Of the organic assistants tartar is undoubtedly the best in general use, and, although slow in its action, leaves a good deposit of oxide of chrome on the wool in a suitable condition to develop the best results on dyeing. Argols are only an impure tartar. They can only be used when dark shades are to be dyed. Oxalic acid does not work as well as tartar, and there is not so much chrome oxide deposited on the wool, while there is a slight tendency for a small proportion of this to be in the form of chromic acid. Of late years lactic acid and (p. 117) lignorosine have been added to the list of assistant mordants; both these give excellent results, they lead to a more complete and more uniform decomposition of the bichromate, and therefore the mordanting baths are more completely exhausted, so that rather less bichromate is required; the shades which are obtained are in general fuller and brighter. Examples of the use of these assistants will be found among the recipes given below.
With fluoride of chrome either oxalic acid or tartar is used, and a deposit of chromium oxide is formed on the wool, the general effect being the same as when bichromate of potash is used with oxalic acid or tartar.
Alumina is applied either in the form of alum or of sulphate of alumina, argol or tartar being used as the assistant, oxide of alumina being deposited on the fibre.
When ferrous sulphate (copperas) is used then tartar is almost invariably used as the assistant mordant, oxalic acid only rarely.
The dyeing with mordant dyes must be done in a special way and with great care, if uniform, level shades and fast colours are to be obtained.
The dye-bath must be started cold, and the wool be entered and worked for twenty to thirty minutes, the object being to cause the dye-stuffs to penetrate well into the substance of the fibre, then the temperature is slowly raised to the boil, not less than three-quarters of an hour being taken in doing so; the temperature is maintained at the boil for fully one and a half hours longer. During the boiling operation the mordant and dye-stuff combine together, and form the characteristic colour lake, and the boiling fixes this firmly on to the wool.
The water used plays a very important part. If too hard in character, the lime it contains shows a tendency to combine with the (p. 118) dye-stuff and form a colour lake, which is deposited in a loose form on the wool or in the bath, tending to make the shades dull and loose to rubbing. This defect can be remedied by adding a little acetic acid to the dye-bath, say about 3 lb. to 100 gallons of the water. It combines with and neutralises the influence of the lime, in so far as the formation of a loose colour lake is concerned; still the lime does unite with the dye-stuff, but the combination is formed more slowly, and in or on the wool fibre so that it is fast.