vat is deficient
in alkalinity, and a little more lime should be added. Soda may be
used in the place of lime, but it is so much more energetic in
character that any additions of it have to be made with great care, or
the vat will become too alkaline in character, and the fermentation
will go on too rapidly, the ammoniacal odour is lost, and a peculiar
putrid smell takes its place. As soon as this is noticed, lime ought
to be added to retard the fermentation and to develop the ammoniacal
smell. The colour of a good well-set vat is olive brown.

When all the indigo is dissolved and the colour of the vat is a (p. 141)
clear olive yellow to brown the vat is then ready for dyeing, and
may be used for a long time, until, in fact, the deposit gets too
large and the wool becomes dirtied. But it must not be continually
worked, or it will give bad shades and loose colours. When in a bad
condition it will usually turn of a dark brown colour, and give dull
greenish shades. To remedy this there should be added some bran,
treacle, and a little madder, as well as indigo, and the vat should be
left for a day, at a temperature of 130 deg. F., to get up to full
strength again. Every night when in work indigo ought to be added to
the vat in proportion to that consumed during the day, with bran and
lime, the latter in not too great amount, just sufficient to keep it
of the necessary alkalinity.

#Hydrosulphite Vat.#--This is one of the best vats to use in dyeing with
indigo on wool, or, indeed, on any textile fabric. It is easy to
prepare and cleanly to work. While depending solely on chemical action
for its preparation and use, it is freer from those peculiar defects
to which organic vats, like the woad vats, are liable.

There is a further advantage about this vat, it is not necessary to
prepare each individual vat separately, but a strong mother liquor or
concentrated indigo solution may be prepared, and this only requires
letting down with water to produce a vat of any required strength.

In the preparation of this vat, which was devised by Schutzenberger
and Lalande, bisulphite of soda and zinc dust are used with either
quick-lime or caustic soda. The bisulphite of soda is allowed to act
on the zinc as will be detailed when an acid solution of sodium
hydrosulphite NaHSO_{2}, more strictly hydrogen sodium hydrosulphite,
is obtained. The acid solution of hydrosulphite has the property of
rapidly reducing and dissolving indigo, a