Page:The chemistry of paints and painting.djvu/56

22 same quantity of infusorial (silicious) earth; the whole being moistened with a sufficient quantity of lime-water to render working easy. For the undercoats the sand, etc., introduced may be coarser; while a small quantity of the most silky and whitest asbestos, cut with scissors into short uniform lengths, will prove a desirable addition. The asbestos lessens the risk of any lack of continuity in the undercoats.

Fresco-Secco and Tempera.—For fresco-secco the same ground as that required for true fresco may be used, but it is allowed time to dry and harden. So long as it contains any caustic lime this ground is unfitted for work in tempera, as its alkaline nature seriously limits the variety of pigments which may be employed in this method. When carbonation of the lime is complete it may be employed for tempera-painting, the surface being first treated with warm size. Many Greek and Byzantine paintings were, however, executed upon a caustic lime ground, but the pigments employed consisted chiefly of those natural earths which are unaffected by alkalies. In these Greek tempera-grounds slaked lime mixed with chopped straw, flax, or cotton, formed the basis of the plaster. It is scarcely necessary to remark that these vegetable materials are liable to decay and to cause discoloration of the ground.

The ordinary ground for Italian and Spanish tempera-paintings consisted either of whitening and size, or of burnt gypsum (that is, plaster of Paris), stirred well with