When the surface to be painted is of a mineral nature, such as the exterior of a house, the pigments may be mixed with a vehicle consisting chiefly of water-glass, or soda or potash silicate. This method of painting requires some care, and a knowledge of the chemical nature of the pigments used. Some colours are completely destroyed by the alkali contained in the water-glass. Among those pigments which are not altered by the alkali may be mentioned lime carbonate, baryta white, zinc white, cadmium yellow, Naples yellow, baryta chromate, chrome red, red ultramarine, blue ultramarine, cobalt blue, cobalt green, chrome green, ivory black. When a wall is to be painted, it should first be prepared with a mortar composed of pure fat lime and clean sharp sand. The water used should also be free from saline impurities, as these might subsequently effloresce and destroy the surface of the paint. When the surface of this plaster is dry, a weak solution of water-glass should be applied, and the operation repeated several times. A strong solution cannot be used, because it forms a thin skin on the surface of the plaster, which closes the pores, and prevents the penetration of the water-glass. The pigments are rubbed down with a very weak solution of water-glass, and applied in the ordinary manner.

When thoroughly dry, the painted surface is treated with a warm solution of potash silicate applied in the form of a spray. Soda silicate may also be used, but the soda carbonate which is then formed is liable to cause efflorescence. A pigment fixed on the surface of a wall in this manner is as durable as the wall itself, and can be exposed to the weather without any fear of deterioration.