The foundation or back of pictures of smaller size, consists of hammered iron or copper, instead of stone, the surface of which is interspersed .witlt small GIt cramps or brackets, like reversed staples, for the pur pose of retaining the cement. They are merely slips of metal, bent up at each end, and soldered by the mid dle to the plate, thus producing inequalities, which ope rate in the same manner as the grooves of the former Mosaics. The foundation of pavements is of the most solid description, consisting of strong beds of masonry, rising towards the surface of the ground, and are fitted to receive the cement and materials of the represen tation.
'After the whole picture is composed, its surface is ground down to a perfect plane, in a manner similar to what is practised in grinding mirrors, and a polish is given to it with putty and oil. During the progress of these operations, any crevices betrayed at the joints are filled with pounded marble or enamel, mixed with wax, which penetrates by passing a hot iron over them. The kind of polish, however, is regulated by the position and use proposed for the Mosaic, and of late it is not so high as formerly.
Although we have said that cubes or parallelopipeds are employed, the figures are not invariably angular, because curves are sometimes required ; and from the ductility of glass, the portions forming a rose, for ex ample, are curvilinear.
Large compositions in Mosaic are tedious, requiring several years to execute ; and the grinding and polish ing of the surface of a picture are extremely laborious. But it is not necessary that the whole parts should be the work of one individual, as equal skill throughout is scarcely ever indispensable, both because large portions are of a uniform description, and because some may be less artificially arranged. The qualities of Mosaic are various, often being rudely designed, and coarsely exe cuted. Sometimes so little attention is paid to the inti mate union of the cubes, that the cement rises between them to form part of the surface, and in general it can not bear narrow inspection. The most skilful artists, however, have produced works so delicate as to be com pared to the finest painting.
From this general description, the substance and structure of Mosaic will be easily understood. It may be carried to an unlimited extent ; and in fact spacious pavements, the incrustation of walls, the ornamenting of cupolas, and lining of arches, appear in it in various countries. Italy, always the scat of the arts, engrosses
almost the whole Mosaic manufacture of Europe. In Rome there is .a large establishment belonging to the Pope, where this kind of painting is conducted on a i great scale. The different materials are arranged in numerous apartments, from which they arc removed by the artists as having occasion for them. Besides this establishment, there are many artists in Rome occupied in smaller works, such as pictures, full length figures, whose dimensions do not exceed two or three inches ; birds, insects, and baskets of flowers, all in miniature, of exquisite execution. The Mosaics for personal or naments and toys arc innumerable, and they are now common in Britain. The most celebrated modern Mo saic picture has been just completed at Milan, after Leonardi de Vinci's painting of the last supper, pre served in a suppressed convent of that city. This great work is about 24 feet in length, by 12 in breadth, imbed ded on 12 slabs of marble obtained from the Lago Mag giore. It is the production of Rafaelli, an artist of the Roman school, by whom eight or ten men were employ ed on it daily during eight years ; and it cost 75001. Sterling. Originally commissioned by Bonaparte, his expulsion is said to have thrown it into the hands of the artist, from whom it was purchased by the present emperor of Austria.
Mosaic appears to greatest advantage at some dis tance; nor can it be compared in general to the deli cacy, smoothness, and uniformity of painting. But its superiority is affirmed to consist in greater durability, and resisting the influence of damp, and the attack of insects; its not suffering from exposure to the sun ; in preserving the permanence of colour, and requiring no particular light for embellishment. Yet all these pro perties are so far counterbalanced by the weight of the materials, tediousness and expense of execution, toge ther with other obstacles, as will prevent Mosaic from competing, iw most countries, with painting.
Analogous to Mosaic is the pietra Jura, or Florentine work, which consists of the union of irregular portions of hard stones containing the gradation of colours in each, instead of obtaining that gradation by the union of multiplied fragments. Some fine specimens of this are preserved at Florence, such as a slumbering Cupid re posing on a lion.