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NATURAL HISTORY OF GEMS.
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Renaissance
gem-engravers, however, has availed itself of this substance : there is
in the Galleria at Florence an excellent head of Leo X., an intaglio on
a large circular seal, set in iron after the manner of a coin-die, and
designed to impress the leaden bulls of that pontiff.
The
art by which the Romans worked * these vast masses with such evident
facility is entirely lost. At present, when made use of in
architectural decoration, the only method of subduing the stone is to
steep it for many weeks in urine (as a Roman architect who had had
recent occasion to employ the stone informed me), and even then it
speedily turns the edge of the best steel tools. It has therefore been
conjectured that the old sculptors worked both this and Basalt by means
of emery-powder and chisels of soft metal. It is known that the early
Egyptians executed their stupendous works in hard stones with bronze
chisels, some of which have been found in the quarries.
The
Grand-Duke Cosimo I. was mightily vain of a discovery made by himself
of how to temper steel tools by some chemical means so as to cut
Porphyry (Galluzzi, iii. 127), and several statues executed by his
order still adorn the entrance to the Boboli Gardens ; but their
execution is rough and clumsy, not approaching in the least to the
beautiful finish of the ancient works in Porphyry.
*
From its extensive dedication to the splendour of the old capital the
Byzantines only knew Porphyry by the name of the ' Roman stone."
Codinus quotes a letter of a noble lady, Mareia, accompanying the gift
to Justinian for the fabric of Sta. Sophia, of certain columns so
designated, stolen from Aurelian's Temple of the Sun.
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