Lastly, mosaic work in hard stone owes its rise in Florence to this epoch; and after gradually improving during two centuries, is now everywhere known as a work of this capital, and almost exclusively its own. In a letter of Teofilo Gallaccini,[226] we read that this species of mosaic "had been invented in Florence, in the time of Ferdinand I.;" an assertion which is not true. Before that period it flourished in Lombardy. The Carthusian Monastery of Pavia had in its pay a family of the name of Sacchi; which has existed there to our own times, and has filled the great church with this kind of mosaic. There are specimens of it in Milan of very ancient date. In that place Giacomo da Trezzo, who executed the tabernacle for the church of the Escurial, which is esteemed the most beautiful and magnificent in Christendom,[227] received his instruction. About the time of Cosmo I., Florence herself witnessed the rudiments of this art in a "small picture composed of gems" which she possessed, as is recorded by Vasari.[228] A similar one was executed for Francis I., from a design of Vasari, by Bernardino di Porfirio of Leccio, (a district of the Florentine state) "composed of oriental alabaster, and large slabs of jasper, heliotrope, cornelian, lapis lazzuli, agate, and other stones and gems, which they estimate at 20,000 crowns." But pictures so wrought in large pieces, were not of that perfect kind of mosaic that contained a vast variety of colours and middle tints. Such are executed in every shade of colour, from the natural stains of the stone itself; and the tints are lowered, heightened, and managed, so as almost to rival painting. For this purpose, every species of hard stone is collected and sawed; innumerable colours are thence selected, graduating from the deepest to the lightest shade, which are kept ready for use. This art was in request at Milan; where, on account of the vicinity of Alpine countries abounding in every species of hard stone, it arrived at great perfection. Francesco I. meditating the erection of the magnificent chapel for the sepulture of the royal family, in the church of S. Lorenzo, and the ornamenting it with urns and altars wrought in hard stone, invited Giovanni Bianchi from that city to his court, in the year 1580, and committed the works in mosaic to his direction. Soon after Ferdinando ascended the throne, and the new art gained ground under him; it was promoted by Constantino de' Servi, and afterwards by other artists, who progressively improved it. The tables, cabinets, and coffers, small landscapes, and architectural pieces which were there executed, and sent as presents to princes, are dispersed over Europe. In one cabinet of the ducal gallery there is an exquisite octagonal table, the round central piece of which was designed by Poccetti, and the ornamental border by Ligozzi. Jacopo Autelli executed the work, on which, with numerous assistants, he was employed for sixteen years, and finished it in 1649.