From processes in which the painting is normally composed of two layers, paint and support, turn to those in which there are three layers, due to the use of a ground laid on the support. By far the most important and numerous of these are employed in mural painting, though, as noted above, examples occur when a ground is introduced in water-colour painting, or, as will be seen later, when one layer is omitted in what is ordinarily a four-layer process.
It is true that there are examples in north European medieval work of paint being applied direct to a wall, a notable case being the paintings in Eton College Chapel. Ordinarily, however, the surface of a wall is too rough, and its texture unsuitable, to receive the paint; and almost invariably a coat of plaster is laid on the wall as a ground. It should be noted, however, that certainly in early practice, this coat of plaster was considered preparation of the wall to receive the paint, and not as a means of concealing major undulations and inequalities in the wall, and so producing an absolutely flat surface. Indeed, skilful use might be made of major undulations in the wall surface, on which the light would fall at slightly differing angles, to produce variations in colour and especially in the appearance of gilding. An example is the famous Adoration of the Magi by Benozzo Gozzoli in the edici Palace, Florence.
Once the plaster is on the wall, a wide range of mediums is open to the painter. He may use a wax medium, oil in various mixtures, size, casein, or half a dozen other materials; but up to the end of the seventeenth century and especially in Italy, if he wanted to complete his work merely by adding a paint layer, he would almost invariably use some form of fresco.
Fresco painting is of two types, fresco secco (dry fresco) and buon fresco (good fresco). The plaster used in both cases is lime plaster, made with slaked lime (calcium hydrate) and sand mixed with water. Lime by itself is liable to shrink and crack, hence the presence of the sand, round the particles of which the lime crystallizes. The setting of lime plaster has two stages; and to understand these is also to understand the basic character of fresco.
In the first the bulk of the water in the plaster dries out; in the second carbon dioxide from the atmosphere acts upon the calcium hydrate to produce calcium carbonate, which is practically chalk or limestone, and leads to a further evaporation of water. This process of forming calcium carbonate is very slow, and is helped by Feriodically wetting the plaster.
In fresco secco, the surface of the plaster was allowed to dry. The pigments were mixed with slaked lime and water, the surface of the wall plaster clamped, and work proceeded. The damping of the plaster allowed some penetration by the paint layer, and made it fast to the plaster; the lime mixed with the pigment was converted into chalk, and held the pigments enclosed.
As a process, this is akin to water-colour. The pigments are mixed with a medium, with water as a diluent, and applied to the ground. From the number of surviving examples it is clear that this was the prevailing method of wall painting in the Middle Ages in northern Europe and in Spain. The exact procedure is, however, unknown, though its essentials are given by Theophilus in the twelfth-century manuscript already mentioned, when he says, ‘When figures or birds or representations of other objects, are drawn on a dry wall, the wall must be immediately sprinkled with water until it is quite wet. And all the colours which are to be put on, must be mixed with lime, and laid on at one wetting, in order that they may dry along with the wall, and may adhere to it.’
Concerning the process of buon fresco, unlike that of fresco secco, a great deal of information has come down to us, since a long succession of Italian painters have described it in detail. The use of buon fresco is indeed almost limited to Italy; and in that process some of the greatest masterpieces of the world from the fourteenth century onwards were executed.
In the nineteenth century a determined effort was made to bring buonfresco into use in northern Europe. At the time of the so-called Gothic revival and of the newly aroused enthusiasm for early Italian painting, there was much research into methods and materials, and a number of reasonably successful applications of its results made. Such were the paintings by William Dyce in the Houses of Parliament in London, and of Peter von Cornelius in the Glyptothek at Munich, and elsewhere. But the tradition of using the process had been lost.
It is one thing to learn a method from books, another in a workshop; and in practice, buon fresco proved difficult to handle and uncertain in results, so that even such great painters as Delacroix and Puvis de Chavannes in their mural work adopted the method of painting in oil (or some adaptation thereof) sometimes direct on the wall, but often on canvas which was later attached to the wall, a process sometimes known as marouflaging.
To return to buon fresco. The essential element of this is that the painting is done while the plaster is still wet. The pigments are mixed with water only, and on being applied are carried with the water into the plaster and locked up in it as it hardens into carbonate of lime. In the great days of fresco much attention was paid to the plastering of the wall.
At least two coats were applied. The business of the first was to adhere firmly to the wall, and its surface was roughened so that the second coat, on which the painting was to be carried out, should also hold fast. This is the system recommended by Cennino Cennini , in his Libro dell’ Arte, one of the most famous handbooks of the Middle Ages on painting and related arts. It was written some time between 1396 and 1437. Cennini was a pupil of Agnolo Gaddi, son of Taddeo Gaddi, the pupil of Giotto, and he takes pride in recording the workshop tradition based on the practice of that great master, which it is tolerably certain ruled for many years in Italy. 1
When the first coat of plaster is dry, Cennini tells the painter to draw his design thereon in charcoal, this design apparently being based on a small drawing. It is interesting to note that various mechanical aids, such as plumb-lines and compasses, are recommended as a means of placing figures, &c. The charcoal is then to be gone over with ochre and red, with a small brush, some shading being introduced.
Then comes the stage which has puzzled some later students. Over the design, another coat of plaster often called the intonaco, is to be applied, this being fairly thin, and to be made quite smooth; and to this, the pigments mixed only with water are to be applied. Evidently, the second coat of plaster, while wet, was sufficiently thin for the underlying design to be seen. Any doubts as to Cennini’s method being as described, were dispelled as a result of the damage done in the Campo Santo at Pisa during the Second World War. There, as the results of explosions and subsequent rain, the final coat of plaster on certain frescoes, which carried the painting, was removed, to reveal the drawing on the underlying coat, and some of the lines used to place the figure.
The importance of the plaster on which the painting is made being wet is emphasized by Cennini’s instruction to ‘consider in your own mind how much work you can do in a day; for whatever you plaster you ought to finish up’. Cennini does not mention it, but it was the practice to undercut the edges of the section of plaster used for each bout of painting, so that the succeeding sections, when they came to be applied, could be neatly and firmly fitted to the previous ones.
The joints, however, are always clearly visible in buon fresco, and afford one of the easiest means of distinguishing it from fresco secco, in which the jointing was unnecessary. The major difference between the two types, however, lies in their degree of permanence, and sometimes in quality of colour. In fresco secco, the pigments are held only in a thin film of chalk, which when applied in the form of lime water, never penetrates far into the plaster of the wall, and is therefore liable to scale and flake; while in buon fresco, the waterborne pigment goes deeper into the wet layer of plaster, which can be more firmly bound to the underlying layer. Also, in fresco secco, any excess of lime water is liable to cloud and muddy the colours, while in buonfresco they remain clear.
In both fresco secco and buon fresco, it was usual to complete and touch up. the work in secco, that is, after the painting was dry. A medium sometimes used was size; but Cennini recommends two forms of egg tempera (for this, see the chapter on Tempera). This not only enabled detail and subtleties to be introduced unattainable in fresco, but since the size or egg allowed pigments to be used which could not be used in fresco (as explained below), richer and more varied colour was possible.
Sometimes, too, gold would be applied to certain parts of the fresco, as in the haloes and the capitals of the columns in the fresco by Melozzo da Forli (fifteenth century) now in the Vatican. In time, however, some of these additions have changed colour or darkened more than the fresco itself.
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