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.
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