at of the indigo. The vat should be stirred very well
and then left to stand. The changes which occur are probably the
following: The lime acts upon the copperas and produces ferrous
hydrate--this is unstable and tends to take up oxygen and hydrogen from
the water, particularly when there is some indigo present, and forms
ferric hydrate; hydrogen is at the same time liberated, and combines
with the indigo to form the soluble indigo white. It takes about
twenty-four hours to make an indigo vat. When properly made and in good
condition, the liquor will be clear and of a brownish-yellow colour, a
bluish scum may collect on the surface. If the liquor appears at all
greenish it is an indication that the indigo has not been completely
reduced, and the vat needs a further addition of lime and copperas,
which should be of good quality. Too much of each should not be used,
because with them there is formed at the bottom of the vat a sediment of
calcium sulphate and ferric hydrate, and it is not wise to increase this
to too great an extent, which would be the case if too much lime and
copperas were added.

_To Use this Vat._--Any scum on the surface is raked on one side, the
cotton yarn immersed for a few minutes, then it is taken out, wrung,
allowing the excess liquor to flow back into the vat, and the yarn hung
up in the air for the blue to develop. The depth of shade which is dyed
depends chiefly upon the amount of indigo in the vat, and also upon the
time during which the hanks are dipped in the liquor. Light and medium
shades can be readily and conveniently got by a single dip, but deep
shades are best got by repeating the dipping once or twice as occasion
demands. Deep shades got by using a strong bath at a single dip are
found to rub badly, while by repeated dips the dye gets more into the
substance of the fibre, and therefore the colour is more firmly fixed
and it rubs less.

Some indigo dyers have quite a range of vats, using those fresh made for
dyeing deep shades, while the old vats being nearly exhausted are used
only for light shades and finally when completely exhausted are thrown
away. After the day's work the vat should be stirred up and then allowed
to stand. If necessary it may be strengthened by the addition of fresh
quantities of indigo, lime and copperas, the next morning it will be
ready for use. Generally a lime-copperas vat will remain in good working
order for about a month, when it will be necessary to