Page:The chemistry of paints and painting.djvu/55

Rh Btion' Fresco.—A good mixture for the first application to the moistened wall consists of 2 parts (by weight) of clean sharp sand to one of lime-putty. When one or more coats of this mixture have been duly laid and have set, then the surface is ready to receive the final coat or intonaco, the actual painting-ground. Before this is applied, the rougher plaster below must be thoroughly wetted with distilled or lime water. The sand in the intonaco is of finer and more uniform grain than that previously employed; the intonaco itself is only one eighth of an inch in thickness. All the coats must be laid without having recourse to scraping or 'floating'; the latter operation brings too much lime up to the surface. Considerable practice and manual dexterity are needed in these operations. The work of painting is at once commenced when the intonaco has been laid, no more being spread at one time than the artist can cover in the day. Upon the wet soft plaster the cartoon is laid, and the outlines and other important parts pounced in, transferred, or impressed by an ivory point. Rapidity and firmness of execution, with the distribution of a uniform thickness of pigment, are matters to which special attention must be paid. The chemistry of this method of painting will be discussed in Chapter XXIII.

Many modifications in the preparation, proportions, and materials of fresco painting-grounds have been introduced or suggested from time to time. I have found the following mixture to yield an excellent plaster for this purpose: Three parts of burnt lime in very fine powder are ground up with 2 parts of whitening or prepared chalk; the mixture is grouted, and then strained through hair-sieves; 5 parts of the putty thus obtained are mixed with 5 parts of sifted crushed marble, or with 5 parts of sharp, fine, sifted sand, or with 3 parts of sifted pumice, or with the