for use when it has a good
yellow colour.

This forms what may be called a "mother," or stock vat, from which (p. 147)
the dyeing vat is made in the following manner: Take a sufficient
quantity of water to make the dyeing vat, add some hydrosulphite of
soda (see below) to destroy any oxidising action the vat liquor may
have, then add sufficient of the stock vat to give the required shade,
this point is one which must be determined by experience. The vat is
now quite ready for use, and the wool is entered and treated in the
usual manner.

After dyeing each lot of wool it is advisable to add some of the stock
vat to replace the indigo abstracted by the goods. When a number of
dyeings have been done, it is possible that the vat may become charged
with oxidised indigo and lose its clean, yellow colour. It may be
restored to its former conditions by adding some hydrosulphite of
soda. Of course, after considerable use this, like all other indigo
vats, becomes too highly charged with sediment, etc., to give
excellent results, in which case the only thing that can be done is to
throw the old vat away and start a new one.

The hydrosulphite of soda referred to above is made in the following
way: 4-1/2 lb. zinc dust are ground into a paste with 5-1/2 gallons of
water and then mixed with 4 gallons bisulphite of soda at 55 deg. Tw.,
stirring well so as to keep the temperature down. Then add 3 pints
caustic soda lye of 72 deg. Tw., and 3-1/2 pints liquor ammonia. Finally,
add sufficient water to make 13 gallons. After standing for two or
three days the preparation is ready for use. It should be alkaline in
property; if not, add a little ammonia to make it so. This vat gives
very good bright shades, from sky blue to dark navy, which are equally
as fast as pure indigo shades.

Sometimes woollen goods dyed with indigo rub badly. The causes of this
defect vary from time to time, and in many instances are often obscure
in their origin. All goods intended for indigo dyeing, and more
especially when shades fast to rubbing are desired, should be (p. 148)
thoroughly cleansed, and before passing into the indigo vat should be
thoroughly freed from any soap which may have been used in the boiling
out. Then, after dyeing, they ought to be well rinsed in water and
passed through a sour made with sulphuric acid (2 lb. in 10 gallons),
and then washed again. Vats highly charged with sedimentary matter, or
with zinc or lime, are often the