3. In stereochromy, or water-glass painting, a process introduced more than sixty years ago, the fixative employed is an alkaline silicate dissolved in water. From time to time different experimenters have improved the painting-grounds, the preparation of the pigments, and the mode of applying the fixing liquid; but the main chemical actions involved in this method of painting are identical in all the modifications which have been introduced. The constituents and preparation of painting-grounds adapted for this process have been discussed in Chapter II (Plaster, Gesso, Stone, Slate, Etc). The pigments should be treated, as recommended by Kuhlmann, with some of the fixing liquid, and then reground; in some cases they require the previous addition of oxide of zinc, powdered marble, powdered glass, carbonate of baryta, soluble silica, hydrate of alumina, etc., in order that their natural inaptitude for equal fixation by the alkaline silicate should be remedied. Opinions differ as to the desirability of treating the painting-ground with some of the water-glass solution before laying on the colours; but it is essential that if a solution of this silicate be used at this stage, it should be very dilute.

The finished painting is sprayed with a warm dilute solution of potash water-glass or potash-soda water-glass, to which has been added liquor ammoniæ. The surface is shortly afterwards washed repeatedly with hot distilled water; and, if necessary, the application of the water-glass solution, and the subsequent washing, are repeated. The final result of these operations is to bind the particles of pigment to one another, and to the ground, by means of an insoluble double silicate. This silicate, formed partly out of some of the constituents of the ground, of the pigments, and of the water-glass, mainly consists of silica, lime, and potash; it often contains zinc, magnesia, and alumina. The soluble salts removed by washing the painting with water are the carbonates of potash and ammonia; when, however, soda is present in the water-glass, carbonate of soda has been formed, and is removed at the same time. The pigments employed in stereochromy are more limited in number even than those available in fresco-painting, and consist chiefly of natural oxides and earths, the artificial oxides and hydrates of chromium and iron, cobalt green, ultramarine, cobalt blue, and ivory-black.