I do not refer to the splendid decorative work on the wooden sarcophagi when I say little or no binder was used, for in these coffins and on the outside of the linen wrappings there are some really wonderful decorative paintings in which binders were used. The portraits outside of the mummy wrappings in the second century were done with wax and resins and are excellent works of art.
The primitive Italians were past-masters at the art of making Tempera Medium, as evidenced by their paintings still in existence, a large number of which are in absolutely perfect condition.
Tempera Medium, generally speaking, is a mixture of either the whole egg—or, more correctly speaking, the white of the egg—and a small quantity of drying oil and water. When the white of the egg is beaten up with linseed oil or poppy oil, it forms a very weak emulsion or soap, from a chemical standpoint. This is really nothing more or less than a solution of flexible adhesive, or glue. Tempera Medium is also made from Casein, which is an adhesive element held in solution in milk. This is separated from the milk, chemically, and dried, dissolved in weak ammonia water, and mixed with linseed oil. In other words, Tempera Medium is a Water Color Medium, to which a little drying oil, or other adhesive, has been added. On account of the weakness of the binder and the large quantity of water contained, which entirely evaporates, the color is left absolutely flat and without the slightest sign of gloss, unless piled on too thickly. After this has been allowed to dry for a few days, it may be varnished with a number of varnishes, which I will describe later on, when it has all the appearance of an oil painting. There are such a large number of tempera tube colors made that it is unnecessary for the artist to make his own. Tempera Colors do not change, excepting, of course, those which are inherently defective, and it is unfortunate that in looking over the list of Tempera Colors for sale, Aniline Lakes should be manufactured and sold at all. Colors, however, which really have inherent defects, like Flake White, Chrome Yellow and Paris or Emerald Green (which turns Black and Brown when submitted to sulphur fumes of the atmosphere) remain absolutely permanent as soon as they are varnished. Tempera Medium made with the whole egg contains a very large percentage of sulphur, and for this purpose the Tempera Medium, when pigments like Chrome Yellow, Flake White and Paris Green are used, should be replaced by a Tempera Medium made of White of Egg and Oil, or Casein and Ammonia. The Ammonia, it must be understood, plays no role, since it is only used as a solvent for the Casein, and during the process of cooking or boiling is driven off, so that a boiled Casein emulsion of Linseed Oil has no effect on any color. The white of the egg contains a very little oil, while the yolk of the egg contains as high as twenty per cent. of fat or oil, thus adding to the flexibility of the medium. But, since the yolk of the egg ranges from a light Yellow to an Orange, it can be readily seen that it is not suitable for the manufacture of white or light shades of Tempera Colors.
Cennini recommends, alternately, the yolk and the white, depending upon the purpose, mixed with the juice of figs. This, evidently, must refer to the sap of the fig tree; for, it is well known that rubber is obtained from the fig tree. A slight incision in any fig or rubber plant gives a milky excrescence, which has considerable adhesive power, and which contains about half rubber and half water, in the form of an emulsion. The yolk of the egg was, however, mostly used for fresco, panel and metal.
Dry white of egg may be bought as a commercial article, as eggs are gathered for this purpose in large quantities in China. This may be dissolved in a weak alkaline solution, and of course this solution must be made in the cold, the alkali, if it be ammonia, being allowed to evaporate at room temperature, even after the oil is added. Otherwise, as anyone knows, the boiling of any white of egg solution, coagulates it, leaving hard boiled white of egg as the result. The ideal Tempera solution for manufacturers to use would, therefore, be Casein, to which the requisite amount of oil should be added—or, white of egg and a drying oil.
Unless a preservative be added to a Tempera emulsion, it will rot and decompose in a very short time. For this purpose there are many preservatives. Oil of Cloves has been used from the time of the Egyptians. In fact, boiling or mixing with spices has been regarded as the natural disinfectant and preservative. Modern chemistry, however, teaches us that materials like Benzoate of Soda, Salicylic Acid, Boric Acid and Carbolic Acid are the best preservatives, although if insufficient amounts are added, the Tube Colors will decompose in time. If Tempera Colors are used in the place of Water Colors for painting on paper, care must be exercised that the colors are applied very thinly; for, if the slightest attempt at impasto is tried, the colors will crack, and in many instances fall off after they have thoroughly dried out. The binder in Tempera Colors is really so weak that it will not support by gravity a large quantity of pigment. Therefore, thin painting in Tempera, especially on paper, is always essential.
The Permanent Palette for Tempera Colors is the same as that for Oil Colors. If no interaction between Tempera Colors takes place while they are in the process of drying, there can be no reaction that will ever take place after they are once dried, since dry chemicals do not react. Hence, after Tempera Colors are varnished, they may be regarded, if permanent to light, to be permanent forever. There comes, then, the question which has been mooted so often among artistic painters—Why varnish a Tempera painting at all, since the idea is to produce the flat effect?—to which the answer must be made that a painting, no matter of what it is made, unless hermetically sealed, either in a glass covered frame or with varnish, or both, is bound to be acted upon by our modern indoor atmosphere, and varnishes can be made, and are made, which dry with an absolutely Matte finish, so that these may be used with perfect success over a Tempera picture.
There are several types of Tempera Colors on the market. Strictly speaking, a Tempera Color should be a mixture of Albumen, made either of dried or fresh white of eggs, linseed oil and water beaten up into an emulsion. It is not generally known among the laity that all vegetable oils, whether they be drying or non-drying, will make an emulsion with lime water or any other alkali, but lime water is probably the best emulsifying agent to use. This medium, when mixed and ground with dry colors, forms the well-known Tempera, similar in all respects, to that used by the primitive Italians. One has only to see the remarkable permanence of the early Italian Tempera painting to conclude beyond peradventure that this medium must be regarded as one of the most permanent in existence.