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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, burnt lime, or quicklime, and slaked lime. The first of these is neutral and nearly insoluble in pure water, the second and third are alkaline and caustic. When burnt lime unites with water to form slaked lime it becomes slightly soluble in pure water. In chemical language these three compounds are called respectively calcium carbonate, calcium oxide, calcium hydrate (or hydroxide). From the first substance the others are readily obtained.

If calcium carbonate, often called carbonate of lime, be heated to a sufficient temperature, it is decomposed, being resolved into carbon dioxide (carbonic acid gas) which escapes, and calcium oxide (lime) which remains: from 100 parts by weight of the carbonate 56 parts of lime, that is, burnt lime, are obtained. Placed in water or exposed to moist air this burnt lime combines with water, 56 parts of it uniting with 18 parts of water to yield 74 parts of slaked lime, calcium hydrate. In the ordinary country atmosphere, which contains no more than 3 measures of carbonic acid gas per 10,000, slaked lime or calcium hydrate loses its combined water, slowly becoming once more the carbonate from which it was originally produced: 74 parts of hydrate lose 18 parts of water and combine with 44 parts of carbonic acid, and yield 100 parts of carbonate. Thus mild lime is formed once more from caustic lime. By this change, if it be effected in the presence of a sufficiency of free water - that is, if the hydrate of lime be in the state of a firm paste - the whole substance becomes a hard crystalline solid, like an opaque marble. Advantage may be taken of this hardening or cementing process to firmly incorporate other substances with the lime.

Silicious sand, infusorial earth, pumice, marble powder, and many other mineral substances, may be thus introduced. Such of these materials as are silicious may 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.

Last Update: 2011-01-23