The Chemistry of Paints and Painting is a free textbook on chemical aspects of painting. See the editorial for more information....

Spirit-Fresco

The ground recommended by the late Mr. Gambier Parry for that modified form of varnish-painting to which he gave the name of 'Spirit-Fresco' is identical with that required for true fresco. All the usual precautions as to the dryness of the backing, and its freedom from soluble salts, must be taken. The plaster must be allowed to dry completely before the operation of saturating it with the medium is commenced; the lime in it should also have become mild - that is, carbonated. (See Chapter XXIII. for tests for alkalinity and moisture.) Syringing the plaster with distilled water previously charged under pressure with carbonic acid gas, though it delays the drying, hastens the carbonation of the lime materially. To complete the preparation of the ground, it should, when quite dry, be soaked with a mixture of two parts of the medium (Chapter XII (Varnishes And Vehicles).), and three of turpentine. After two days, this treatment must be repeated. A third application may be needed for very porous grounds. Another period of forty-eight hours having elapsed, the surface receives a coat of white paint, made of equal parts of white lead and gilder's whitening, ground up with the medium diluted with one-fourth or one-third its bulk of turpentine. This priming is repeated when the first coat is dry.

After three weeks, the painting may be commenced. Stone and terra-cotta, if sufficiently porous, may be primed in the same way as plaster. Under no circumstances should cements containing plaster of Paris be introduced into the grounds used for spirit-fresco.

During the last twenty years a considerable number of large mural paintings have been executed either in Gambier Parry's medium or in the paraffin-copal medium. Some of these works have been painted directly on plastered walls, some on canvases which have been afterwards affixed by marouflage to the surfaces prepared to receive them. To the latter category belong nearly all the paintings in the Ambulatory of the Royal Exchange, London. Each of the compartments has been very carefully arranged with a view to secure dryness and freedom from soluble saline matter. In front of the wall itself has been fixed a slate slab slightly inclined forwards at the top and having a ventilated airspace behind it. Upon the slate the finished picture has been attached (or marouflé) by means of a thick paste of white-lead, oil, and copal-varnish, spread not only upon the slate, but simultaneously upon the back of the canvas. It may be affirmed that paintings so secured are free from all risk of injury from the back. In an atmosphere like that of London the surface of the painting must either be protected by glass or be periodically cleansed from deposits of dust, soot, tarry matters, and the other impurities which are described in Chapter XXV (Conservation Of Pictures And Drawings). of this handbook.

Several fresh materials have been recently employed as painting-grounds. They are either patent or secret preparations, dependent in general for their solidification upon reactions between insoluble earthy and alkaline earthy matters, such as china-clay, asbestos, and compounds of lime and magnesia, with solutions of such salts as magnesium chloride, aluminium sulphate, and alum. There is sometimes a lack of tenacity, and always a lack of toughness in these mixtures, but some artists find them to possess precisely the texture and absorptive character they desire in grounds not only for tempera, but also for oilpainting, and they may be spread on canvas as well as on more rigid supports. There is some danger of want of adhesion between the paint and the ground. It is also necessary to make sure that the materials of the ground do not affect sensitive pigments such as ultramarine. The hardening or petrifying liquids which in most cases are used in association with solid preparations to make the grounds in question, are invariably acid to test-paper, unlike the alkaline silicates described in Chapter IX.

Slate may be used as the ground for spirit-fresco and oil-painting; but its freedom from crystals of iron-pyrites, which present a brass-yellow colour, must be first ascertained. The firm adhesion of any priming, or other layers of oil-paint which may be applied subsequently, to slate may be secured in the following manner. The slate is slowly warmed in a water-oven, and thus becomes quite dry. While still warm, it receives a very thin coat of oil-copal varnish, largely diluted with turpentine or with toluol, and applied warm. When this film is hard, the painting may be carried out as in the ordinary way of using oil-colours; a priming of flake-white ground in oil and mixed with a little copal-varnish and turpentine, may be first applied, if desired. Terra-cotta and stone may be treated in the same way, but, being more absorbent than slate, the process recommended on p. 31 is preferable.

Owing to the presence of sulphuric acid in urban air painting-grounds containing calcium carbonate are liable to an injurious change, the carbonate being turned in part into the hydrous sulphate (gypsum) with a considerable increase of bulk. Then, through such expansion, the surface-pigment becomes fissured and even detached. It will be readily understood that grounds consisting chiefly of sulphate of lime are not susceptible of such change. So, where damp can be excluded, they may be used for mural paintings, ground flints or fine sand being admixed with the burnt gypsum employed. The paintings of the buried cities of Chinese Turkestan explored by Sir Aurel Stein were executed on grounds of this kind - grounds, that is, of nearly pure plaster of Paris.


Last Update: 2011-01-23