The substance sold by druggists under the name of potassium is very violent in its action and would burn the leather as well as the brushes used to apply it. It should only be employed, and that very sparingly, for such purposes as black lettering. Pearl-ash should be used in preference, but also with great caution, or it will not only destroy the surface, but rot the substance of the leather.
Potash is soluble in water to saturation point, that is to say, a given quantity of water will not dissolve more than a given quantity of potash.
It is very difficult to measure the exact strength to use; that which is barely sufficient to penetrate and colour a surface artificially made smooth by rolling, will sink in where the leather has been cut or punched and immediately darken it considerably. It is a good plan therefore to experiment on a spare piece of the leather. Potash lye is more easily used in the right proportion, and may be employed for staining very light coloured leather.
Soda has the same effect as potash, except that it is a little less strong; both substances have always been much employed in dyeing skins. Permanganate of potash is supposed to dye a mouse-grey colour, but it is most uncertain in its action, as it depends entirely on the porosity of the leather. Xanthin and catechol browns are pleasing in appearance, but their effect is less rich than that obtained with potash. They become improved by hard rubbing with encaustic.
Sulphate of iron has a strong and persistent odour and is also somewhat injurious. It is useful, however, and produces a colour varying from a light shade to an intense
violet black. When used over potash, a dead black is obtained.
REDS
Alizarin, the colouring principle of madder, which can also be obtained from coal-tar, is employed in many colour combinations and gives a fine durable red. Hydrochloric acid changes it to violet. Carthamine and cochineal also produce good reds; the latter is turned orange by the action of hydrochloric acid.
Hydrochloric acid turns ammoniacal cochineal pink, but merely discolours the red made from archil. Direct extracts of red wood are unaffected by it.
The colouring matter of logwood, extracted by treating the powder obtained from an aqueous decoction of the wood with spirit, is sold as hematoxylin. Metallic salts convert it to a violet black, salts of tin change it to pink, alkalis to blue, and acids to yellow. Durable pinks are obtained directly from carthamine, the colouring matter of Carthamus Tinctorius.