Page:The chemistry of paints and painting.djvu/65

Rh thorough seasoning, first of all to reduce the panel, by planing and glass-papering both sides equally, to the desired thickness. The panel is then soaked in water heated to 50° C, and then steamed. When dry, it receives a wash on both sides of a solution of corrosive sublimate in methylated spirit; it is again dried and seasoned in a warm air-chamber. After these operations, the panel should not require more than a slight rubbing with fine glass-paper, in order to render both surfaces plane. For panels to be used for oil-pictures, a priming is now applied, consisting of white lead, a little copal-varnish, and drying linseed-oil prepared by means of borate or oxalate of manganese. Allow this coat, which is intended to fill up the cavities and pores of the wood, to dry thoroughly, and then apply another coat in the transverse direction; subsequent coats should contain nothing but white lead (or other pigment) and the drying oil. Repeated smoothings of each coat, when hard, with fine pumice-powder are necessary; the last coat may consist of zinc-white and drying-oil. Both sides of the panel should be treated, as far as possible, alike, so that they may be equally loaded, and equally protected; but the pumice-rubbings are, of course, not required for the back of the panel. The object of priming the back is twofold—the prevention of decay and of the attacks of insects; and the avoidance of that gradual curvature whereby the protected front becomes convex, and the unprotected back concave. This change occurs through the slow loss of water from the back of the panel—a loss which is generally accompanied by a loss of some of the organic constituents of the wood through oxidation. Here it may be mentioned that the original steaming of the panel removes some of the extractives, and coagulates the albuminoids present, which are generally the first cause