VEGETABLE DYES III 
dant, to fix them to either animal or vegetable fibres. 
The mordants usually employed are salts of aluminium, 
iron and chromium ; copper and tin salts are also some- 
times used, while salts of arsenic or antimony may be 
added in small quantities to modify the colour, but are 
too poisonous to be used in bulk. There has been a 
good deal of controversy about the nature of the action 
of mordants, but a few general principles can be safely 
formulated. 
The dye itself is feebly acidic, and forms soluble 
salts with the alkali metals, sodium, potassium, etc., 
but insoluble coloured “‘ lakes ’”’ with the alkaline earths, 
calcium, barium, etc., and also with the mordant metals. 
To be a successful mordant a metal must first form an 
insoluble “‘ lake ”’ with the dye and, secondly, it must 
*be what is called an ‘‘ amphoteric electrolytic,” i.e. it 
must be capable of making either weak acids or weak 
bases. This second property enables the mordant to 
form a linkage, partly acidic and partly basic, between 
the material on the one hand and the dye on the 
other. 
The mordant is usually applied in a separate opera- 
tion either before or after the dye bath, though very 
occasionally the two may be applied in the same bath. 
With vegetable fibres, impregnation with tannic acid 
must precede the absorption of the mordant, so that 
the whole process of adjective dyeing is rather tedious. 
One important property of all mordant dyes is that, 
by changing the metallic radicle, considerable modifica- 
tion in the colour of the dye may be introduced so that, 
from a single dye radicle, several distinct colours may 
be produced. 
