indigo dye liquor.
By "vat," therefore, we understand not only the vessel used for dyeing
indigo, but the solution of alkali salts of indigo white in water.
This definition distinguishes the _indigo vat_ completely from indigo
extract, or any other improper purposes to which indigo may be put.

The zinc lime indigo vat is better than any other for dyeing cotton
and linen. It is also very good for dyeing silk. It has many
advantages over the hydrosulphite vat, as it is not nearly so much
affected by changes of temperature and weather. It can be put to work
after a six months' rest.

The disadvantage which it shares with the copperas vat, though in a
less degree, is that there is a sediment which must not touch the
stuff during the dyeing. This is avoided by hanging a net in the vat
after the sediment has settled, or by dipping the skeins on rods.

It is essential that the indigo used should be of the best quality,
and ground to so fine a powder that it will float on water. Coarsely
ground indigo will never reduce and can be found at the bottom of the
vat unchanged. It should be so fine that no roughness is felt with the
tongue. Buy the best quality indigo ready ground, and if possible
mixed to a paste with water. A 20% paste, _i.e._ 20% of indigo and 80%
of water, is a usual quantity. If indigo powder must be used it must
be mixed to a paste very carefully, as it will, if properly ground,
fly about like dust. The easiest method of mixing is to pour the
required amount of boiling water into a jar (previously heated), then
put in the indigo. Close the vessel tightly. The steam which rises
will moisten the indigo so that it loses its tendency to fly about.
After 10 or 15 minutes it can easily be mixed with a stick. The zinc
dust should be dry and not caked.

_The lime_ should be in hard lumps. It should be bought from a
reliable chemist in a sealed container, and kept sealed till wanted.
If it is crumbling and cracking it has been exposed to damp air, and
is partly slaked already, and therefore more or less useless.

As the indigo is more quickly reduced in a concentrated solution, a
stock vat is first made and this is added to the dye vat as required.
The vessel for the stock vat should have a well-fitting lid. A
stoneware jar with a bung will do very well. To make a stock vat
sufficient to furnish a dye vat containing 15-20 gallons use:--

10 oz. Indigo 20% paste (or 2-1/2 oz. indigo pasted with
7-1/2 oz.