painting, and they may be spread on canvas as well as on more rigid supports. There is some danger of want of adhesion between the paint and the ground. It is also necessary to make sure that the materials of the ground do not affect sensitive pigments such as ultramarine. The hardening or petrifying liquids which in most cases are used in association with solid preparations to make the grounds in question, are invariably acid to test-paper, unlike the alkaline silicates described in Chapter IX.
Slate may be used as the ground for spirit-fresco and oil-painting; but its freedom from crystals of iron-pyrites, which present a brass-yellow colour, must be first ascertained. The firm adhesion of any priming, or other layers of oil-paint which may be applied subsequently, to slate may be secured in the following manner. The slate is slowly warmed in a water-oven, and thus becomes quite dry. While still warm, it receives a very thin coat of oil-copal varnish, largely diluted with turpentine or with toluol, and applied warm. When this film is hard, the painting may be carried out as in the ordinary way of using oil-colours; a priming of flake-white ground in oil and mixed with a little copal-varnish and turpentine, may be first applied, if desired. Terra-cotta and stone may be treated in the same way, but, being more absorbent than slate, the process recommended on p. 31 is preferable.
Owing to the presence of sulphuric acid in urban air painting-grounds containing calcium carbonate are liable to an injurious change, the carbonate being turned in part into the hydrous sulphate (gypsum) with a considerable increase of bulk. Then, through such expansion, the surface-pigment becomes fissured and even detached. It will be readily understood that grounds consisting chiefly of sulphate of lime are not susceptible of such
