In the treatment of wool or fur with bichrome (potassium bichromate) we start with an acid salt, a bichromate (K_{2}Cr_{2}O_{7}) and a strong oxidising agent, and we finish with a basic substance, namely, oxide of chromium, in the fibres of the wool or fur.  If we desire to utilise the whole of the chromic acid in our mordanting liquor, we must add to it some sulphuric acid to set free the chromic acid from the potassium with which it is combined.  Bichromate of potash with sulphuric acid gives sulphate of potash and chromic acid.  The question of the proper exhaustion of bichromate baths is an important economic one.  Now we must remember that this chromic acid (CrO_{3}) oxidises our wool or fur, and must oxidise it before it can of itself act as a mordant by being reduced in the process to hydrated chromic oxide, Cr_{2}O_{3} + 3 H_{2}O. [2 CrO_{3} (chromic acid) = Cr_{2}O_{3} (chromic oxide) + O_{3} (oxygen).] It is this hydrated chromic oxide in the fibre that yields with the Haematein of the logwood your logwood black dye.  Mr. Jarmain finds that it is not safe to use more than 3 per cent. (of the weight of the wool) of bichromate; if 4 per cent. be used, the colour becomes impaired, whilst if 12 per cent. be employed, the wool cannot be dyed at all with logwood, the phenomenon known as “over-chroming” being the result of such excessive treatment.  I think there is no doubt, as Professor Hummel says, that the colouring matter is oxidised and destroyed in such over-chroming, but I also think that there can be no doubt that the wool itself is also greatly injured and incapacitated for taking up colour.  Now the use of certain coal-tar black dyes in place of logwood obviates this use of bichrome, and thus the heavy stress on the fibre in mordanting with it.  It also effects economy in avoiding the use of bichrome, as well as of copper salts; but even thus, of course, other problems have to be solved before it can be finally decided which is best.

LECTURE IX

DYESTUFFS AND COLOURS

Classification.—­In classifying the different dyestuffs and colouring matters it is, of course, necessary to consider first the properties of those colouring matters generally, and secondly the particular reason for making such classification.  The scientific chemist, for example, would classify them according to theoretical considerations, as members of certain typical groups; the representative of medical science or hygiene would naturally classify them as poisonous and non-poisonous bodies; whilst the dyer will as naturally seek to arrange them according to their behaviour when applied to textile fabrics.  But this behaviour on applying to textile fibres, if varied in character according to the chemical nature of the colouring matter, as well as the chemical and physical nature of the fabric—­and it is so varied—­will make such classification, if it is to be thorough-going, not a very simple matter.