Table showing Value of Solutions of Aluminium Sulphate.
| Twaddell. | Pounds per 100 gallons. | ||
| Al2O3. | SO3. | Sulphate of Alumina containing 15 per cent. Al2O3. | |
| 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.
Alum is added to solutions of animal size or gelatine in order to thicken the solution and render it more viscous. It also acts as a preservative, and is used for regulating the absorption of the gelatine by the paper, the penetration effects being materially varied by the extent to which the alum is utilised.
In the process of engine sizing, a term applied to the application of rosin size on account of the fact that the process is completed in the beating engine, alum plays an important part. The mere addition of the prepared rosin soap to the mixture of pulp and water in the beating engine does not size the paper, but the alum precipitates the rosin from its solution, producing a complex mixture said to consist of resinate of alumina and free rosin particles, and subsequently the heat of the paper machine drying cylinders renders the paper more or less impermeable to moisture.
The appearance and tone of paper, more particularly of coloured papers, are brightened by the use of an excess of alum over and above that necessary to precipitate the rosin soap.
Rosin Size.—This substance is used chiefly for the sizing of news and cheap printing papers, and is also employed together with gelatine for the commoner writing papers. It is prepared by boiling rosin with carbonate of soda under various conditions.