Page:The chemistry of paints and painting.djvu/57

Rh water so as to lose the power of setting, strained, and mixed with size. Sometimes both whitening and slaked burnt gypsum are found together as constituents of the ground. The ground was laid directly on the panel, or on the cloth which had been previously glued to the wood. Great care was taken by sifting and washing to secure the fineness and purity of the whitening (calcium carbonate) and of the slaked plaster of Paris (calcium sulphate united with two proportions of water). Various kinds of size were used ; one of the best was made partly from parchment, partly from the finer kind of fish-glue. An excess of size will cause the ground to crack; it must never contain such a quantity as to be rendered non-absorbent. All tempera-grounds of gesso were originally absorbent; in course of time they have become more so owing to the decay of the size. Whether they were afterwards to be painted in tempera or oil they were always first sized. This sizing preserved the luminous whiteness of the ground, which was unable to absorb the oil of oil-paints or that present in the egg-yolks employed in tempera. A proof of the existence of this layer of size above the ground proper is obtained in the process of transferring old tempera and oil pictures to canvas, for in such cases we find discoloration of the ground under cracks only where both the size and the paint above it have become fissured.

It will have been gathered from what has been stated in the preceding paragraph that a non-caustic tempera-ground is suitable for work in oils. In the latter case, however, it must be perfectly dry before the painting is commenced. It should be gently warmed and rubbed with a little clean spirits of turpentine before laying on the first coat of oil-paint.