The octagonal Cappella dei Principi surmounted by a tall dome, 59 m. high, that is the distinguishing feature of San Lorenzo when seen from a distance, stands centrally sited with respect to the nave, to which it provides the equivalent of an apsidal chapel. Its entrance is from the exterior, in Piazza Madonna degli Aldobrandini, and through the low vaulted crypt planned by Bernardo Buontalenti before plans for the chapel above were made.
The opulent Cappella dei Principi, an idea formulated by Cosimo I, was put into effect by Ferdinand I de' Medici; it was designed by Matteo Nigetti, following some sketches tendered to an informal competition of 1602 by Don Giovanni de' Medici, the natural son of Cosimo I, Grand Duke of Tuscany, which were altered in the execution by the aged Buontalenti; thus, a true expression of court art, it was the result of collaboration among designers and patrons.
For the execution of its astonishing revetment of marbles inlaid with colored marbles and semi-precious stone, the Grand Ducal hardstone workshop, the Opificio delle Pietre Dure was established. The art of commessi, as it was called in Florence, assembled jig-sawn fragments of specimen stones to form the designs of the revetment that entirely cover the walls. The result was disapproved of by 18th and 19th century visitors, but has come to be appreciated for an example of the taste of its time. Six grand sarcophagi are empty; the Medici remains are interred in the crypt below. In sixteen compartments of the dado are coats-of-arms of Tuscan cities under Medici control. In the niches that were intended to hold portrait sculptures of Medici, two (Ferdinando I and Cosimo II) were executed by Pietro Tacca (1626–42).
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