Panel

Wood, 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 shrinkage; 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.

Wood contains (1) water, (2) ligno-cellulose, (3) extractives, (4) ash or mineral matter. The water, in thoroughly-seasoned and air-dried wood, generally constitutes about one-eighth part of its weight. The main constituent of wood is the so-called ligno-cellulose, which is present to the extent of from 75 to 85 percent It may be resolved into two substances, which, for convenience' sake, are here called cellulose and lignose. The extractives belong to two groups - one, soluble in alcohol and ether, consists chiefly of resins; the other, soluble in cold or hot water, or else in very dilute alkalies, includes tannin, albuminoids, gum, and colouring-matters. The following analyses of three kinds of wood in an air-dried state will convey a fair idea of their constitution in 100 parts:

The preparation of panels for painting requires much time and trouble. The directions given by ancient authorities are numerous, and not always accordant. One author tells us to boil the wood; another says we are to coat it with mastic dissolved in twice-distilled turpentine and mixed with white. Then it is to be treated twice or thrice with spirits of wine, in which some white arsenic or corrosive sublimate has been dissolved; coats of boiled oil, of liquid-varnish¹ and white, and of verdigris and yellow are subsequently mentioned. Probably the best method of treating the harder woods intended for pictures is, after 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 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 mixture consisting partly of parchment-size, partly of fish-glue, and whitening.

It is very probable that some of the hard, fine-grained woods of British India and of North Borneo will furnish excellent materials for picture-panels. At present experiments in this direction cannot be regarded as more than tentative and promising.

1	Made by boiling 1 part of sandarac in 3 parts of linseed-oil.