+------------+--------------------------+-----------------------+ |Temperature.|Crystallised Ammonia Alum.|Anhydrous Ammonia Alum.| +------------+--------------------------+-----------------------+ | 0° C. | 5·22 | 2·62 | | 10° ” | 9·16 | 4·50 | | 20° ” | 13·66 | 6·57 | | 30° ” | 19·29 | 9·05 | | 40° ” | 27·27 | 12·35 | | 50° ” | 36·51 | 15·90 | | 60° ” | 51·29 | 21·95 | | 70° ” | 71·97 | 26·09 | | 80° ” | 103·08 | 35·19 | | 90° ” | 187·82 | 50·30 | | 100° ” | 421·90 | 70·83 | +------------+--------------------------+-----------------------+ Of the different alums, ammonia alum contains the largest and potash alum the smallest proportion of alumina. The composition of the three commonly occurring alums is given in the following table:— +--------------------+------------+----------+-------------+ | |Potash Alum.|Soda Alum.|Ammonia Alum.| +--------------------+------------+----------+-------------+ |Potash, K₂O | 9·95 | | | |Soda, Na₂O | | 6·80 | | |Ammonia, NH₃ | | | 3·89 | |Alumina, Al₂O₃ | 10·83 | 11·20 | 11·90 | |Sulphuric acid, SO₃ | 33·71 | 34·90 | 36·10 | |Water, H₂O₄ | 45·51 | 47·10 | 48·11 | +--------------------+------------+----------+-------------+ Thus ammonia alum is to be preferred to soda alum and soda to potash alum, whilst the latter is used on account of its greater purity. In a colour works, in which large quantities of alum are used, it is advantageous to work with ammonia alum which is recrystallised on the works. The alums and aluminium sulphate are the alumina compounds in ordinary use in colour works; aluminium acetate could also be used if it were to be had at a reasonable price. =Alumina=, Al₂O₃ = 102, =and Hydrate of Alumina=.—Pure alumina, or rather hydrate of alumina, is required in the preparation of many colours. When the solution of an aluminium salt is precipitated by soda, the carbonic acid escapes with effervescence, and a gelatinous precipitate is formed which it is extremely difficult to wash clean. The precipitate, which consists of hydrate of alumina, shrinks very greatly in drying, and turns to a horny mass; when strongly heated it loses water, and becomes a white, insoluble powder of anhydrous alumina.