e.

Its peculiarity is that in the form in which it comes to the dyer it is
insoluble in water, and to enable it to be dissolved and therefore to be
used as a dye, the indigo has to go under a special treatment. The
colouring principle of indigo is a body named indigotin, to which the
formula C{16}H{16}N{2}O{2} has been given. When indigo is mixed with
substances like lime and copperas, lime and zinc, zinc and bisulphite of
soda, which cause the evolution of nascent hydrogen, it takes up this
body and passes into another substance which is called indigo white that
has the formula C{16}H{12}N{2}O{2}, leuco, or white indigo; this
substance is soluble in water, and so when it is formed the indigo
passes into solution and can then be used for dyeing. But indigo white
is an unstable substance on exposure to air, the oxygen of the latter
attacks the hydrogen which it has taken up, and indigotin is reformed,
the indigo white changing again into indigo blue.

Indigo dyeing consists of three operations:--

(1) Preparation of the indigo solution, or, as it is called,
setting the dye vat. (2) Steeping the cotton in this vat. (3)
Exposing to the air.

[Illustration: FIG. 30.--Indigo Dye-vat for Cloth.]

There are several methods of preparing, or setting the dye vat, and of
each of these modifications are in use in every indigo dye-house:--

(1) With lime and copperas. (2) Zinc and lime vat. (3) Zinc and
bisulphite of soda.

In all cases it is necessary for the indigo to be ground to the form of
a fine paste with water; this is usually done in what is known as the
ball-grinding mill. The finer it is ground the more easy is it to make
the dye-vats.

The dye-vats may be either round tubs or square wooden tanks; when
cloths or warps are being dyed these may be fitted with winces and guide
rollers, so as to draw the materials through the liquor. In the case of
yarns in hanks these appliances are not necessary.

Fig. 30 is a sketch of an indigo dye-vat for cloth or warps.

(1) =Lime and Copperas Vat.=--To prepare this vat take 75 gallons of
water, 4 lb. of indigo, 8 lb. copperas, and 10 lb. of good quicklime.
Put these into the vat in the order shown. The amount of indigo is added
in proportion to the shade which is required to be dyed: for pale
shades, 2 lb. to 3 lb. will be sufficient; while for deep shades, 6 lb.
to 7 lb. may be used. The amount of copperas should be from one and a
half to twice th