Page:The chemistry of paints and painting.djvu/59

25 dryness of the wall and purity of the materials, already noted in the case of fresco-grounds, must be observed in reference to those intended for stereochrome painting.

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.), 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