8 per cent. of copperas and 20 per cent. of cream of tartar is a mordant used for some colours.
4 per cent. copperas, 10 per cent. cream of tartar gives good olive colours with weld.
8 per cent. copperas without tartar with single bath method, for dark olive brown with old fustic.
2 oz. copperas and 2 oz. cream of tartar to 2½ lbs. wool.
2 oz. copperas, 1½ oz. oxalic acid to 2½ lbs. wool.
TIN.—(Stannous chloride, tin crystals, tin salts, muriate of tin.)
Tin is not so useful as a mordant in itself, but as a modifying agent with other mordants. It must be always used with great care, as it tends to harden the wool, making it harsh and brittle. Its general effect is to give brighter, clearer and faster colours than the other mordants. When used as a mordant before dyeing, the wool is entered into the cold mordanting bath, containing 4 per cent. of stannous chloride and 2 per cent. oxalic acid: the temperature is gradually raised to boiling, and kept at this temperature for 1 hour. It is sometimes added to the dye bath towards the end of dyeing, to intensify and brighten the colour. It is also used with cochineal for scarlet on wool, in the proportion of 6 per cent. of stannous chloride and 4 per cent. of cream of tartar. Boil for 1 to 1½ hours. Then wash well. The washing after mordanting is not always essential. Also 6 to 8 per cent. of oxalic acid and 6 per cent. of stannous chloride, for cochineal on wool. This mordant produces bright fast yellows from old fustic, by boiling the wool from 1 to 1¼ hours, with 8 per cent. of stannous chloride and 8 per cent. of cream of tartar. One recipe gives 2 oz. tin and 4¼ oz. cream of tartar to 2½ lbs. wool in 10 gallons of water. It is not a suitable mordant alone for cotton, but can be used to brighten the colour in combination with other mordants. "The nitro-muriate of tin (dyer's spirit) although it produces good yellows with quercitron bark, produces them in a much weaker degree than the murio-sulphate of that metal, which is really the cheapest and most efficacious of all the solutions or preparations of tin for dyeing quercitron as well as the cochineal colours." —Bancroft.
CHROME. (Potassium dichromate, Bichromate of Potash.)
Chrome is a modern mordant, unknown to the dyer of 50 years ago. It is excellent for wool and is easy to use and very effective in its action. Its great advantage is that it leaves the wool soft to the touch, whereas the other mordants are apt to harden the wool. In commercial dyeing it is now almost exclusively used, as it has proved itself the most generally convenient. By some it is said not to be so fast to light as the other mordants, but it produces brighter colours. The wool should be boiled for one to one & a half hours with bichromate of potash in the proportion of 2 to 4 per cent. of the wool. It is then washed well and immediately dyed. Wool mordanted with chrome should not be exposed to light, but should be kept well covered with the liquid while being mordanted, else it is liable to dye unevenly. An excess of chrome impairs the colour. 3 per cent. of chrome is a safe quantity to use for ordinary dyeing. One recipe gives 1½ oz. of chrome to 2½ lbs. of wool. It should be dissolved in the bath while the water is heating. The wool is entered and the bath gradually raised to the boiling point, and boiled for three quarters of an hour.
In the dyeing of cotton, it is used for catechu browns and other colours. The cotton is soaked in a decoction of catechu, and afterwards passed through a boiling solution of chrome, or it is worked for half an hour in a bath of chrome at 60°C., and then washed. It is usual to wash wool or cotton after mordanting with chrome, but some dyers do not think it necessary.