Indigo is the great substantive dye: the indigo has to be de-oxidised and thereby made soluble, in which state it loses its blue colour in proportion as the solution is complete; the goods are plunged into this solution and worked in it "between two waters," as the phrase goes, and when exposed to the air the indigo they have got on them is swiftly oxidised, and once more becomes insoluble. This process is repeated till the required shade is got. All shades of blue can be got by this means, from the pale "watchet," as our forefathers called it, up to the blue which the eighteenth-century French dyers called "Bleu d'enfer." Navy Blue is the politer name for it to-day in England. I must add that, though this seems an easy process, the setting of the blue-vat is a ticklish job, and requires, I should say, more experience than any other dyeing process.
The brown dyes, walnut and catechu, need no mordant, and are substantive dyes; some of the yellows also can be dyed without mordant, but are much improved by it. The red dyes, kermes and madder, and the yellow dye weld, are especially mordant or adjective dyes: they are all dyed on an aluminous basis. To put the matter plainly, the goods are worked in a solution of alum (usually with a little acid added), and after an interval of a day or two (ageing) are dyed in a bath of the dissolved dye-stuff.
A lake is thus formed on the fibre which is in most cases very durable. The effect of this "mordanting" of the fibre is clearest seen in the maddering of printed cotton goods, which are first printed with aluminous mordants of various degrees of strength (or with iron if black is needed, or a mixture of iron with alumina for purple), and then dyed wholesale in the madder-beck: the result being that the parts which have been mordanted come out various shades of red, etc., according to the strength or composition of the mordant, while the unmordanted parts remain a dirty pink, which has to be "cleared" into white by soaping and exposure to the sun and air; which process both brightens and fixes the dyed parts.
Pliny saw this going on in Egypt, and it puzzled him very much, that a cloth dyed in one colour should come out coloured diversely.