Page:The chemistry of paints and painting.djvu/52



PLASTER, GESSO, STONE, SLATE, ETC.

The painting-grounds to be considered in this chapter consist mainly of mineral substances. However their constituents may be varied, in accordance with the process to be used in painting upon them, the wall or backing upon which they are spread should fulfil certain conditions. It must be naturally dry, free from soluble saline matters, and not very porous. A damp-proof course above the level of the ground is necessary, and the wall should be well-built, and free from tremors. A double wall well-bonded has been recommended: in this case the air enclosed between its two divisions should not be stagnant.

Before being plastered, the wall, whether its surface be of stone, bricks and mortar, roughened slate, or tiles, must be thoroughly wetted with lime or baryta-water. The plaster is applied in two or more coats, the coarsest and thickest first. In the case of a ground for fresco the two ingredients usually employed are (or rather were) pure slaked lime, and clean sharp silicious sand. The sand must be uniform in grain, white, and free from soluble salts. The slaked lime is so important an ingredient in the majority of plasters, that it is expedient to describe its preparation once for all. Before doing so we may state the relations subsisting between the three compounds known generally as carbonate of lime (chalk), or mild lime,