The final coloring or whitening coat is left untreated so as to be least absorbent. In all, a maximum of eight priming coats is sufficient. Each one must be applied very quickly in order not to soften the previous coat which has been allowed to dry thoroughly before being covered.
This priming used to be called "chalk ground" as distinct from a ground of equal parts of chalk and oil. The use of the word chalk is quite misleading, for what is meant is actually a pure glue ground, irrespective of whether chalk, bole, anal in, or other filler is used. A ground of equal parts chalk and oil would be better called glue and oil ground, the upper coats being given an oily binder, while oil grounds use oil alone except for the first sizing.
The two oil grounds have been mentioned only because they are often the only ones found on the ready-prepared panels and rolled canvas offered for sale. They are very convenient, especially for amateurs and painters with little technical experience, but they have no other advantage. Oil grounds are responsible for the unfortunate habit of rolling primed, and worse, painted, canvases, because the elastic oil film seems to suffer no harm from it. In fact, even a fully oxidized oil film cracks or crumples when it is rolled up, and cracks when it is unrolled.
A mixed glue and oil ground suffers even worse. Old pictures which have been left rolled for a long time or are rolled up after many years generally crack right through to the canvas and are a great problem to picture restorers. It sometimes takes weeks to scrape away the canvas from the picture layer and stick on a new canvas-a process called in the trade "relining."
Oil grounds absorb no binder, but this is a dubious advantage. There are excellent technical measures to be taken against excessive absorption by soft glue grounds. Pure glue grounds cannot be bought on the market, for they are very sensitive, and primed canvas must be already stretched, which is impossible on a commercial scale with all the varied sizes in demand. It is better not to try to buy ready-primed supports; no one can, or will, say what has been used on them.
METAL remains alien to any painting with pigments and binder. Although paintings on thin sheets of copper survive in good condition from the eighteenth century, it is totally inadvisable as a support. Only enamel colors will hold securely on metal, and they are only really secure when fused on in patches, separated into little cells formed by soldered-on wire, a technique far removed from the character of painting.
NATURAL STONE, on the other hand, forms a good support, both on facades and indoors. Either wax colors can be used, which are made to penetrate the pores of the stone by heating (encaustic), or watery pigments can be made to petrify with the stone by means of waterglass, which is sprayed over the painting. Stone is not primed, although sometimes for mineral painting it is etched.
BRICK WALLS and any other similar artificial stone must be primed for painting. Plaster is generally used for this. At least three coats are applied before it is painted: an undercoat or rough rendering, an upper coat, and a top coat. Each layer contains finer grit and sand than the one below, whether the work is done in pure lime plaster, lime and cement, or pure cement plaster.
Plaster containing cement is suitable only to be painted on when it is quite set and dry. Casein or silica paints can be used on it, and glue colors are suitable indoors. Fresco-secco was done with egg yolk as a binder.
True fresco, fresco-buono, is done only on pure white lime plaster while the plaster is still quite wet. The pigments are mixed exclusively with water and bound into the top plaster by a coating of crystalline calcium carbonate, the "sinter skin," which exudes from it. As soon as the fresco plaster begins to set, the pigment no longer takes on it. Thus, large frescoes are always painted piecemeal; the fresh plaster is added to the brick wall for each section as needed.
We can see that there is no universal support for all painting techniques. At any rate the priming must be suited to the subsequent technique, and the technique is determined primarily by the binding material.