The artist who uses the mixed technique must feel drawn toward it and be willing to forego the somewhat facile charm of impasto paint with a plastic texture. He must also use a smooth ground. As only the best ground will do for such meticulous work, you will undoubtedly take canvas-covered board. The canvas must be finely and smoothly woven, however, and the primer must be polished with sandpaper, producing a ground as smooth as paper, whose linen texture will in time be completely obscured.
Hence, the mixed technique is absolute painting, the method which rejects any external material appeal. It is the method which gives precedence to content, what the picture is about, over means, how it is done. The tendency to value these nonessentials at the expense of the subject is one found in all fairly recent paintings.
The subject matter of a picture painted in mixed technique is brought out by rich color effects, not by striking mannerisms. The color effect is based on a gray brought about partly by optical color mixture, which in turn consists partly of an additive light mixture. The glaze is like colored glass through which reflected light shines strongly at different angles of refraction; this brings out interfering colors such as exist in genuine pearls. Thus it is impossible to copy a picture painted like this in oils, using alia prima technique. Anyone who has attempted to do this will have discovered how primitive any kind of alia prima technique is when compared with mixed technique.
So far scientific research into painting methods has been able to ascertain, mixed technique was discovered by the brothers Hubert and Jan van Eyck, and they are among the select few, the greatest painters of all time. They do not owe this to their technical ability alone, to which the amazing state of preservation of their works bears such eloquent testimony.
If the mixed technique is out of favor at the present time, one does not need to be a prophet to predict that in the changing cycle of fashion it will again playa great part. That is, of course, unless technical progress provides us with a further simplified medium with allembracing artistic potentialities. The first step would be the discovery of a completely foolproof binding agent, capable of making the paint layers into a homogeneous whole and liable to no further change.
Synthetic materials are constantly being improved and might in time make such a discovery possible. From a technical viewpoint, the mixed technique rests essentially on the contrast between the use of resinous glazes and lean tempera colors thinned with water, i.e., an "O-W" emulsion: it lies embedded like trellis work between the films of resin.
When once you have grasped this notion, you will see how worthless would be the use of a fatty "W-O" emulsion in mixed technique. An unnecessary layer of fatty paint would result. The same is true of casein tempera. The strong binding power of its glue constituent renders it unsuitable for painting in several layers. Casein tempera is a good binder for a weather-proof "O-W" tempera applied in a single layer.
An artist can tackle these problems only against a background of wide practical experience, and he must know what his aim is. He will then find his way by means of systematic experiments. All that can be given here are basic ideas, but these can be subtly adapted to any and every form of painting.