Page:The chemistry of paints and painting.djvu/54

contain silica in a form which is known as ' soluble silica.' This substance further strengthens the plaster by forming with a part of the lime an insoluble compound called silicate of lime. To return to the preparation for artistic purposes of hydrate of lime. White or black marble, limestone, chalk, or other fairly pure forms of carbonate of lime are first of all burnt, and then the quicklime produced is slaked with clean water. This is done in a grouting box, having a sluice 1 or 2 inches from the bottom. Run the thick cream of lime into a tank of slate and keep it, covered loosely, for two months. At the end of this time it will be ready for all the rougher purposes of plastering. For finer work the grouting operation is to be repeated, and the cream of lime strained through hair-sieves, and preserved in screw-top stoneware jars. Some water will accumulate above the lime-putty, as it may be called, in these jars; it should be poured off or drawn off, from time to time. The jars are kept tightly closed to prevent further carbonation of the lime hydrate. This change, if carried beyond a certain point, is undesirable, since the binding and hardening powers of the lime would thereby be lessened seriously, or even vanish altogether by its conversion into mild lime: not more than one-third or at most two-fifths of the lime should be converted into the carbonate. The lime-putty thus prepared may be used for plaster and intonaco with the certainty that it will not give rise to defects in the painting-grounds made therewith. Much lime paste of this kind was prepared for the works in fresco in the Houses of Parliament, and was kept in the cellars under that building, where probably some of it still remains. I have made many experiments with samples from that source, and can speak with confidence of its excellent quality.