Cennini {Trattalo della piltura, chs. 67 seq., ed. Milanesi, 1859;
Eng. trans, by Christiana J. Herringham, Lond., 1899); Leon
Battista Alberti (De re aedificaloria, bk. vi. ch. g; early and middle
15th century); Vasari (Operc, ed. Milanesi, i. 181; middle of
i6th century) — all refer in general terms to the fresco process, as
one generally understood in their times. Armenini {Dei veri
precetli della pitlura; Ravenna, 1587), and Palomino (El Mitsco
piclorico; Madrid, 1715-1724), give more detailed accounts of
the actual technical procedure, of which they had preserved
the tradition. Much information of the highest value and
interest was collected at the time when, in the forties of the
igth century, the project for the decoration in fresco of the new
EngUsh Houses of Parhament was under discussion. This is
contained in various communications by Sir Charles Eastlake,
Mr Charles Heath Wilson, and others, printed with the successive
Reports of the Commissioners on the Fine Arts from
1842 onwards. The experience obtained in the revived modern
work in fresco by Cornehus, Hess, and other German artists
encouraged by King Ludwig I. of Bavaria, which began at Rome
in the second decade of the iQth century, was also drawn upon
for the purpose of these Reports. A useful compendium was
issued at the time by W. B. Sarsfield Taylor, A Manual of Fresco
and Encaustic Painting (Lond., 1843). F. G. Cremei sV all stand igc
Anleitung zur Fresco-Malerei (Düsseldorf, 1891), may also be
mentioned as a recent manual. The chemistry of the process
is well explained by Professor Church in his Chemistry of Paints
and Paintings.
The fresco process is generally regarded as a method for the production of a picture. It is better to look upon it in the first place as a colour-finish to plaster-work. What it produces is a coloured surface of a certain quality of texture and a high degree of permanence, and it is a secondary matter that this coloured surface may be so diversified as to result in a pattern or a picture.
We do not know among what people the discovery was first made that a wash of Uquid pigment over a freshly laid surface of lime plaster remained permanently incorporated with it when all was dry, and added to it great beauty of colour and texture. The Egyptians, the Assyrians, the Phoenicians, the Mycenaean and later Greeks, the ancient Italians — all made extensive use of plaster as a coating to brickwork or masonry, but when they coloured it this was done after it was dry and with the use of some binding material or tempera.
The earhest notice of the fresco technique that we have in extant literature is contained in the third chapter of the seventh book of Vitruvius, and it is there treated as a familiar, well understood procedure, the last stage in the construction and finish of a wall. Pliny also in several passages of his Natural History treats the technique as a matter of common knowledge. In Vitruvius the processes of plastering albaria opera are first described (vii. 2, 3), and it is provided that after the rough cast, truUissatio, there are to follow three coats of plaster made of lime and sand, each one laid on when the one below is beginning to dry, and then three of plaster in which the place of the sand is taken by marble dust, at first coarse, then finer, and in the uppermost coat of all in finest powder. It might now be (i) finished with a plain face, but one brought up to such an exquisite surface that it would shine like a mirror (chs. 3, 9); or (2) with stamped ornaments in relief or figure designs modelled up by hand; or (3) it might be completed with a coat of colour, and this would be applied by the fresco process, for which Pliny uses the formula udo illinere, " to paint upon the wet." The reason why the pigments mixed with water only, without any gum or binding material, adhere when dry to the plaster is a chemical one. It was first clearly formulated by Otto Donner von Richter in conne.xion with researches he made on the Pompeian wall-paintings and published in 1868 as an appendix to Helbig's Campanische Wandgemdlde. He demonstrated that when limestone is burnt into hme all the carbonic acid is driven out of it. When this hme is " slaked " by being drenched with water it drinks this in greedily and the resultant paste becomes saturated with an aqueous solution of hydrate of lime. When
this paste is mixed with sand or marble dust and laid on to the wall in the form of plaster this hydrate of lime in solution rises to the surface, and when the wet pigment is applied to this the liquid hydrate of lime or hme water, to use Professor Church's phrasing, " diffuses into the paint, soaks it through and through, and gradually takes up carbonic acid from the air, thus producing carbonate of hme, which acts as the binding material " (Church, p. 278). It is a mistake to speak of the pigment " sinking into the wet plaster." It remains as a fact upon the surface, but it is fixed there in a sort of crystalline skin of carbonate of limethe element originally banished when the lime was burned that has now re-formed on the surface of the plaster. This crystalline skin gives a certain metallic lustre to the surface of a fresco painting, and is sufficient to protect the colours from the action of external moisture, though on the other hand there are many causes chemical and physical that may contribute to their decay. If, however, proper care has been taken throughout, and conditions remain favourable, the fresco painting is quite permanent, and as Vitruvius says (vii. 3, 7), " the colours, when they have been carefully laid upon the wet plaster, do not lose their lustre but remain as they are in perpetuity ... so that a plaster surface that has been properly finished does not become rough through time, nor can the colours be rubbed off, that is unless they have been carelessly applied or on a surface that has lost its moisture."
In the passage from which these words are taken Vitruvius gives useful hints as to the aesthetics of the fresco technique. Italian writers on the subject, such as Vasari, are generally so taken up with the pictorial design represented on the wall that the more essential characteristics of the process in itself are lost sight of. To Vitruvius the work is coloured plaster, not a picture on plaster, and he shows how important it is that the plaster should be finished with a fine surface of gleaning white so as to light up the transparent film of colour that clothes it. It is the result of such care in classical times that a surface of Pompeian plastering, self-tinted " a fresco, " is beautiful without there being any question of pattern or design.
This beauty and polish of Pompeian, and generally of ancient Roman plaster, has recently been made the ground for calling in question the view accepted for a generation past that it was merely hme plaster painted on " a fresco, " and for substituting a totally different technical hypothesis. The reference is to the treatment of ancient wall-painting generally in the first part of Berger's Beitrage (2nd ed., 1904, pp. 58 seq.). This writer denies that the well-known classical wall-paintings in question are frescoes, and evolves with great ingenuity a wholly new theory of this branch of ancient technique. It is his view that the plaster was prepared by a special process in which wax largely figured and which corresponds to, and indeed survives in, the so-called " stucco-lustro " of the modern Itahans.
The process in question is described by L. B. Alberti (Dc re acdificatoria, vi. 9), who says that when the plaster wall surface has been carefully smoothed it must be anointed with a mixture of wax, resin and oil, which is to be driven in by heat, and then polished till the surface shines Hke a mirror. This is a classical process referred to by Vitruvius under the name " ganosis, " as applied to the nude parts of marble statues, possibly to tone down the cold whiteness of the material. Now Vitruvius, and Pliny, who probably follows him, do as a fact prescribe this same process for use on plaster, but only in the one special case of a wall painted " a fresco " with vermilion, which was not supposed to resist the action of the light unless " locked up." in this way with a coating of this " Punic " or saponified wax. Neither writer gives any hint that the process was appUed to plaster surfaces generally, or that the lustre of these was dependent on a wax polish, and Vitruvius's description is so clear that if wax had been in use he would certainly have said so.
Vitruvius prescribes so many successive coats of plaster, each one put on before the last was dry, and on the wet uppermost coat the colouring is laid. How can we with any reason substitute for this a method in which the plaster has to be made quite dry and then treated with quite a different material and
