ifferences in the manner
of setting the vat are brought about not by any radical differences in
the materials used, but by some unnoticed differences in other
surroundings; differences in the mean temperature of the water used,
in the general conditions of the atmosphere of the indigo shed and in
other similar circumstances, all of which have a material influence on
the development of the vat, but which are, in the majority of cases,
overlooked by the indigo dyer, the result being that a method of
working which is successful in one place would not be so in another.
The fermentation processes depend upon the reducing action brought
about by certain organisms of the nature of the yeast plant which grow
and develop in such vats.

To ensure the proper growth and development of these organisms every
condition must be perfect, correct temperature, proper proportions of
food for them to live on, and a certain degree of alkalinity or
acidity of the vat, and these points are most difficult to regulate as
they will vary very much from time to time.

A successful vat maker is one who closely observes his vats, and the
way in which they are working, and who, as the result of such (p. 140)
observations, is able to tell in what way his vats are deficient,
so that he may know how to supply that deficiency.

The following method of setting a woad vat may be adopted. It is
calculated for 100 gallons of liquor. The vat is filled with hot
water, and 80 lb. of woad are allowed to steep overnight in it, having
first been well stirred into the water, so as to ensure that every
part is wetted out. The next morning there is added 8 lb. madder,
12 lb. bran, 5 lb. quick-lime (previously slaked with water), and
2-1/2 lb. soda. These are thoroughly stirred together, then from 5 to
7-1/2 lb. indigo is stirred in. The indigo should have been previously
ground into a fine paste with water. The temperature of the vat should
now be maintained at from 115 deg. to 125 deg. F. for two to three days,
at the end of which time it ought to be in a state of quiet working.
Should it be found that the fermentation is going on too rapidly, a
little lime may be thrown in, which will retard it. On the other hand,
if it should not be going on with sufficient energy, this may be
remedied by adding a little bran, or better, a little treacle.

When in perfect condition the vat should have a slight smell of
ammonia. If this is not noticed it indicates that the