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Porphyrites, Porphyry

Porphyrites, Porphyry Page of 384 Prasius, Plasma Text size:minus plus Restore normal size   Mail page  Print this page
204
NATURAL HISTORY OF GEMS.
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.
Porphyrites, Porphyry Page of 384 Prasius, Plasma
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