His way of smearing or buttering the wall answers pretty well on a very rough surface, but on smooth stone or tiles it would not do at all. In Italy it is not at all uncommon to see marble columns coated with frescoes more than four hundred years old. The intonaco in these cases is very thin, not above one eighth of an inch in thickness.
As a rule the thinner the intonaco the better it will stick.
We will suppose now that we have painted our flat background and finished our first day’s work. We now get our pricked tracing, and holding it so as to fit the panel, we apply our charcoal bag to the outline of the heads. When we remove the tracing-paper we find a black dotted line which gives us the outline against the sky. With a knife or a sharp spatule we cut away the superfluous mortar. The cut should not be at right angles with the wall, or the outline will be sure to be injured next day when the fresh mortar is joined on to it.
It should be inclined at an angle of fifty or sixty degrees. I always make a point of doing this cutting job myself. The dotted line is sometimes indistinct, and I have to cast a glance at the cartoon. Where, therefore, there is any complication of outline or the least indistinctness, this operation ought to be done by the artist.
Before leaving, we make a charcoal mark as before, which will completely cover our next day’s work and leave us a remnant to cut away. Our plasterer fits in the new mortar up to the charcoal mark the next morning, and so we proceed bit by bit as if we were putting together a puzzle, until the whole is completed.
It is hardly necessary to say that it is very desirable that each cutting should correspond with some natural division of the work. Thus, in painting a female head, we might paint the hair and diadem the first day, and go on with the face and neck the next, stopping at the necklace. In real fresco nothing can be retouched. Every day’s work must be finished and complete in the minutest detail.
I will now say something about the colors and execution of fresco.
In fresco (as in distemper) the colors in drying become of a much lighter shade. It is, therefore, very desirable to have a piece of some very absorbent material at hand to try the tints on. There are two distinct modes of painting fresco. One is the solid body-color method, as practised by M. Angelo, Raffaelle, and all the other masters of that period. The other is the thin water-color method.
If we adopt the first mode, we get a porcelain or metal palette, and set the colors on it just as we do for oil-painting. Lime takes the place of white lead. The only yellow it is safe to use, at least in England, is raw sienna; probably, however, Mars yellow, which is derived from iron, might be used with safety. Light red of various kinds and burnt sienna are the principal reds. Oxide of chromium is the green. Raw and burnt umber are quite safe, as is also black. Blue is a very difficult color to manage in fresco.
It seems very antagonistic to lime, and it is almost impossible to paint a blue sky properly graduated. On the other hand, raw umber takes very kindly to fresco. Lakes and all vegetable colors are to be strictly avoided.