The general effect depends on a good texture for the top layer of plaster. The sgraffito coats are better made of a fine plaster with little texture effect. The coat must be applied according to the size of granulation and
should be about double the thickness of the largest grain. Coarse granulation is thus possible only if there is to be no more than a single sgraffito layer. If the top layer of plaster is coarse, it is difficult to make subtle outlines; they will crumble, which looks unpleasant unless the design is done in very heavy drawing.
This consideration must be weighed against our desire for a coarse texture to stress the picture plane. It is particularly important in sgraffito to keep the design flat, as it must form part of the whole building. The sense of depth must embrace the whole architectural unit, not just the surfaces surrounding the design, and effects of perspective
are totally out of place. At most, perspective should be confined to a simple arrangement of individual elements one behind the other as in a stage set, for which Egyptian wall paintings give the classical example.
There are three types of finish: one is tubbed flat with a float or felt, one is scraped, and one is thrown on. The strongest and most naturally varied texture is obtained with a float finish, mixing largish round pebbles into a relatively fine plaster. The pebbles make grooves and ridges and then fall
off. It can be clearly seen how the float moved, in circles, up and down or horizontally.
The texture of scraped plaster is dependent entirely on the size of granulation. When the plaster is applied it is smoothed flat and then, when set hard, nearly scraped with a metal edge or a nailed board that
works like a coarse brush. The result is uniform, without any direction in the texture, and is always darker in tone than a smoothed surface. The contrast is strong enough to be able to make a design recognizable if carried out in scraped against floated plaster.
If a finer plaster is
smoothed with the trowel alone, not with a float, it falls into ridges and waves. This is certainly the most craftsman like of treatments. It has a mellow effect, especially on rural buildings when it is whitewashed every few years. After a time patches of earlier whitewash peel off leaving a natural looking relief patina. A two-color sgraffito looks very well in this technique, as long as the
design does not get splashed with whitewash.
Thrown-on plaster, rough cast or wet dash, is the lowest grade of plastering and shows up - in contrast to a fine sgraffito layer - almost palpably for what it is, a thin skin covering the building. It is a comparatively coarse but watery plaster thrown on
with a trowel, or more often squirted. It remains very rough and is rather fragile. While it is new it looks quite well, but it quickly accumulates dust.
Many cheap products have been developed in recent years to give a very smooth plaster finish. Any texture that is put on them looks quite artificial
and arbitrary sgraffito on these surfaces is reminiscent of an antimacassar on a plush chair.