LIME and WATERGLASS are inorganic binders. They are never used directly together with pigments; at most they are added, in a very dilute form, to the water in which the colors are ground. This is, however, not sufficient for fixing the pigments onto the ground lime in the form of thick lime water can be used in a limited manner as a binder mixed with the pigment, but only for light colors much dimmed by white, and even they can be used only in a single layer, which is rarely sufficient in painting. If more than one coat is put on, the paint flakes off either as a whole or in separate layers.
True fresco, fresco buono, is very different. The pigment is ground up with water or lime water, or is bought ready mixed in bottles. It is painted onto the fresh plaster and the hardening lime covers it with a thin transparent skin.
We have already dealt thoroughly with lime plaster in the section on sgraffito. The use of pure lime (without cement) which alone gives fresco plaster its binding power is the reason why today paintings on facades in fresco buono technique can no longer be durable; for lime in every form reacts strongly to all acids, however weak they be.
The author experienced this first as a young boy: he left a half lemon on the marble rim of his washstand and found a few hours later that the pretty star pattern of the fruit section had eaten into the "eternal" stone. His mother was less impressed with the charm of the design, since it was quite indelible. lime plaster is equally sensitive even to the sulphurous acids in the air from the smoke of coal fires.
In a few years they can destroy the crystalline lime layer over the fresco and bring about the deterioration of the paint as well. This is the situation in northern latitudes-at any rate since the introduction of coal firing. The greater moisture in the air is an added enemy, making even the carbonic acids in the air destructive. The only hope of preserving fresco in northern climes is to use it indoors.
The only really weatherproof binder is waterglass, which has only recently been used by artists. It was first tried as a binder for painting in 1842 in Germany.
It was at that time used mainly as a protective coating against fire for stage scenery, the stage then being illuminated by open lights. In 1880 A. W. Keim began to study and improve the technique of painting with waterglass. He called the technique "mineral painting"; it is nowadays sometimes called silica painting.
There are three kinds of waterglass: potassium, sodium, and double watergloss, which is made of a combination of the other two. The formulae are simple: K2SiOa, and Na2SiOa. Potassium waterglass is the only one suitable for painting purposes. The pigments must contain a certain percentage of zinc oxide (zinc white) and magnesia in order to petrify with dry plaster or natural stone through the agency of the waterglass. They are painted on the ground, as in fresco painting, with only water or with very dilute waterglass.
The waterglass is sprayed on afterwards like a fixative. It then combines with the carbonic acid in the air to make potassium carbonate and silicic oxide. The potassium carbonate is water soluble and is removed by washing or simply by the rain. If sodium waterglass were used, sodium carbonate (soda) would be produced, which does not wash off easily and would veil the painting.