In the course of the fifteenth century and especially during the sixteenth century, refinements and elaborations of Cennini’s methods were introduced. This appears from various sixteenth-century descriptions of the process, notably that of Giorgio Vasari in the introduction to his Lives of the Painters, by which he is better remembered than by his work as painter and architect.
A minor development was the greater care taken over plastering the wall, one great authority recommending at least three layers, with special attention given to the final coat of intonaco on which the painting was to be made. Much more important was a change in standard methods of transferring the design to the wall.
The older plan still persisted of dividing the wall into geometrical areas, and copying from smaller sketches divided up in the same way, so that the copy could be made section by section; but the usual and recommended way was to prepare from the sketches cartoons, consisting of large sheets of paper on which the design or portions of it were drawn of the same size as they were to be on the wall.
These, too, were made by squaring up (as it is called) and copying the smaller drawing square by square. The design was then transferred to the intonaco or final layer of plaster in one of two ways. Either the cartoon was placed on the newly laid iatonaco and the lines traced with a metal point, which since the plaster was soft, made an indented line; or the design was pounced on to the intonaco by pricking the lines of the cartoon with a needle, placing the cartoon on the wall, and shaking some finely divided dark substance, such as ground-up charcoal, through the holes.
The pounced outlines would then be gone over with a brush and colour. That a cartoon was made is evident from the incised lines on the fresco, which indicate how the design was transferred to the plaster. But the cartoon has disappeared and was probably destroyed, as was often the case, since they were fragile, liable to be damaged in the course of work, and bulky to store.
It is to be noted that the sixteenth century did not, as Cennini recommends, make the design on dry plaster, and cover it with intonaco, but on the intonaco direct. By devices such as these, by attention to the fineness of the intonaco, and by skilful handling of the paint, the painters of the time believed that additions in secco were unnecessary. In the words of Vasari, ‘therefore let all those who wish to paint upon walls, paint in fresco, like men, without retouching in secco; which besides being a most vile practice, shortens the duration of the pictures’.
In well-trained hands fresco can yield results of great breadth and simplicity, combined with luminosity. The very nature of the process calls for quick and decisive handling, and forbids labour over details; while the crystalline character of the medium and its mat surface, help to give an airy lightness to the colours. Moreover, if the painter observes certain rules, these results are reasonably enduring.
These rules, however, set very definite limits to when fresco can be used, and the effects it will yield. First is the difficulty of getting wall, plaster, and colour into a cohesive whole. Defects in plastering may easily lead to scaling and flaking. It is particularly hard in buon fresco to judge the right consistency (or degree of wetness) of the intonaco on which the painting is made.
If this is too dry it is liable not to adhere firmly to the underlying plaster, and the pigment will not penetrate. If it is too wet, however, the brush used for the colour will disturb the plaster, and the colour itself may run. An example of these difficulties are the above-mentioned nineteenth-century frescoes painted by Dyce in the Houses of Parliament.
The first of the series began to flake and peel soon after it was finished; and as there was no workshop tradition to guide them, the artist and his plasterers had to learn as they went on, and full mastery was only achieved towards the end. The later frescoes, however, are good examples of how resistant fresco can be to what might ruin other forms of painting.
The windows of the room they decorate open on the river Thames, and oscillations of temperature and humidity are considerable, while from across the river, smoke-laden air from factories enters. The frescoes became filthy; but when cleaned, the ones painted when the process was fully understood were found to be in fairly good condition. Nevertheless, exposure to air containing sulphur is likely to be harmful by converting the carbonate of lime of the fresco into sulphate, with disintegration as the result.
The major enemy of fresco, however, is damp, less on the surface of the painting than from behind. This not only disintegrates the plaster, but carries salts to the surface of the painting where they crystallize and form blotches, while providing favourable conditions for the formation of mould. So, stringent precautions are needed against damp from outside the wall, and, above all, from below the wall. This is perhaps the chief reason why fresco secco fell into disuse in northern Europe, and why buon fresco was never extensively used. But if damp can be guarded against, and extensive pollution of the air, fresco could as well be used in northern climates as elsewhere.
A third difficulty in the use of fresco comes from the restricted number of pigments that can safely be used. The lime carbonate is not completely effectual in protecting the pigments from the influence of light and atmosphere, and, as in water-colour, pigments such as white lead, verdigris, and some of those derived from vegetable sources, cannot be used.
In the great period of fresco painting the white used was lime putty (made by keeping lime with water for a long time) and later lime carbonate (chalk) made by exposing lime putty to the air. An additional limitation is that the alkali in lime changes, the colour of some pigments, those which are organic being particularly liable to change. The painters of the fifteenth and sixteenth centuries held that only natural as distinct from manufactured or artificial pigments should be used.
One of them, G. B. Armenini, in his De Veri Precetti della Pittura, 1587, says, artificial pigments never do well in fresco, nor can any art make them last long without changing’; and adds later ‘you may leave to foolish painters those secrets of theirs, which no one envies them, of using vermilions and fine lakes; because,… in the long run, their pictures become ugly daubs’. The distinction between natural and artificial in relation to fresco no longer holds good, since there are some comparatively modern manufactured pigments (among them cobalt and green oxide of chromium) which can safely be used; but there still remain many colours on the oil painter’s palette which the fresco painter should not use.
Lastly, the dark, rich, intense quality of colour possible in oil, cannot be secured in fresco. It was to widen the range of colours from dark to light, and the number of tints available, that finishing in secco with tempera took place. But even so, the strong contrasts and vivid colour juxtapositions of the oil painter were not available. Nor was another means of emphasis and variation, that of loading the pigment in certain places to form what is called an impasto.
Sometimes, certain parts of a fresco, such as the haloes of saints, or the jewels on a robe, may be in relief; but this is achieved by building up the relief in plaster or similar material, and not by the thickness of the paint. Thus the fresco painter, like the water-colour painter, works with more restricted technical means than the painter in oil. Just as in the case of water-colour, no safe or effective means of putting a protective layer over the surface of fresco has been found.
Recently, wax has been used for the purpose; but it is apt by penetrating the paint layer to make it more transparent, and so to change its colour, while it robs the painting of its distinctive mat surface. Moreover, the chief enemy of fresco is damp from behind and below, against which no surface layer is a protection.
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