_Orders must be obtained from the Rev. Monsignor Fiorani, at the
Sacristy._
_It is entered by a corridor from the ground floor at the left-hand
corner of the Court of S. Damaso._ The mosaics in S. Peter's, S.
Paul's, and other churches, were manufactured here. Some mosaics take
a long time to execute, as great patience and art are required in
blending the shades, &c., upwards of 27,000 different shades of the
coloured _vetri_ being kept in stock.
A plate, generally of metal, of the required size, is first surrounded
by a margin rising about three-quarters of an inch above the surface.
A mastic cement, composed of powdered stone, lime, and linseed oil, is
then spread over as a coating, perhaps a quarter of an inch in
thickness. When set, this is again covered with plaster-of-Paris
rising to a level with the margin, upon which is traced a very careful
outline of the picture to be copied, and just so much as will admit of
the insertion of the small pieces of smalto or glass is removed from
time to time with a fine chisel. The workman then selects from the
trays, in which are kept thousands of varieties of colour, a piece of
the tint which he wants, and carefully brings it to the necessary
shape. The piece is then moistened with a little cement, and bedded in
a proper situation, the process being repeated until the picture is
finished, when the whole, being ground down to an even face and
polished, becomes an imperishable work of art. The process is the same
for making the small mosaics so much employed at the present day for
boxes, covers, or articles of jewellery, and this work is sometimes
upon almost a microscopic scale. The Florentine mosaic, which is
chiefly used for the decoration of altars and tombs, or for cabinets,
tops of tables, coffers, and the like, is composed of precious
materials, in small slices or veneers, and by taking advantage of the
natural tints which characterize the marble, the agate, or the jasper,
very admirable effects may be produced in imitation of fruits,
flowers, or ornaments. The use of this kind of mosaic is extremely
restricted, on account of the great value and expense, not only of the
materials, but of the labour employed upon them. None but the hardest
stones are used; every separate piece must be backed by thicker slices
of slate or marble to obtain additional strength; and every minute
portion must be ground until it exactly corresponds with the pattern
previously cut.