be added from time
to time.

After a day's work it is well to add a little of the zinc and bisulphite
mixture, to stir well and allow to stand overnight; the next morning
strengthen up the vat by adding fresh stock liquor.

In place of using lime in making up the vat it is possible to use a
mixture of caustic soda and ammonia. The lime will tend to cause some
sediment to form in the vat, whereas the soda and ammonia will not. When
they are used the following mode of working may be followed:--

_Stock Liquor._--Soda zinc vat: Put in a tub 26 gallons cold water, 15
lb. zinc powder ground into a paste with 6 gallons water, then stir in 8
gallons bisulphite of soda at 60 deg. Tw., stir well, keeping the heat down
as much as possible, after which add 8 pints caustic soda lye at 70 deg. Tw.
and 14 pints 20 per cent. liquor ammonia. When all is thoroughly mixed
add 30 lb. indigo, ground into a paste with 7 gallons water, allow to
stand for half an hour, then add water to make 100 gallons, stir at
intervals for twelve hours or so, when the stock liquor will be ready
for use.

This is used to make the vat in the same way as the first above
described. It is needful before adding the stock liquor in making a vat
to destroy any oxygen or air which is present in the vat. This may most
conveniently be done by adding thereto a solution of hydrosulphite of
soda, which may be made by mixing 4-1/2 lb. zinc dust with 5 gallons of
water and 3 gallons bisulphite of soda at 70 deg. Tw., stirring well, so
that the temperature does not rise too high, there is then added 2 pints
caustic soda and 3 pints liquor ammonia, 20 per cent.; when all is
dissolved, water is added to make up 13 gallons.

Should the vat show signs of becoming charged with indigo, indicated by
its becoming of a green colour, a little of this hydrosulphite added
from time to time will correct it.

The hydrosulphite-indigo vat made by either of the two methods indicated
above works well, and with due care may be kept in work for months. It
gives good shades of indigo, although some dyers consider that these
have not the rich bronze hue got from the lime and copperas vat. The
shades are rather faster to rubbing.

It has been proposed to employ the dye-stuff indophenol in conjunction
with indigo, in which case the method of making the vat is with zinc,
bisulphite of soda, caustic soda and ammonia as last described, only in
place of using all indigo a mixture of 22 lb.