Touched by Indigo - Royal Ontario Museum

1
beat the foam down. The foam will gradually disappear and the blue
liquid becomes clear. It is then left undisturbed for more than five
hours, letting the indigo settle. A final step is required to get rid of
impurities. The big pond has two small holes dug out on its wall, one
higher and the other close to the bottom. The liquid in it is drained
by removing the stopper in the upper hole from outside. When the
water level reaches just above the lower hole (used mainly for drainage),
bottom residue is scooped up with a
small bucket and poured into a rectangular
sieve placed across the long side of the
smaller pond located below the big pond,
allowing the indigo to strain into the pond.
This strained content is again left to settle
for at least seven days. When the time is
considered to be right, excess water is carefully
sucked up by a rubber hose and the
still wet indigo dyestuff is retrieved from
the bottom of the pond. It is placed in a
pre-soaked woven bamboo basket dusted
with a layer of plant ash and lined with a
wet cloth. After the moisture in the dye has
seeped through the cloth and basket the
indigo is taken out using the cloth and is
allowed to harden on its own into a cake.
When dyeing textiles using indigo, a dye vat is filled with warm
alkaline solution to dissolve and reduce the insoluble dyestuff.3 The
dye liquid turns yellowish-green when indigo is temporarily transformed
by hydrogen into a leuco-derivative called "indigo white."
Textiles immersed in the dye first appear yellowish-green. However,
after they are lifted out of the dye vat for brief airing, oxygen converts
the dye back to its permanent blue form again.
Indigo is the most versatile of all natural dyes. It is particularly
well suited to dyeing cotton, hemp, and ramie because these cellulosic
fibres are unaffected by the alkaline medium of the indigo vat.
Animal fibres of silk and wool, too, can be dyed by indigo beautifully,
but they need a less alkaline vat. Whatever the fibres used, the
desired shade of blue is controlled by the number of dippings and
aeration. The built-up layers of colour not only strengthen the fibres,
but also give indigo its unique durable quality. Once dyed a deep
blue, indigo is so colourfast that it can last for centuries, as is evident
in textiles discovered in archaeological excavations. If there is any
undesirable quality about indigo, it is its susceptibility to rubbing.
After a fabric has been dyed repeatedly with indigo, the dye is deposited
on its fibres as sheets of flat molecules. Whenever the fabric is
rubbed, friction will cause them to slide apart relatively easily. As a
result, indigo blue tends to fade after extensive washing or rubbing,
but it never loses its hue completely.
Indigo can be mixed with other colorants to achieve different
hues. It is also used as an antidote to poisoning in China. The
Chinese still use indigo to treat bleeding, boils, insect bites, fever, and
respiratory ailments.
10