Plate 2.—Fragment of Ancient Fresco from Tiryns
The wall paintings of the Egyptian tombs were executed in tempera on a gesso or stucco white ground, the same method being followed in the decoration of the mummy cases and other objects. In some instances these tempera paintings of the Egyptians were varnished, which was not an advantage to their appearance, as the varnish darkened, and in a great measure destroyed, the beauty of the original colours.
Some of the wall paintings found at Pompeii are said to have been executed in veritable fresco, since lime has been found in mixture with most of the colours used. On the other hand, this has been disputed, and some authorities classify them as tempera or secco paintings; but perhaps the truth of the matter is, that a certain amount of the first colouring was really executed on the wet lime plaster, and that, in some instances, certain colours, used in the finishing of the work, were applied afterwards in a tempera medium when the wall surface had become quite dry. This method of procedure, according to the statements of Vasari and Cennini, was not an uncommon practice with the Italian frescanti of the fourteenth and fifteenth centuries.
CHAPTER III
FRESCO-BUONO PROCESS—COLOURS—PREPARATION OF THE WALL—METHOD OF EXECUTION
Fresco-buono, or true fresco, is so called because the painting is executed “a fresco,” that is, directly on the fresh, or wet plaster, which forms the painting surface of the wall. This plaster, or mortar, is a mixture of lime and sand, and the colours used in the painting are such as will remain unchanged in hue when in contact or in mixture with the lime. When all necessary details relating to the method of procedure connected with the work are carefully carried out, true fresco paintings may be said to have a higher degree of permanence and durability than those executed by any other method or medium. The fine surface texture and luminous quality of buon fresco pre-eminently distinguishes it as the most beautiful colour finish for mural decoration.
The great permanence of fresco paintings is due to the formation of carbonates, and sometimes silicates, of lime on the surface of the plaster, which takes place during the drying of the latter. The carbonic acid contained in the natural limestone is driven out of it by the process of burning in the kiln, and after the burnt lime has been slaked into lime “putty,” it has then become what is known as a hydrate of lime. As long as the plaster is wet on the wall the lime in the plaster exists in the state of a hydrate, and the applied colours of the painting become saturated with this form of lime; but during the process of drying, the lime on the surface, and to a slight depth below the surface of the plaster, rapidly absorbs carbonic acid gas from the atmosphere, and becomes a carbonate of lime, which is formed as a hard and crystalline skin, or surface covering, under which the colours are locked up, and so protected from any atmospheric influences. In addition to this carbonate surface a silicate of lime is sometimes formed, especially when in the painting some of the more earthy colours are used which may have silica in their composition. These thin coverings of carbonates and silicates of lime render fresco paintings impervious to wet or damp on the surface, so that they may be occasionally washed without injury; but bad air, such as sulphuretted hydrogen gases, or the sulphur products given off by gas and coal combustion, will in time convert the carbonate covering into a sulphate of lime, a substance which disintegrates and destroys, not only the colours, but the plaster surface also. This would suggest that in buildings which contain fresco paintings coal fires and gas should not be used for heating and lighting purposes, and also that the buildings should be properly ventilated.