The reader will be able later to judge to what extent it is worth his aiming at a professional standard in the mastery of this technique and how artistically rewarding it can be. To form an idea of the technique, the student must at least have a theoretical understanding of the composition of wall renderings and the use of mortar or plaster as a wall covering. Its manufacture and application is, of course, the work of an experienced builder, but some acquaintance with the builder's work when he prepares the wall for an artist will give the beginner confidence, and make him feel less of an amateur.
Every mortar is a combination of three materials: sand, water, and a binding element. Lime, cement, or a mixture of both, serves as the binder, the choice depending entirely on the function of the rendering, which also determines the choice of the sand. Whatever its granulation, sand must be sharp and free of
earth. Plaster, a harder preparation for the top coat and containing more lime, is also a general term used for any wall coating.
The lime is obtained by heating broken-up limestone, such as marble, until it is red hot, and thus allowing the carbonic acid and water of crystallization to escape. When the burnt limestone is then brought into contact with water it generates great heat and forms a creamy paste which hardens immediately upon absorbing sufficient carbonic acid from the air. It never becomes as hard as the original limestone; however, and needs the addition of sand and grit to make it firm.
Cement is made in much the same way I as lime, but clay, dross from smelting furnaces, and other materials are added in the heating; the product is then finely ground. Cement does not need carbonic acid from the air to harden it; it can set in an airless environment - under water, for instance. It is always harder than lime. If greater firmness is required, or there is likely to be insufficient air, cement is added to lime.mortar, or replaces the lime entirely.
Besides making it harder, the additional material forms the main body of the mortar. The proportions vary between 1:5 and 1 :3, the higher number being the sand content in relation to lime or cement, both measured dry.
"Fat" plaster has a lower sand content. In plastering a wall, the same principle applies as in oil painting: always put fat over thin, never the other way around. Water is added to give the right consistency. It does not require any special measuring, but plaster will not adhere to a wall if it is too wet. However, excess water evaporates. The mixture is too friable to take at all if it is not wet enough.
Even if he leaves all the plastering to the builder, the student must himself execute the actual work of scoring and carving. The plasterer would work in sgraffito with the same precision he has been trained to for cornices and mouldings; his training in the use of level and float would produce the same effect as a portrait drawn with ruler and compass. Even if at first the strange position close to the wall on a scaffold makes the student lose his sense of the vertical, and he has to use plummet and spirit level, he should put them aside after the first sketching in of the design. The cutting tools must ibe used freehand, except, perhaps, for work on large lettering.
A good outdoor wall plaster is composed basically of three layers. The first coat seals up any joints, cracks or seams in the wall and makes a rough but even surface, to which the second, finer coat can be keyed. The top coat forms a permanent texture and gives the wall the desired color. For sgraffito the colored layer is put in either as the second (middle) coat or as an extra coat above it. As each color requires a separate coat, it is technically impossible to do sgraffito in anything but a limited color scheme. If plaster is applied too thick to a wall it does not hold securely; it falls off of its own weight whenever frost or mechanical causes give it a chance. Polychrome designs are thus better carried out in a modified form of sgraffito, called plaster intarsia.
Sgraffito can be worked only while the plaster is soft enough to be easily cut and scored. If it has hardened so that it can be cut only with a mason's chisel and hammer, it makes lighter patches and the whole work will look faulty. The carving has to be done rapidly; there is no time to reconsider the design. Plaster is rarely in a condition suitable for sgraffito tools for longer than 24 hours, and then only if the under layer was well dampened before the application of the top coat, and the weather is dull or a screen of damp cloths protects the working surface from the sun. Hence, all preparation must be done beforehand so that the artist can concentrate on the technical execution of his design as soon as the top coat of plaster has been applied.