Francesco Borromini's renovation of the basilica of San Giovanni in Laterano in Rome is one of his grandest conceptions. It remains frustratingly incomplete, however, because Borromini was not able to vault the nave as he had hoped. The nature of his design for a vault has been much discussed. The only surviving visual evidence consists of a drawing in a private collection, not heretofore studied in depth. It shows a section through the basilica with two superimposed roofing structures, a flat ceiling and a coffered barrel vault. Comparison to other Borromini drawings for the renovation of the basilica establishes that the drawing is from Borromini's own hand and shows an intermediate stage of his design, probably dating from April 1647. In conjunction with recently published documents from the Spada archive, the drawing demonstrates that as of mid-1647 the vaulting of the basilica was seen as a two-phase process: the existing flat ceiling was to be retained for the present, but the basilica was prepared to support a vault in the future. It also shows that at the Lateran Borromini was consciously emulating not only Michelangelo's Saint Peter's but also Alberti's Sant'Andrea. The prominent use of a classical form on a monumental scale, especially striking in view of Borromini's reputation as an anticlassicist, is attributable to the unique nature of the commission, Borromini's only basilica. The design shows Borromini's boldness in seeking to accommodate the classical form to his own personal concerns for skeletal structure and new effects of motion and light.

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