Page:The chemistry of paints and painting.djvu/63



PANEL

, as a backing for the painting-ground of works in tempera and oil, presents some advantages over plaster and canvas. Its chief merit lies, perhaps, in its comparative immunity from mechanical injuries. The wood selected must be hard, that its surface may resist blows and abrasion; and it must not contain much resin, gum, colouring-matter, or other 'extractives,' as they are called, or else discoloration of the painting-ground, or priming, may occur. Wood grown in poor soils, in temperate climates, and felled in winter, is the best. The Flemings used oak; the Italians white poplar. But oak often proves treacherous, through irregular sh mkage; while poplar is too soft. Italian painters employed, also, the wood of the stone pine and chestnut. Leonardo da Vinci recommended cypress, pear, and service-tree. Mahogany, which was unknown to the old painters, is now generally employed. Teak and cedar, and also American or black walnut, deserve further trial.

The specific gravity of wood varies from 0-3 to 1-3; the lighter kinds contain large volumes of interstitial air. The longitudinal contraction of wood is much less than the transverse; the distribution, form, and number of the cracks in old panel-pictures is often to be traced to this cause.