Raphael's _Transfiguration_, cost about 12,000
crowns, and the labour of nine years, ten men constantly working at it.
The late works, are, however, of very inferior execution.
The slab upon which the mosaic is made, is generally of Travertine, (or
Tiburtine) stones, connected together by iron cramps. Upon the surface
of this a mastic or cementing paste, is gradually spread, as the
progress of the work makes it wanted, which forms the adhesive ground,
or bed, on which the mosaic is laid. This mastic is composed of fine
lime from burnt marble, and finely powdered Travertine stone, mixed to
the consistence of a paste, with strong linseed oil. Into this paste are
stuck the _smalts_, of which the mosaic picture is formed. They are a
mixed species of opaque vitrified glass, partaking of the mixed nature
of stone and glass, and composed of a variety of minerals and materials,
coloured for the most part, with different metallic oxydes. Of these no
less than 1,700 different shades are in use; they are manufactured in
Rome in the form of long, slender rods like wires, of different degrees
of thickness, and are cut into pieces of the requisite sizes, from the
smallest pin point to an inch.
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