2l6 DYEING 



solution, is red, and the aluminium lake crimson. The oxazine dyes 

 are still less affected in colour by combination with mordants. 



The oxazine dyes do not generally work well if applied in simple 

 solution after mordanting ^^^ and the single-bath method is 

 commonly used with them. Carminic acid forms very stable 

 soluble lakes, and here again the single-bath method is appro- 

 priate. Haematein can be used as a soluble lake, but the two-bath 

 method is generally used when iron is the mordant for this dye. 

 In Hansen's so-called Trioxyhdmatein ^^^ the dye is mixed with 

 the iron mordant, but precipitation is liable to occur unless pre- 

 cautions are taken. It was Ehrlich ^^^ who first overcame the 

 instability of the aluminium haematein lake, by the addition of 

 acetic acid. The effect is to keep the dye separate from the mordant. 

 The acidified solution shows the reddish colour of the dye, not the 

 blue of the lake. How the acid acts is not perfectly clear. The 

 mordants themselves, as we have seen (p. 212), are themselves 

 strongly acid. It must be presumed that the additional hydrogen 

 ions stop the loss of hydrogen from the phenolic -OH group of the 

 dye, and thus prevent its replacement by the metal of the mordant. 

 (See p. 221.) 



Strong acid is needed to prevent the formation of an iron lake 

 or break it when once formed. Sulphuric acid may be added at 

 2% to prevent the precipitation of the iron lake of coelestine 

 blue.^^^ Since this lake has the same colour as the dye, the course 

 of events is not so easy to follow as when aluminium and haematein 

 are kept apart by acidity and then allowed to join by neutralization. 

 With iron and coelestine blue the tissue/mordant/dye complex can 

 be formed without removal of the acid. The presence of tissue 

 seems to favour the formation of the lake. 



It is a curious fact that some lakes have greater powers of 

 penetration than their parent dyes. Thus gallamine blue diffuses 

 into gelatine more slowly than its lake with chrome alum.^^^ It is 

 evident that the dye by itself is somewhat aggregated, and that the 

 mordant disperses it. 



We turn now to the attachment of the metal to the tissue. 



It is desirable to say at the outset that the metal will eventually 

 be held to the tissues by bonds that are very similar to those that 

 hold it to the dye. The primary linkage is with acidic groups 

 in the tissues, and this is the reason why dye-lakes act as though 

 they were basic dyes. 



