Many years ago a substitute for alum was introduced, called Pochin’s aluminous cake. It consisted principally of sulphate of alumina Al_{2}3 SO_{4}, and was, provided it was pure, as suitable for the papermaker as the more expensive potash alum. It was more economical at equal price, as it contained about 14 per cent. of alumina, whereas potash alum, even if absolutely pure, could only contain 10·85 per cent. Pochin’s aluminous cake however, was liable to contain a considerable amount of free sulphuric acid and soluble sulphate of iron. Of late years, a number of different varieties of sulphates of alumina have been introduced into commerce, of so pure a nature, and at such a low price, that it is a matter of wonder that they have not entirely superseded the more expensive alum. They are prepared either from very pure native alumina (bauxite), or from China clay of good quality. Methods have been discovered of eliminating almost the whole of the iron and free sulphuric acid. The following analyses of different specimens sufficiently indicate their purity.[12] Alumina (Al_{2}O_{3}) 14·84 14·70 14·95 14·85 16·00 16·20 Ferric oxide (Fe_{2}O_{3}) ·06 ·12 ·05 trace nil trace Sulphuric acid (SO_{3}) 35·00 34·60 36·09 34·94 38·00 38·00 Free sulphuric acid ·32 ·40 nil ·29 nil nil Lime (CaO) ·11 ·11 ·17 ·14 ·16 ·14 Water 49·42 49·95 48·72 49·60 45·50 45·43 ───── ───── ───── ───── ───── ───── 99·75 99·88 99·98 99·82 99·66 99·75 [Footnote 12: Journ. Soc. Chem. Ind., Jan. 1886, p. 16.] Sulphate of alumina possesses this advantage over alum, that it is more soluble in water, and thus a stock solution of considerable strength can be prepared. It is soluble in {141} 2 parts of cold water, alum requiring 18 parts for complete solution. The amount of alum or sulphate of alumina added to a pulp is largely in excess of the quantity necessary to precipitate the rosin soap; as a matter of fact, in the case of esparto or straw pulps, for the bleaching of which considerable quantities of bleach have been employed, and which therefore contain a certain amount of basic lime, together with calcium chloride, complete precipitation of the size is effected without the addition of alum. A certain amount is also required to precipitate the starch. The excess of alum appears to be necessary, however, not only to brighten the colour of the paper, but also to render it capable of resisting the action of ink. From experiments made by the authors, it appears that one part of rosin requires 2·9 parts of alum for complete precipitation from its solution in soda. One part of starch requires 0·40 part alum.