This imitation is the most complete that could be con ceived ; and, w hen the bases and capitals are made of real marble, as is the common practice, the deception is beyond discovery. When not exposed to the weather, it is also little inferior to real marble in point of durability, retains its lustre full as long, and is not one-eighth of the expense of the cheapest imported.
There is another species of plastering, though done by a distinct set of persons, known to the public by the name of composition ornament, used not only for the decorative parts of architecture, but also for the frames of pictures, looking glasses, &'e. This composition. which is very strong when quite dry, and of a brownish colour, consists of the propor tion of two pounds of powdered whiting, one pound of glue in solution, and half a pound of linseed-oil, mixed together in a copper, heated, and stirred with a spatula till the whole is incorporated. After being suffered to stand to settle and cool, it is laid upon a stone, covered with powdered whiting, and beaten till it assumes a tough and firm consistence; after which it is put by for use, and covered with wet cloths to keep it fresh. The ornaments to be cast in this composition are modelled in clay, as for common plastering, and after wards a cameo, or mould, is carved in a block of box-wood. This carving requires to be done with the utmost neatness and truth, otherwise the symmetry of the ornament to be cast Crow it will be spoiled. When the composition is to be used, it•is cut with a knife into pieces adapted to the size of the mould, and forced with the hand closely into every part. It is then placed in a press worked by an iron screw, by which it is farther compressed into every crevice : after being removed' from the press, the mould is turned upside-down, with a smart tap on a board, which dislodges the composition, and the mould leaves it with its face upwards. One foot in length is as much as is usually cast at one time, and when the ornament first drops from the mould, all the superfluous composition is pared away with a knife, and thrown into the copper towards a fresh supply for the next east. The orna ments, when formed, are glued upon wooden, or other grounds, or they are affixed by means of white-lead, &c., after which they are painted or gilt, according to the purpose for which they are intended. This composition is at least SO per cent. cheaper than carving, and in most cases is equally well
calculated to answer all the purposes of that art.
It were much to be wished, that the art of plastering could be again brought to its ancient perfection. in best buildings, the plastered walls and ceilings crack and fly, and, in is little time, grow damp, or moulder to decay.
The Romans had an art of rendering their work of this kind much more firm and durable ; and there is no reason to despair of reviving this art by proper trials.
The ancient plastering of these people, preserved to this where it has not met with violent blows, or injuries from accidents, is still found as firm and solid, as free from cracks or crevices, and as smooth and polished on the sur face, as if made of marble. The bottoms and sides of the Roman aqueducts were lined with this plastering, and endured many ages without hurt, unless by accidents : witness that whereof some yards are still to be found on the top of the Pont de Gard, near Nismes, for the support of which that celebrated bridge was built to earry water to the said town. The roofs of houses, and the floors of rooms, at Venice, are covered with a sort of plaster, of later date, and yet strong enough to endure the sun and weather for several ages, with out cracking or spoiling, and without much injury from being trod upon.
The secret of preparing this Venetian plaster is not among us ; but it would be worth m bile to try whether such a sub stance might not be made by boiling the powder of gypsum dry over the fire (for it will boil in the manner of water); and when this boiling or recaleining is over, mixing with it resin, or pitch, or both together, with common sulphur, and the powder of sea-shells. If these were all mixed together, the water added to it hot, and the matter all kept upon the fire till the instant of its being so that it might be laid on hot, it is possible this secret might be hit upon.
Wax and oil of turpentine may be also tried as additions; these being the common ingredients in such cements that we have accounts of as the firmest. Strong ale-wo•t is, by some, directed to be used instead of water, to make mortar of lime stone of a more than ordinary strength. It is possible that the addition of this tenacious liquor to the powdered ingre dients of this proposed plaster, might greatly add to their solidity and firmness. See CEMENT, MORTAR, and STUCCO.