If the binder used is an emulsion, a
somewhat more advanced painting technique is required. All the methods dealt with so far have been such that, provided the ready-made materials were of the finest quality, the resultant picture would be as durable as if one painted with homemade painting materials. Any deterioration observed in following the methods so far described should be attributed to inexpert handling rather than to the materials themselves.
If the ground and paint layers contain a good deal of oil, the picture will yellow and blacken and develop cracks. Colors which contain too much glue are apt to falloff. Both phenomena accom
pany the use of a fatty medium or a solvent containing too much glue to thin the colors, in place of turpentine or water.
Admittedly, genuine tempera colors are also to be found on the market, but only an experienced chemist can say how the emulsion used was compounded. The names "egg tempera" or "oil tempera" merely tell you that you have an "O-W" (oil-water) emulsion in egg tempera, which you can dilute only with water, and which, when painted, will remain soluble in water; an oil tempera color, on the other hand, will be a "W-O" (water-oil) emulsion, and can be diluted only with turpentine.
You now know enough to guard against the mistake of adding oil to a fatty oil tempera to make it more runny, a process recommended in many old art textbooks. With the exception of casein colors, which are deliberately kept short of binder, all homemade colors should contain just enough binder to make the pigments adhere.
Experience shows that all bought colors contain too much binder. The reason for this is that the purchaser would find fault with a margin too narrow to guarantee indelibility. He would not know what to do, and on the next occasion would buy a different make.
Thus, it is a mistake to add extra binding agent to bought paints. Solvents which evaporate are the only ones to use. If, despite this, the makers continue to market every conceivable "art material," with no more precise indication of the drying time than "slow drying" or "quick drying," it is because over-fatty colors can be smeared on with such gay abandon. Due to their fatty oil content they take on a sheen when they harden; and, when a glaze is used, they do not attack and dissolve the underpainting. In principle, therefore, bought colors have no place in the supplies of the artist who wants to develop a really good technique.
This fact should be particularly borne in mind in handling true tempera. Bought materials only deprive a compound with this type of binder of the advantages peculiar to it. The advantages of the lean "O-W" emulsion and of the fatty "W-O" tempera are, as you know, their ability to dry rapidly and the fact that normally they do not crack or turn yellow. Their drawbacks are the need either to use up the tempera colors made with egg or a casein within a few days, or else to substitute a preservative to forestall deterioration.
Materials of this kind, to be found at any art supplier's, are very difficult to measure accurately for oneself; they also diminish the binding power of the medium, and this in turn entails the use of more binder than is strictly called for. However, the inconvenience of frequently making up easily perishable tempera should not deter you, once you are convinced of the abiding artistic value of your work.
The easiest ways to handle lean or fatty tempera are much the same as for oil or glue colors. But even with lean tempera, such as the commercial egg tempera, you will not obtain the best results on paper without a primer. After a while the oily constituents seep through the paper.
You may test this for yourself by studying the back of the unmounted sheet of paper, some weeks after a painting is completed. It will be seen, too, that the whole effect of the picture is grayer and yellower, as the luminosity of the originally white paper has been lost. When using lean tempera, therefore, such as those containing vegetable gum or resinous oil, the paper must first be given at least one or two coats with a glue-bound preparation. Conversely, fatty tempera should only be used on the same sort of ground as oil painting.