There is one remarkable difference in the action of the lime plastering and of the gypsum plastering, arising from the fact that the crystalli sation of the latter is accompanied by an expansion so energetic, that if it be not provided for very serious accidents would occur ; whereas the lime plastering, as a general rule, has a tendency to contract. Of what ever materials the body of the plastering may be formed, the finer mouldings, and all the cast ornaments, are executed in fine plaster, unless the cements, or the prepared sulphates of lime, known as the Parkin, Keene's, or Martin's cements, are used. The class of materials last named are obtained by the calcination of a previously dehydrisod sulphate of lime, soaked in solutions of alum, borax, &c.; and they produce remarkably hard and smooth surfaces, admirably adapted to receive paint ; unfortunately, their use is costly.
Stuccos are made by the application, as it setting coat, of lime mixed with calcareous powder, plaster, and other ingredients, which is trowelled up by dint of excessive labour to a fine surface resembling marble, and able, like marble, to receive a polieh. Different colours are given by mixing with the lime the metallic oxides of the required tints; and the polishing is effected only when the whole of the moisture has worked out of the lime. The mode of polishing is by the use of fine grits, tripoli powder, and oil rubbers, in nearly the same manner as for marble. Scagliola is in fact a species of stucco, but it is executed
by the introduction of mall particles of fine spars, or of coloured marble, into the rendering coat, in addition to the simple lime powder, so as to produce the effect of the snore ornamental descriptions of marble breccias. Both these ornamental kinds of work are adapted for internal deco ration, but externally they are exposed rapidly to decay. It may also be added that, in the construction of concert, or even of debating rooms, it is preferable to finish the walls with ordinary plastering than with stucco, or scagliola, for the latter materials are so elastics that they reflect sound in a manner to interfere with the purity of the tone.
W'hatevcr mode of plastering may be adopted, it is•essential that the walls on which it is applied should be perfectly dry, and perfectly impermeable, if any elaborate surface decorations are to be applied to them. Great precautions must also be taken to secure the absence of organic matters in the materials used, for if any of them should be present, the nitrogen salts will be thrown out, under such circa:as etanoos as to destroy any painting or papering on the walls, It is precisely to the neglect of this precaution that we may attribute the loss of so many noble fresco paintings.