colour biological 

 material substantively. 



The great advantage of the use of mordants in microtechnique is 

 that when once the tissue/mordant/dye complex has been formed, 

 it is insoluble in all the neutral fluids ordinarily used, so that 

 subsequent colouring with other dyes is easy and there is no hurry 

 in dehydration. The lakes are basic in action. Their fastness to 

 alcohol is a very great advantage over ordinary basic dyes and 

 renders them particularly suitable for the colouring of whole 

 mounts, which cannot be dehydrated quickly. Certain dyes, par- 

 ticularly haematein, produce w^eak, indefinite, or unsuitable colours 

 w^hen used substantively, but give brilliant or intense results when 

 used with a mordant. 



In the textile industry the mordant is sometimes used first and 

 the dye afterwards (two-bath method); sometimes the two are 

 used together (single-bath method) ; sometimes the dye is used 

 first and the mordant afterw^ards ('afterchrome' method). '^^ In 

 microtechnique the mordant is very seldom used after the dye, 

 though two examples can be quoted. ^'^^' ^^^ The other methods are 

 both in common use. 



Two problems at once present themselves. How does the mor- 

 dant attach itself to the tissue ? How does the dye attach itself to 

 the mordant (or, in other w^ords, what is the lake)? 



In trying to answer these questions one gets far less help from 

 the textile chemists than might be expected. There are several 

 reasons for this. In microtechnique the most important dyes used 

 adjectively are haematein and carmine, but in the textile industry 

 the great majority of mordant dyes belong to the azo-group. No 

 azo dye is used with a mordant in ordinary microtechnique. A far 

 more important reason, how^ever, concerns the nature of the 

 mordants used. The industrial dyer uses chromium mordants 

 almost exclusively nowadays, but these are much less used in 

 microtechnique than iron and aluminium. T'his would perhaps 

 not matter very much in itself, for knowledge gained about the 



