acids. When heated with strong and fuming
sulphuric acid it dissolves, forming a blue liquor from which the
colouring matter may be obtained on addition of soda in the form of a
paste, which is used in wool and silk dyeing under the name of indigo
extract. But dissolving in sulphuric acid materially affects the
properties of indigo as a dye-stuff, as will be seen later on.

By the action of reducing agents the insoluble blue indigo is
converted into a soluble white indigo. This body is rather unstable,
and on exposure to the air it rapidly becomes oxidised and converted
back again into the blue indigo. Upon this principle is based the
application of indigo in dyeing by means of the vat.

Various methods may be adopted to cause the indigo to become
dissolved. These may be divided into two groups: (1) Fermentation
vats, in which the action of reducing agents is brought about through
the influences of the fermentation of organic bodies, such as woad,
bran, treacle, etc; (2) Chemical vats in which the reducing effect is
brought about by the reaction of various agents on one another.

Of such vats the copperas and lime and the hydrosulphite vats are
examples. The fermentation vats, when in good order, work well and
give good results, but they are most difficult to prepare or set. The
chemical vats are the easiest to work, and (especially the
hydrosulphite vats) are coming to the fore, and are gradually driving
out the fermentation vats.

The actual method of dyeing with the indigo vat is the same with all
methods of preparation. The material to be dyed is well wetted or
wrung out in water. It is then dipped into the vat, handled a few
minutes to ensure its thorough impregnation, then lifted out, the
surplus liquor wrung out, and the material exposed to the air, (p. 139)
when the indigo white on it soon absorbs oxygen and turns into blue
indigo.

With these few preliminary remarks the methods of setting the various
indigo vats will now be described in detail.

#Woad Indigo Vats.#--This is one of the most difficult of the various
methods of setting vats. There are so many opportunities for it to go
wrong, and to be able to set a woad vat successfully will go far to
make a man a successful indigo dyer. No two woad vat dyers use exactly
the same recipe in setting a woad vat, and each considers he has a
secret art by means of which he ensures the successful working of this
vat, and this he jealously guards. All these d