Tempera-Painting

Tempera-painting, or painting in distemper, is generally assumed to include two, if not three, methods of procedure, in which different vehicles or media are employed. These vehicles all contain a nitrogenous constituent; but in one of them - and that the most important - oil or fat is present in addition. Tempera-grounds must be rigid, tenacious, and firm; they need not be dry, but if organic pigments are to be used, they should not contain caustic lime. Thus, a surface of plaster made with slaked lime and sand must have been so long exposed to the air as to have absorbed the amount of carbonic acid necessary to convert the hydrate of lime present into 'mild lime' - that is, the carbonate. To detect the existence of caustic lime in such a painting-ground recourse may be had to test-papers. Three kinds are available for this purpose. Thus, yellow turmeric-paper, first wetted and laid upon the surface of the plaster, should show no change of colour; if it become reddish, the presence of caustic lime is indicated. Under the same circumstances red litmus-paper turns blue or purple, while phenolphthalein-paper acquires a crimson hue.

If these tests show the absence of caustic lime, the painting may be commenced, otherwise the surface must be carbonated by syringing it or washing it with water charged with carbonic acid gas. These precautions are, of course unnecessary in cases where the painting-ground has been prepared with plaster-of-Paris or other neutral compositions of which caustic lime is not a component. Before commencing work the painting-ground must be slightly and uniformly moistened with distilled water, and then coated with weak size. The pigments to be employed are those recommended for use as water-colours; they are thoroughly mixed with the medium to be employed, namely, egg-yolk emulsion, or size, or prepared white of egg. These media serve not only to bind the pigments to the ground, but also the coloured particles to one another. To render the egg-yolk more tractable, its alkaline reaction should be exactly neutralized by the cautious addition of a very few drops of white vinegar - fig-tree sap or white wine was sometimes formerly employed for the same purpose. Some artists content themselves with diluting the egg-yolks with a little water, others add a small proportion of white of egg, previously shaken with a little water and filtered.

To keep the egg-emulsion sweet, a lump of camphor or a few cloves may be put into it. Size and also white of egg have been employed in tempera-painting. The white of egg needs dilution with water, thorough shaking, and then filtering through muslin. When egg-yolk is used in this method of painting, the oil in it gradually hardens, while the albuminoid matters which accompany it become partly insoluble and coagulated. As the amount of oil in egg-yolk is twice as great (31 per cent.) as the albuminoid matters (15 per cent.), this vehicle presents considerable resemblance to those employed in oil-painting, the albuminoid matters corresponding in a measure to the resins often used in the latter method. This vehicle does not act so effectually as oil and varnish in 'locking up' pigments, and so the protection against change which it affords is less. Moreover, instances have been observed in which the sulphur present in the albuminoids of egg-yolk has acted injuriously upon some of the pigments of the picture; but by excluding, as we now do, all paints containing lead and copper from the tempera-palette, accidents of this kind are prevented.

A finished tempera-picture was often - one might almost say generally - rubbed with a cloth and then varnished, the varnish being often made by dissolving sandarac in oil. The tone of the colours was thus warmed, while further protection was at the same time afforded against moisture and impure air.