_Preparation of Size._—Very pure gelatine can now be obtained in the form of light-coloured cakes. It is, however, more economical for the paper-maker to prepare his own: this is therefore done in the majority of cases. A great many animal substances, such as clippings of hides, horns, bones, &c., yield gelatine when heated with water. Any of these substances may be used, the first being the chief raw material. They are first softened by soaking in cold water for some days: they are then cleansed by washing in a stream of water. The next operation is that of heating with water. This is usually done in iron or copper vessels provided with a false bottom or outer jacket, into which steam may be passed. The hide cuttings are covered with water, and the mass gradually raised to a temperature of about 85° (185° F.). In from 10 to 15 hours nearly the whole of the cuttings will have passed into solution as gelatine. This solution is then drawn off and any insoluble substance suspended in it is removed by subsidence or filtration. The hides contain a certain quantity of fat, which collects on the surface of the solution. The residue in the boiler is again heated with water and the solution added to the bulk. It is of the greatest importance that too high a temperature be avoided, as gelatine is liable, when strongly heated with water, to lose its power of gelatinising. {163} To the filtered solution of gelatine, which should be, if properly prepared, of a pale colour, a quantity of alum solution is added. The effect of alum upon gelatine is very remarkable. If added slowly it will be found gradually to render it stiff until at a certain point the mass will become almost solid; a further addition renders it fluid again. It is in this state ready to be used for sizing purposes. The amount of alum necessary to produce this effect is about 20 per cent. of the weight of the raw material. The alum is also useful in preventing the decomposition of the gelatine, but its chief characteristic is to render the gelatine a more efficient sizing material. There is no doubt that, besides its action upon the gelatine itself, it has a considerable effect upon cellulose. It is of the greatest importance that the alum or the sulphate of alumina, which can be substituted, should not contain any free acid. This is especially necessary in the case of papers made from rags, in the bleaching of which an acid has been used; one effect of which is to remove all basic substances derived from the boiling or bleaching processes. In the case of esparto and similar fibres, a considerable quantity of such substances are present in the pulp, the result being that the free acid of alum is to a large extent neutralised, and its injurious effects prevented. The effect of the free acid is seen in the weakening of the paper and the destruction of metal surfaces with which it comes in contact. The effect of acid upon cellulose will be found more fully described in p. 12.