Natural Substantive Colours.—­Indigo, one of the most valuable dyes, is the product of a large number of plants, the most important being different species of indigofera, which belong to the pea family.  None of the plants (of which indigofera tinctoria is the chief) contain the colouring matter in the free state, ready-made, so to say, but only as a peculiar colourless compound called indican, first discovered by Edward Schunck.  When this body is treated with dilute mineral acids it splits up into Indigo Blue and a kind of sugar.  But so easily is this change brought about that if the leaf of the plant be only bruised, the decomposition ensues, and a blue mark is produced through separation of the Indigo Blue.  The possibility of dyeing with Indigo so readily and easily is due to the fact that Indigo Blue absorbs hydrogen from bodies that will yield it, and becomes, as we say, reduced to a body without colour, called Indigo White, a body richer in hydrogen than Indigo Blue, and a body that is soluble.  If this white body (Indigo White) be exposed to the air, the oxygen of the air undoes what the hydrogen did, and oxidises that Indigo White to insoluble Indigo Blue.  Textile fabrics dipped in such reduced indigo solutions, and afterwards exposed to the 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.