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In fresco-painting - both buon' fresco and fresco secco - the ground must not only be wet, but caustic. In true fresco the pigments are applied to the last and freshly-spread coat of plaster before it has had time to absorb more than a trace of carbonic acid from the air; the painting-ground is in fact saturated with an aqueous solution of hydrate of lime, while there remains a large reserve of this compound in an undissolved condition. When on such a surface a layer of pigment mixed with water is placed, as that water evaporates the lime-water in the ground diffuses into the paint, soaks it through and through, and gradually takes up carbolic acid from the air, thus producing carbonate of lime, which acts as the binding material in this method. As there still exists an ample reserve of hydrate of lime in the ground, wetting the painted surface with pure water will cause more of this hydrate to enter into solution, and so the liquid present in the plaster will be reinforced with a fresh supply of the binding material. Ultimately the ground and the pigment become incorporated and harden together.

If more binding material be required, it may be introduced by means of lime-water itself, or even by baryta-water, which contains about twenty times as much hydrate of baryta as the strongest lime-water contains of hydrate of lime; these liquids or hydrate of lime may also be mixed with the pigments used. Although the chief binding material in fresco-painting is this carbonate of lime, yet with some plasters and with some pigments another substance is produced. This is silicate of lime, produced by the action of caustic-lime in solution upon the soluble silica of the plaster or of the pigments. Some sands, infusorial earths, and ochreous pigments, contain such soluble silica, but it is certainly not present in every case. Silicate of lime as a binding material is more permanent than the carbonate.

In fresco secco the plaster is allowed to harden, and, in some measure, to dry, and the operation of painting may be continued at leisure. The ground immediately before beginning work is moistened with lime- or baryta-water, and the pigments are mixed with one or other of these liquids, or with a little slaked lime. This modified process is far easier of execution than true fresco; but the fixation of the pigments, though resulting from the same cause, is less complete.

In the treatises of Cennini and other later writers the expression 'painting in secco' is generally employed to designate any process of tempera-painting, but the fresco secco described in the preceding paragraph was practised before and during the thirteenth century as the precursor of buon' fresco, and is briefly mentioned in Theophilus ('Schedula', book 1, chapter 15 (red pigments)).

The protection afforded to the pigments by the binding material in fresco-painting is not generally very efficient. In the case of a dry wall, free from soluble saline matter, and exposed to a pure atmosphere, it may remain good for centuries. But in air contaminated with the products of the combustion of coal and gas, and with tarry and sooty impurities, a fresco picture soon perishes. The binding carbonate of lime is converted into the sulphate, breaking up the paint, and becoming itself disintegrated in the process of change. Through the same cause, and through the production of sulphate of magnesia from the carbonate of magnesia in the plaster, even the layer of paint itself may scale off, while the lodgment of dirt and soot upon the surface obscures such colours as still remain in their place. And fresco-paintings often show scalingoff, by reason of the interposition of a film of carbonate of lime between the coats of paint - a film formed during the completion of the picture.

True fresco did not come into use in mediŠval times until the close of the fourteenth century. About the year 1390, Pietro d'Orvieto painted some subjects from Genesis in the Campo Santo at Pisa. In 1503, Pinturicchio, at Siena, began some works in fresco, which he finished in tempera with lakes and other pigments injured by lime. This mixed method was much used in Italy to a late period, as it enabled a greater richness of effect to be attained. For the palette of the painter in true fresco is severely restricted in certain directions, very few colours of organic origin withstanding the decomposing action of lime. It is a good plan to test each pigment intended to be employed in this method: The pure pigment is thinly painted over a slab of plaster-of-Paris, and then half of it is to be moistened with lime- or baryta-water. No change of hue, only a lightening of the tone, should be observed, after drying, in the treated portion. Prussian blue may be named amongst the pigments most quickly and seriously altered by lime; it becomes a mere stain of rust.

Although it might have been expected that the earthy pigments, terre verte, yellow ochre, and raw sienna, would prove peculiarly suitable for use in fresco-painting, the examination of works executed in this method, during the last half-century in England, does not confirm this expectation. Indeed, it is found that the most friable portions of such frescoes are precisely those in which these pigments have been freely employed. This remark applies particularly to terre verte, which is found to have become swollen and easily detachable.

As lime in the caustic state acts strongly upon wood, it is necessary to employ palettes of zinc or glazed earthenware; bone or ivory palette-knives are preferable to those of steel.

Asiatic Fresco

The remarkably successful explorations of Sir Aurel Stein among the buried sites of Chinese Turkestan have brought to light numerous examples of a peculiar variety of fresco-painting. In the brief 'Guide to the Stein Exhibition of 1914,' in the British Museum, the method is vaguely described thus: 'A preparation of lime is spread over a foundation of mud and chopped straw, and the pigments applied to the surface while it is wet.' In reality the process adopted in these works, dating from the third to the tenth century of our era, may be more exactly described in the following words: On a backing of the ordinary local loess mixed with the chopped stems and leaves of the common reed, there was spread a thin flat coating of impure burnt gypsum made into a cream with water. Pigments such as an iron red, malachite, a charcoal grey and an ochre, sometimes mixed with the cream of burnt gypsum, were then painted on while the surface was still moist. On drying the colours became fixed, not by carbonation, as in true fresco work, but simply by loss of the solvent water present and the crystallization of its content of gypsum.

As I made numerous analyses for Sir Aurel Stein of painted plaster, from sites at Kadalik, Miran, and Mingoi, I can speak with confidence of the essential distinction between Asiatic and European fresco: the former is essentially a plaster-of-Paris method.

Last Update: 2011-01-23