Page:The chemistry of paints and painting.djvu/58

24 Steveochromy.—The ground for stereochromy has been modified several times since the first introduction of this method of water-glass painting. Originally it was recommended to use an undercoat containing 2 parts of sharp sand, 2 parts of fine sand, and 1 part of slaked lime in fine powder. Upon this was laid an intonaco of one-tenth to one-eighth of an inch in thickness, made of 3 parts of fine sand and i part of slaked lime. The fineness or coarseness of the sand in the intonaco must, however, be regulated by the nature of the surface required by the artist. I see that nearly fifty years ago I recommended the employment of sifted white marble powder, and of several other substitutes for sand, recommendations which, within the last few years, have been again brought forward by Herr Adolph Keim. Oxide of zinc may be advantageously substituted for a part of the lime in the intonaco, and it may be added to the pigments. Keim recommends the wall to be first coated with a mixture of 1 part of burnt lime (which is to be slaked with distilled water), and 4 parts of a composition consisting of coarse quartz sand, infusorial earth, and powdered marble. The actual painting-ground, which is from one-eighth to a quarter of an inch in thickness, is made of 1 part of slaked lime, and 8 parts of a mixture of the finest quartz sand, marble sand, marble meal, and infusorial earth. Fine asbestos paper, wetted with lime-water, and firmly pressed by rolling into a soft freshly-laid lime and sand-plaster, makes an excellent ground for stereochrome painting; but as a single breadth only of this paper can be used, the size of the work that can be executed upon it is somewhat limited. All the precautions as to