The brushes ought to be hog’s hair tools, but long and soft, so as not to disturb the surface of the wet mortar.
Painting fresco in this opaque, solid method is a very similar process to oil-painting. It is best to begin with the shades and work up to the lights, no scumbling is practicable, but at the end of the day, when the surface is becoming too dry for solid painting, thin washes of color may be used with great advantage.
The Italian terra rossa, burnt sienna, raw sienna, and even vermilion, may be of great service for these light glazings.
It will take three or four days (and often more, if the intonaco is thick and the weather cool) before the colors begin to lose their dark, wet tint.
The beginner must not be discouraged if the colors seem to be drying not as he intended. Some colors take a longer time than others, and it is well to have a little patience. The old masters generally retouched defective parts with what was called fresco secco (dry fresco), but which was simply some compound of white of egg, vinegar, and garlic; but it is much better to cut the defective portions out, to have fresh intonaco laid on, and to repaint them. If once you begin to retouch, the whole work seems to require it, and you never know where to stop.
The second method of painting fresco is totally different. I very much prefer it, as the work is done more rapidly, and the colors hardly change at all in drying. Besides (as far as my experience goes), the result is more durable.
As soon as the fresh intonaco for the day’s work is sufficiently set, you mix some lime with water very fluid, something like milk (good milk, I mean, and not milk and water).
You float this over the intonaco, and in about ten minutes you may give it a second coating of lime-water. This ought to smooth the surface, and remove any little grains of sand.
You now trace your outline as before with the tracing paper and the bag of charcoal. You have no palette, but half-a-dozen small tumblers.
Into one of these you put a small lump of raw umber and about the same quantity of oxide of chromium. You add water, and mix them well together. The result is of course a brownish olive green.[3] You pour half the mixture into another tumbler, and add water, thus getting a weaker solution of the same mixture. You repeat the process into a third tumbler, and get a still weaker tint.