air, become blue through deposit in the fibres of the insoluble Indigo
Blue, and are so dyed. This is called the indigo-vat method. We can
reduce this indigo so as to prepare the indigo-vat by simply mixing
Indigo Blue, copperas (ferrous sulphate) solution, and milk of lime in a
closely-stoppered bottle with water, and letting the mixture stand. The
clear liquor only is used. A piece of cotton dipped in it, and exposed
to the air, quickly turns blue by absorbing oxygen, and is thus dyed.
The best proportions for the indigo-vat are, for cloth dyeing, 4000
parts of water, 40 of indigo, 60 to 80 of copperas crystals, and 50 to
100 of dry slaked lime. The usual plan is to put in the water first,
then add the indigo and copperas, which should be dissolved first, and
finally to add the milk of lime, stirring all the time. Artificial
indigo has been made from coal-tar products. The raw material is a
coal-tar naphtha called toluene or toluol, which is also the raw
material for saccharin, a sweetening agent made from coal-tar. This
artificial indigo is proving a formidable rival to the natural product.

Orchil paste, orchil extract, and cudbear are obtained by exposing the
plants (species of lichens) containing the colouring principle, called
_Orcin_, itself a colourless substance, to the joint action of ammonia
and air, when the oxygen of the air changes that orcin by oxidising it
into _Orcein_, which is the true red colouring matter contained in the
preparations named. The lichens thus treated acquire gradually a deep
purple colour, and form the products called "cudbear." This dye works
best in a neutral bath, but it will do what not many dyes will, namely,
dye in either a slightly alkaline or slightly acid bath as well. Orchil
is not applicable in cotton dyeing. Being a substantive colour no
mordants are needed in dyeing silk and wool with it. The colour produced
on wool and silk is a bright magenta-red with bluish shade.

Litmus is also obtained from the same lichens as yield orchil. It is not
used in dyeing, and is a violet-blue colouring matter when neither acid
nor alkaline, but neutral as it is termed. It turns red with only a
trace of acid, and blue with the least trace of alkali, and so forms a
very delicate reagent when pieces of paper are soaked with it, and
dipped into the liquids to be tested.

Safflower: This vegetable dyeing material, for producing pink colours on
cotton without the aid of a mordant, consists