Red liquor may also be obtained by dissolving precipitated hydrate of alumina in the strongest acetic acid, until the latter is completely saturated.
In whatever manner acetate of alumina is prepared, the absence of iron is essential. Red liquor cannot be indefinitely preserved, especially if freely exposed to the air, as the acid gradually escapes, and the composition of the mordant is consequently altered.
Nitrate of alumina (aluminium nitrate) is not extensively used. It is prepared when heeded by mixing together, in equivalent proportions, solutions of alum or sulphate of alumina, and nitrate of lead. Sulphate of lead is deposited, and the clear liquid is nitrate of alumina.
Muriate of alumina (aqueous aluminium chloride) is also rarely used. It is prepared by dis solving hydrated alumina in hydrochloric acid, by decomposing a solution of the sulphate of alumina with one of chloride of calcium, or more economically under Fournier's patent, by mixing solutions of sulphate of alumina and common salt, and exposing the mixture to a temperature of — 2° to 0° (284 —32° F.). Sulphate of soda is deposited in fine crystals, whilst hydrochlorate of alumina remains in solution.
Hypoaulphite of alumina (aluminium thiosulphite) has been proposed by Kopp as a substitute for red liquor. It is readily obtained on decomposing sulphate of alumina by hypoaulphite of lime.
By the action of sulphurous acid gas upon a mixture of the tank-waste from the soda manufacture, with 10 per cent. of its weight of sulphur and water, previously boiled together, hyposulphite of lime is obtained in fine crystals. Kopp considers that it possesses distinct advantages over red liquor ; but in England, the opinion of practical men is not in its favour.
Oxalate of alumina has been made by dissolving precipitated alumina in oxalic acid, but is rarely used.
The aluminate of soda (alkaline pink mordant) forms an exception to the rest of the aluminons mordants, as it consists of alumina dissolved not in an acid, but in an alkali. It was originally prepared by precipitating alumina from alum or from sulphate of alumina by means of caustic soda, and adding the latter in excess, with the aid of heat, till the precipitate was redissolved. It is now to be met with in the solid state, and nearly pure.
In dyeing, it produces every effect yielded by alum, and, in addition, certain results which cannot be otherwise obtained. Notwithstanding its alkaline nature, it may be used to fix colours upon wool. In calico-printing, it aervea as a pink mordant, the pieces being afterwards taken through a bath of sal ammoniac, or chloride of zinc.