Page:The chemistry of paints and painting.djvu/66

32 of decay. This decay is not primarily a chemical and spontaneous one, but is commenced by certain minute organisms, the growth and increase of which is, in part, dependent upon the presence of available albuminoids, but which involves also the destruction of some of the other extractives, and even of the ligno-cellulose itself. The corrosive sublimate employed helps to sterilize the wood, and to prevent the inroads of animal organisms.

In order to avoid the disastrous effects of transverse shrinkage upon compound panels, the old painters glued linen cloth, or vellum, or parchment, or tinfoil to the front surface of the wood, and on this they spread their gesso or painting-ground. Gesso, made of plaster of Paris and size, or of whitening and size, often lost its cohesion through the decay of the binding material, and in consequence became fragile and powdery ; the panel itself decayed, and thus at last the linen or parchment remained as the best preserved element of the composite structure. Were we to avoid gesso and use lead-primed canvas glued to panel, we should really be painting upon canvas backed or protected by wood. Panel is to be recommended for modern work only when a single piece of uniform and well-seasoned wood of sufficient size can be secured. However, an excellent cement for joining panels together was sometimes used with success. It consisted of lime and cheese, both in fine powder, the latter having been grated, and then washed with water. These materials intimately mixed and then ground into a paste with water, yield a tough and adhesive cement which becomes of rocky hardness.

In order to prepare a panel for tempera work, it should be treated in the manner above described, substituting for the priming with oil, white lead and copal-varnish, a