By the action of rennet on milk the proteid or albumen principle is converted into a curd (casein). This curd, when freed from fats, is insoluble in water, but is soluble in dilute acids, or alkalies, or alkaline carbonates, from which substances, however, it is reprecipitated by acidulation. Instead of the above method, casein may be precipitated from milk by saturation with sulphate of magnesia, and washing the precipitate with a solution of that salt until the washings contain no albumen, and then redissolving the prepared casein by adding water. The salt still adhering to the precipitate enables it to dissolve. On a large scale the casein is usually prepared by treating the milk with acid.
Casein is readily dissolved by alkalies and alkaline carbonates, borax, boracic acid solution, caustic soda, and bicarbonate of soda.
Starch.—This substance is used in many classes of paper for improving the surface and finish. It is added to the pulp in the beating engine in the dry form as powder, or in the form of starch paste, produced by boiling the starch in water.
The viscosity of the starch paste is somewhat increased by the addition of a small quantity of alkali, but due care must be exercised in boiling, which should only be carried out sufficiently to cause the starch granules to burst, as any excessive boiling causes the starch paste to lose some of its viscosity.
The presence of starch in paper is detected by the blue coloration produced when the paper is dipped into a weak solution of iodine. The determination of the exact percentage of starch in a paper is a matter of some difficulty.
Silicate of Soda.—The precipitation of gelatinous silica upon the pulp in the beating engine is generally regarded as favourable to the production of a sheet of paper having what is known as a harder finish. The precipitation is effected by adding a solution of silicate of soda to the beating engine, with the subsequent addition of sufficient sulphate of alumina to react with the silicate of soda.
Analysis of Commercial Alums.
(Griffin and Little.)
| — | (1) | (2) | (3) | (4) |
| Insoluble in water | 0·05 | 10·61 | 0·11 | 0·56 |
| Alumina (Al2O3) | 15·47 | 14·96 | 11·64 | 16·58 |
| Iron protoxide (FeO) | 0·02 | 0·13 | 0·06 | — |
| Iron sesquioxide (Fe2O3) | 0·00 | 1·08 | 1·17 | 0·04 |
| Zinc oxide (ZnO) | — | — | — | — |
| Soda (Na2O) | 1·72 | 0·57 | 4·75 | 0·56 |
| Magnesia (MgO) | — | — | 0·45 | — |
| Sulphuric acid (SO3) combined | 37·26 | 37·36 | 35·98 | 39·17 |
| Sulphuric acid (SO3) free | — | 1·08 | 5·13 | — |
| Water by difference | 45·48 | 34·21 | 40·71 | 43·09 |
| 100·00 | 100·00 | 100·00 | 100·00 | |
| Sizing test (parts of dry neutral rosin size precipitated by one part of the alum) | 3·32 | 3·47 | 3·19 | 3·71 |