PLASTERING. The application of any coat of plastic material to the uneven surfaces of masonry or of wood-work, for the purpose of bringing the latter to a sufficiently smooth surface to receive elaborate surface decoration, is known by the generic name of plastering ; but in practice a distinction is made in order to separate the more elaborate processes of the plasterer's art, which are known by the names of stuccos, scagliolas, &c.
Ordinary plastering is either executed with the pure dehydrised sulphate of lime (or the baked gypsum), or with mortars composed of the carbonates of lime, forming a substratum to receive ultimately a finishing coat of fine plaster ; the choice between those materials being decided by motives of economy, for the gypsum does not occur in large quantities, or over large areas, although it Is found in a groat many countries. When the carbonates of lime are used for plastering, great precautions must be taken to ensure their perfect hydration before they are put upon the walls, as any unslaked lime will be sure to attract. moisture from the atmosphere, and to slake, or, as the workmen say, "to blow." Near London the usual practice is to cover the walls with a rough coat of lime so slaked, mixed with sand and with cows hair, for the purpose of counteracting the tendency of the lime and sand mortar to contract unequally, and thus.to crack, in setting. This coat is called the rendering coat, and in good work It is left with a roughly trowelled face, able to form a key for the further finishing coats ; but iu commoner descriptions of work, known as laid work, the rendering coat is at once Coated, or worked to such a surface as to be fit to receive the finishing coat. In good work the rendering coat is then covered with a second coat, called the )(eating one, with which great pains are taken to secure its verticality and its evenness; and upon this again the setting coat of pure lime is applied, which is trowelled up with more or less care according as tho surface is intended to receive paper or paint. Upon ceilings, or wooden partitions, the several rendering, floating, and setting coats are applied, in the same order as above, upon laths ; and the laths themselves are either pre pared from oak or from fir. In ceilings, however, it is customary to add, after the setting coat, a very fine one, composed of the sulphate of lime, or of what is commonly called the " plaster of Paris," whether obtained from our own or from foreign quarries.
The manner in which plastering is executed in Paris itself may be considered to present the best type of the application of the sulphate of lime for that purpose. The gypsum there used is obtained from the quarries of Belleville, Montmartre, Triel, or faux, and it is usually dehydrised iu kilns heated by wood ; the stone is said to contain a certain proportion of the carbonate of lime, and it certainly is harder than the English gypsum, on which latter account it may bo that the French plastering is so much harder than the plastering executed with the Derbyshire gypsum. Workmen, indeed, consider it an axiom, that " the harder the stone, the harder the plaster." After the gypsum has been burnt, it is grouud or pounded, and applied to the walls as soon as possible after, that operation ; the coarser kinds are need to render the uneven surfaces tolerably level, and the finishing coats are executed with the finer materials. The rough rendering is often laid on with a broom, and the setting coat is applied upon it by the appli cation of rather stiff plaster floated under a rule, but finished imme diately with the trowel ; in fact, ordinary French plastering is no bettor than English laid work, and it is quite as uneven and as irregular as the latter. In the execution of their ceilings, the French plasterers adopt a different system from the one used in England ; for they generally nail their laths at wide intervals, and upon them they lay slabs of old or rough plaster, which they bind together by new materials, so as to produce, as it were, a single solid slab of light plaster over the whole, ceiling, which is subsequently made smooth on the under side by the addition of a setting coat• This description of ceiling is heavier than the English ones; but it resists the transmission of fire more successfully, and it is also less permeable by sound : the latter inconvenience of English ceilings is sometimes obviated by the introduction of sound boarding, or of pugging—which, however, are objectionable on the score of their tendency to develop° the dry-rot in the joists of the floors.