The mixture of size and starch may be added directly to the engine, or it may be previously dissolved in water; the latter method is perhaps preferable. After allowing it to mix thoroughly with the pulp, a solution of alum is run in. It is made up with boiling water in either lead or copper tanks. Iron or zinc vessels must be avoided, as the solution acts rapidly on these metals.
Alum consists of a double sulphate of aluminium and potassium or ammonium, Al2 3 SO4, K2SO4 + 24 H2O; its function being to form with the rosin acids insoluble soaps which are precipitated in intimate mixture with the pulp, the sulphate of potash taking no part in the reaction. The choice of a suitable alum is a matter of very great importance; it should be free from excess of sulphuric acid and from iron. The former is deleterious on account of its action upon the colouring matter used to tone the paper, some colours being completely discharged by it; and because {140} of its effect upon any metal work with which it may come in contact, especially upon the brass wire-cloth on which the paper is made. The iron is objectionable as it forms a dark red precipitate of oxide of iron.
Alum as supplied by makers of repute is generally sufficiently pure for even the best classes of paper.
Many years ago a substitute for alum was introduced, called Pochin’s aluminous cake. It consisted principally of sulphate of alumina Al2 3 SO4, 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into commerce, of so pure a nature, and at such a low price, that it is a matter of wonder that they have not entirely superseded the more expensive alum.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 (Al2O3) | 14·84 | 14·70 | 14·95 | 14·85 | 16·00 | 16·20 |
|---|---|---|---|---|---|---|
| Ferric oxide (Fe2O3) | ·06 | ·12 | ·05 | trace | nil | trace |
| Sulphuric acid (SO3) | 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 |
[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.
Colouring.
In choosing an ultramarine, attention should be directed not only to its tinctorial power but to its behaviour with a solution of alum, inferior qualities being entirely discharged by it. All ultramarines are readily decomposed by free acids, hence the necessity of employing an alum as free as possible from this impurity.