MOSAIC, commonly designed Painting in Mosaic,is a mode of representing objects by the union of portions of stone or glass, reduced to an even surface. But it is always a copy, for artists in Mosaic never compose.
The materials are few and simple, consisting merely of the substance with which the picture is to be repre sented, formed into cubes, parallelopipeds, or other poly gonal figures, as required, and retained by one end in a strong cement, to preserve the union of the whole. Their colour, size, and substance, are arbitrary with the artist, according to the subjects and future position : all, however, are of the most durable kind. The an cient Mosaics consisted chiefly of marble and coloured glass or pastes ; the modern are composed of marble, glass, enamel, and sometimes of the harder stones, as agate, cornelian, lapis lazuli, and even jewels. From the variety of design, and diversity of shades, the works of the latter evince great superiority over the former, for the ancients used few colours, and the same design was frequently repeated.
In general, Mosaics are composed of cubes, or short parallelopipeds. that is, oblong pieces of from two inches of a side, down to fragments of the smallest surface, as is seen in a portrait of Pope Paul V. where the face alone consists of 1,700,000 portions, each no larger than a grain of a millet. The Mosaic of the cupola of St. Peter's at Rome, on the other hand, is formed of unp fished coloured stones, of from half an inch to two inches square ; and pavements appear in pieces still larger. Of this last description, or of what may be called large Mosaic, the pieces are reduced to the ne cessary size by sawing and grinding, after the ordinary operations of the lapidary; those of glass, which is in universal use, are broke down from rods, drawn out of a suitable shape ; but glass being a refractory substance, enamel is substituted for it, as it is more easily manag ed in reduction. The enamel which is used for smaller pictures, is drawn out into long quadrangular rods while fused, and these are broke across with a hammer, or divided by a file into portions longer or shorter as required. It is so fusible, that the rods of inconsider able size, may be drawn out by the flame of a candle, without assistance of the blow-pipe. In using the finer hard stones, they also are fashioned into the proper form, by the art of the lapidary ; but the expense of purchase, and the labour of workmanship, restrict the fabrication of such Mosaics. Most of the enamel played for this purpose was formerly prepared in Hol land and at Venice, in small round cakes, four or fire inches in diameter, and five, six, or eight lines thick, which were sent to the chief places of manufacture in Italy, as Milan, Rome, and Florence. Being dexter ously struck with a hammer, on an anvil of a particular shape, fragments more regular than those of other sub stances fly off in the fracture, and commonly approach ing an oblong form. At Rome, where it is now carried
on extensively, all the materials are compounded.
Such artificial substances are prepared of a vast va riety of different shades, in order to obtain the suitable gradation of colour in the picture. Hence the present number of tints in Mosaic is said to amount to no less than 15,000 or 17,000, proceeding by a nicety almost inconceivable. About 50 years ago they were comput ed at 4000: and it seems evident that the increase has been great and gradual, and most probably an object of more attention in later times ; for ancient Mosaics, on which considerable labour has been bestowed, appear in only three colours.
The numerous fragments from each species of sub stance are all arranged in drawers, boxes, and cases re gularly labelled, from which they are withdrawn by the artist for his work, as the compositor selects types for printing. It is necessary to explain, that, in composing a large picture in Mosaic, the foundation or back of the picture consists of a stone called piperno. Several ob long pieces, together equal to the whole surface, are taken, each some inches thick, whereby wonderful strength and solidity are acquired, and hollowed to the depth of about three inches and a half, leaving a border all around, with which the work, when completed, will be level. The excavated surface is intersected by trans verse grooves, about an inch and a half deep, and some what wider at the bottom than the top, in order to retain a quantity of cement or mastic which fills them, the line of the grooves joining in an inclined direction from each side, so as to form an angle in the middle. The sepa rate pieces are then nicely adjusted together, by strong iron cramps behind. Or let it he supposed that a large marble slab is hollowed to the depth of three inches and a half, leaving a projecting and prominent border, the fragments are imbedded in it, to form the picture. This bed or frame is gradually filled with a strong and durable kind of cement or mastic, on the composition of which we shall not here enlarge, both from the gene ral attention now paid to substances of that nature, and because different artists compound them after a differ ent fashion. As the frame is filled, the picture is deli neated on the cement, the same way as painting in fres co ; and the fragments of enamel being selected for a small portion of it at a time, they are successively beat into the cement with a small flat wooden mallet, until the top of the whole is level, or nearly so. When the artist observes that the fragments so arranged are not suitable to his taste and expectations, he removes them, or substitutes others, which is easily done before the cement hardens, but subsequently it is a more difficult operation. Proper cement remains in a state to receive fragments during fifteen or twenty days, by observing the necessary precautions.