ALUMNA MORDANTS. — Alum, known also as rock-alum, roach-alum, is probably the most ancient and the most widely-used mordant. It is a double sulphate of potassium and aluminium (potash-alum), or of ammonium and aluminium (ammonia-alum). For most purposes, these two kinds differ little in value and utility, ammonia-alum containing a larger proportion of alumina, and dissolving more readily in water. These alums are not only used to a great extent as such, especially in wool dyeing, both alone and along with argol, chrome, &c., but also in the preparation of other aluminous compounds.
The most dangerous impurity which alum may contain is iron, a substance very injurious even in the minutest traces. To detect its presence, a portion of the sample dissolved in water is mixed with a few drops of solution of potassium ferrocyanide and ferricyanide (yellow and red prussiate of potash): an immediate blue precipitate shows the presence of iron. Or a little solution of tartaric acid is added to the solution, then an excess of pure caustic soda, and a drop of ammonium sulphide: a blank coloration shows the presence of iron.
To distinguish ammonia-alum from potash-alum, add to the solution a little caustic soda, and apply heat. Ammonia-alum is at once known by giving off ammoniacal fumes, which may be recognized by the smell, by turning red litmus-paper blue, and by forming a white cloud with s, rod moistened in hydrochloric acid ; ammonia-alum, which, 20 years ago, was by far the more commonly met with, is now rare in the market. The Roman alum, which is now again coming into use, has a reddish cast, derived from the presence of a small quantity of oxide of iron, which, being in a condition insoluble in water and dilute acids, is perfectly harmless. It often contains a proportion of basic sulphate of alumina, and hence deposits its base, hydrate of alumina (aluminium hydroxide), more freely on the fibre. Basic alum is formed by adding ammonia or potash-lye, as the case may be, till the precipitate formed begins to appear permanent on shaking.
Cubic alum is obtained in a very similar manner, by dissolving alum in boiling water, and adding slaked lime in the proportion of A the weight of the alum. It forma cubic crystals.
Potash, soda, or their carbonates, if cautiously added to solutions of alum, withdraw a part of the acid, and form a salt which has a greater affinity for the fibre.
Sulphate of alumina, known also as cake alum, patent alum, or concentrated alum (aluminium persulphate), differs from alum in not containing an alkaline sulphate. It contains much more actual alumina—the really active principle—than either potash or ammonia-alum, and is much more readily soluble in water. Hence it goes farther, and is more convenient in use. Its dis advantages are—not being a crystalline compound, its composition is not absolutely invariable, traces of free acid being not unfrequently present; sometimes also it contains iron to a serious extent. It must be remembered that great improvements have been effected in the preparation of this salt, and it will doubtless be ultimately obtained free from iron and from uncombined acid. When this end is accomplished, alum will have no further claim on the dyer. Sulphate of alumina may be rendered basic in the same manner as potash- and ammonia-alum.
Next in importance to alum, comes red liquor, acetate of alumina, or aluminium acetate, a compound very largely used in cotton-dyeing and -printing ; more rarely with animal fibres. It is prepared in two general methods. A solution of alum, or of sulphate of alumina, is mixed with a suitable proportion of lead acetate (auger of lead) or of acetate of lime. Sulphate of lead (or of lime) is precipitated, and the clear liquid is the acetate of alumina required. Various prescriptions for this mordant will be found in the article on Calico-printing. It may be noted that small variations in the quantities of the ingredients often make an important difference in the results. Carbonate of soda crystals are generally employed to give a more basic product, and many practical men hold that a red liquid prepared from alum is often preferable to one produced from sulphate of alumina, the acetate of potash or of ammonia present in the former case playing a certain part in the production of the colour.