DYES AND DYEING - Archaeological Chemistry - page 372

Blue Dyes
A single blue dye, indigo , seems to have been predominant in the ancient
world since it first became known to humans, over 4000 years ago. Some his-
torians believe indigo to be the oldest known dye. Indigo is a vat dye that
may be derived from either of two different types of plants: indigo or woad .
Indigo plants belong to the genus Indigofera , which grows naturally and was
also cultivated in many areas of the world, as for example, Egypt in Africa,
India in Asia, and Brazil and Peru in South America. The woad plant, Isatis
tinctoria , is native to southeastern Europe, from where it spread quickly
throughout Europe in prehistoric times (Hurry 1930). Although the dye in
both types of plants is the same, namely, indigo, in the Indigofera (indigo)
plants the dye is about 30 times more concentrated than in Isatis (woad)
plants. The dye is composed of a mixture of two different organic substances:
Indigo blue , the major component (see Fig. 77), and small amounts of indigo
red , chemically known as indirubin . Trace amounts of indirubin are probably
present in all samples of the natural blue dye. Some indigo plant varieties,
such as some from Java, are particularly rich in indirubin, containing as
much as 6% indigo red. Indigo blue has been traditionally extracted from
the leaves and to some extent from the midribs of the plant. After the leaves
and midribs have been collected, they are soaked in water; the mixture of
water and plant tissue is then pounded into a paste that is left to ferment for
a few days. Following the fermentation process, the liquor is filtered out of
the solid residue and exposed to air; oxygen in the air oxidizes the dye,
which precipitates out of solution as a powdery dark blue solid that is then
separated by filtration from the liquid and finally dried.
Since it is a vat dye, indigo cannot be used for dyeing without
prior chemical modification to make it soluble. Thus, during the dyeing
process indigo is first chemically reduced to its colorless intermediate, leuko
indigo , which is soluble in water. Yarn and/or textiles dipped in the
colorless solution absorb the reduced leuko indigo, which, when exposed to
air, is oxidized and yields the characteristic indigo blue color (Seefelder
1982).
Ancient people in such widely separated regions as India, Egypt, and
Peru independently discovered the preparation of the blue dyestuff from
different indigo plants indigenous to the regions they inhabited. The dye has
been identified in woolen fabrics from upper Egypt dating from 200 to 700
C.E., in cloth from the Judean Desert in Israel, and in Inca textiles from Peru.
The ancient Celts tattooed their bodies with designs colored with indigo that
they obtained from the woad plant (Balfour-Paul 2000; Saltzmann et al.
1963).