110 EXPLOITATION OF PLANTS 
concentrated sulphuric acid, which converts it into a 
soluble blue derivative, capable of dyeing directly. 
But this procedure is not satisfactory, as it yields a 
fugitive colour. 
The most usual method is to reduce the blue dye to 
the soluble colourless leuco-compound. This reduc- 
tion used to be carried out by means of a complex 
fermentation lasting over several days, or even weeks. 
The preparation of the reducing vat required great 
skill and judgment ; the materials used were stale wine, 
dock roots, bran, madder, woad, etc., whilst an even, 
moderate temperature had to be maintained. Now- 
adays chemical reducing agents are generally used. 
The chief of these are: (a) for vegetable fibres, hydro- 
sulphite of soda made by the reduction of sodium 
bisulphite with zinc dust and lime water, tannic acid 
is also added to the vat; (6) for woollens, etc., either 
ferrous sulphate, quicklime and water, or zinc dust, 
quicklime and water. These vats can be prepared 
in from four to six hours; the indigo is then added, 
and dissolves in the solution with a yellowish brown 
colour. 
The cloth is boiled or warmed in the vat for some 
time, which may be anything up to a month for navy 
blue. Then it is oxidised in the air by various me- 
chanical devices. As the oxygen penetrates the fibres 
the insoluble blue colour gradually develops there, and 
in about twenty-four hours the process is complete ; 
then no amount of washing or milling will remove the 
colour. 
Mordant dyes.—The majority of plant dyes belong to 
the third class and require some metallic base or mor- 
