---------+------------------------------------------ | Pounds per 100 gallons. +------------------------------------------ Twaddell.|Al_{2}O_{3}. | SO_{3}. |Sulphate of | |Alumina | |containing 15 per | |cent. Al_{2}O_{3}. ---------+-------------+---------+------------------ 1 | 1·4 | 3·3 | 9·0 2 | 2·8 | 6·5 | 19·0 3 | 4·2 | 9·8 | 28·0 4 | 5·6 | 13·0 | 37·0 5 | 7·0 | 16·3 | 47·0 6 | 8·4 | 19·6 | 56·0 7 | 9·8 | 22·8 | 65·0 8 | 11·2 | 26·1 | 75·0 9 | 12·6 | 29·4 | 84·0 10 | 14·0 | 32·6 | 93·0 11 | 15·4 | 35·9 | 103·0 12 | 16·8 | 39·1 | 112·0 14 | 20·3 | 47·3 | 135·0 16 | 23·1 | 53·8 | 155·0 18 | 26·2 | 60·3 | 172·0 20 | 29·4 | 68·5 | 196·0 25 | 37·1 | 86·5 | 247·0 30 | 44·8 | 104·4 | 299·0 35 | 53·2 | 124·0 | 355·0 40 | 60·9 | 142·0 | 405·0 45 | 68·6 | 159·9 | 456·0 50 | 77·7 | 181·0 | 578·0 55 | 86·1 | 200·6 | 575·0 60 | 95·2 | 221·8 | 635·0 ---------+-------------+---------+------------------ _Alum._--Alum is one of the most important substances required in the manufacture of paper, its chief function relating to the sizing of paper. Various forms are utilised for this purpose, the purest being sulphate of alumina, required for high grade papers, and the cheaper form known as alum cake, for news and common printing. The alum is manufactured on a large scale by heating china clay or bauxite with sulphuric acid. This reaction gives sulphate of alumina together with silica. If the mass is heated to dryness, it is sold under the name of _alum cake_. If the mass is extracted with hot water and the insoluble silica filtered off, the solution can be evaporated down for the production of _sulphate of alumina_, which is sold in the form of large cakes or in the form of crystals. By careful selection of raw material a sulphate of alumina can be prepared almost entirely free from iron. The presence of the latter is undesirable, since on exposure to air the sulphate of iron produced during the manufacture of the alum is slowly oxidised and turns brown. Ultimately this affects the colour of the finished paper.