throw it away.

=(2) Zinc and Lime Vat.=--Zinc dust is a bye-product in the process of
zinc extraction. It is a grey, very heavy powder, consisting mostly of
finely divided metallic zinc, with traces of oxide and sulphide of zinc.
Of these only the metallic zinc is active in reducing the indigo, the
rest of the ingredients are not of any consequence. The valuation of
zinc dust is a very difficult operation, but it is desirable that this
be done, as the product is liable to be very variable in the proportion
of actual zinc it contains, and it will pay large buyers always to have
it tested. Zinc dust must always be kept in a dry place.

For the reduction of zinc powder lime is chiefly used. The following are
two good mixtures.

Vat with zinc and lime:--

10 lb. indigo, dry and ground fine;
5-1/2 lb. zinc dust;
22 lb. slaked lime, dry.

The vat is set as follows, a part of the lime is mixed with the indigo,
and the two bodies are well mixed together and allowed to stand for ten
minutes, then the zinc powder is added. It is best to make this into a
smooth paste with water before adding it to the other ingredients, then
the rest of the lime is added and the whole is thoroughly stirred
together with the necessary quantity of water.

Vat with zinc powder, lime and soda:--

10 lb. indigo, dry and ground fine;
10 lb. zinc powder;
10 lb. slaked lime, dry;
35 lb. caustic soda at 11 deg. Tw.

Add the lime to the ground indigo, then add the zinc and finally the
soda lye.

Soon after the various ingredients of the vats are added together the
whole mass becomes hot, when it must be well stirred. It soon begins to
evolve gas and the mixture froths. In from two to four hours the
evolution of gas ceases. The dark blue solution now becomes yellow and
the liquor shows all the characteristics of the indigo vat. It is
necessary to keep the vat well stirred up during the time of setting,
which takes from five to six hours. If there is much evolution of gas
after this time it indicates that too much zinc powder has been added;
this is a common fault with dyers, and such excess causes the vat to be
too much disturbed and to work dirty. A lime-zinc vat, with occasional
additions of new materials, keeps good for three months, and even then
is in a better condition than the copperas vat.

This vat is used in precisely the same way as the copperas vat; as it
contains no sediment, or but little, it works cleaner than the copperas
vat