Indigo is derived from several plants of warm climates. In the plant
the color exists as a yellowish liquid; but when extracted and exposed
to the action of the air it becomes insoluble, and takes an intensely
blue color. The cultivation of the plant is carried on chiefly in
India, Java, Egypt and Louisiana. Indigo comes in the market in lumps,
which, if of good quality, presents a deep bluish purple color, and
exhibits a fine reddish coppery lustre if rubbed with a hard, polished
body. If very hard or heavy, or when the color is very dull, blackish,
greenish or brownish, the quality is below the standard. It is,
however, of very little consequence in ostrich feather dyeing, and
its impurities would scarcely at any time be noticeable. It should,
however, dilute thoroughly in boiling water, and if there remain a
sediment of any proportion, the indigo is impure. Sulphuric acid is
generally used to develop the color.