Preparation of the Wall in Buon-Fresco:
The wall on which a work in buon-fresco is to be painted should be of good brick, or if a stone wall, it should be lined with brick on its inner face, or, better still, it should be what is known as a hollow brick wall—that is, one having an air-space between the outer and inner linings. The joints between the bricks should be scored out, so as to leave a key for the first coating of rough plaster; this should consist of old lime and coarse, gritty, well-washed river sand, mixed with ox-hair, or white asbestos cut into small bits, so as to bind the mixture more effectually. For this first coating of plaster the usual proportions are one part of lime and two parts of the gritty sand; it should be about three-quarters of an inch in thickness. This coating should have a roughened surface, made by scratching it with a coarse-toothed kind of wooden comb, and should be left for the best part of a year, so that it may thoroughly harden, before it receives the second, and last, coat of plaster, that on which the painting is to be executed, which is known as the intonaco. This plaster mixture must be prepared with great care, and more than sufficient to cover the whole wall space should be made before beginning the painting, so as to get the whole surface evenly tempered, and of the same mixture; for if different mixtures were made, and at different times, the lime and sand might vary in proportions, which would possibly affect the colours of the painting, and prevent them drying uniformly, as to tint or tone.
The proportions of lime and sand for the intonaco is, one part of lime and three parts of fine and well-washed river sand. Pit sand must not be used, on account of the clay and earthy matter which it contains; nor sea sand, which of course contains a certain amount of salt that would attract damp and cause the lime to perish. The lime must be well slaked, and must be fairly old in the “putty” state, so that there may be no fear of its blistering and blowing off here and there in round flakes on the finished surface, which it will do if it is too new or not properly slaked, even six or eight months after the plaster has been spread on the wall.
The method of preparing lime for fresco work is, first to select the best white variety which has been properly calcined. The lime is put into a large wooden trough with sufficient clean water to slake and dissolve it into a thin creamy consistency. The mixture is then strained through a fine sieve into a brick-lined pit, roofed over to keep out the wet and dust. A thick coating of clean river sand should be put over the lime when it has cooled down and thickened into a paste, and has become what is known as “lime putty.”
There are many tales and legends concerning the extreme old age of lime putty before it has been used in the plaster of wall surfaces, or as a painting material in fresco.
It has been stated that for some of the Italian frescos the lime used was eighty years old. As a matter of fact it is quite ripe for use about one year after it has been slaked. The lime used by Sir Edward J. Poynter in his fresco in St. Stephen’s Church at Dulwich was about two years old, and that used in the Houses of Parliament frescos was three years. The quality of the lime, however, is of more consequence than its age. The common grey lime used by the London builders should be avoided; the best English variety is the pure white limestone, which is quarried and burnt near Buxton in Derbyshire.
Before describing the method of laying on the last plaster ground, or intonaco, a few words must be said about the preparation of the design or subject to be painted. A finished coloured drawing or painting of the design, to a smaller, or even to the same scale as the fresco painting, must be prepared, and the artist should endeavour to make an exact copy of this on the plaster surface, as there is no time to experiment in colour schemes, or to make alterations from the original coloured design in the short period that is at the disposal of the painter when he is at work on the wet plaster. If the colour sketch of the work is prepared on a small scale it will be also necessary that a cartoon in light and shade drawing should be made to the full scale of the fresco painting, before beginning to paint on the wall.
As the colours, and everything else that may be required, must be in readiness, and close at hand before laying on the plaster ground and before commencing the painting, it will be as well to indicate here the proper colours which may be safely used on the lime plaster.
Although the palette in buon-fresco painting is very simple and restricted as to the number of the colours which may be trusted to withstand the caustic action of the lime, at the same time a fairly rich and luminous colour scheme may be obtained, notwithstanding the limited range of the palette. The following list may be safely relied upon:—