THE GEOMETRY BEHIND THE OCTAGONAL HALL OF SMALL THERMAL BATHS AT HADRIAN’S VILLA

ABSTRACT: Hadrian's Villa was a place of architectural experimentation, wanted by the Emperor Hadrian during the 2nd century AD in order to express a very personal and revolutionary idea of art, of architectural design and in particular his new formal conceptions of vaulted spaces. The first ribbed vaults of the Roman Empire were conceived there, at his Tiburtine villa, where the incessant sequence of promenades and naturalistic environments culminated into innovative vaulted spaces. Some of those pavilions, at that time, must have appeared so strange and futuristic that Apollodorus of Damascus, famous architect of the previous Emperor Trajan, did not appreciate their shapes, in his opinion more similar to "pumpkins" rather than domes (Cassio Dio, Roman History, 69, 4). Among the numerous domes of the Villa, one in particular, still adequately preserved, represents a unique example in the history of architecture, since its rare tent-shape no longer appeared in other Roman and Late Antiquity examples. This dome is the one that covered the octagonal hall of small thermal baths of the Villa. Thanks to new surveys, carried out through laser scanner and processed by means of reverse modelling applications it was possible to obtain highly reliable contour lines and sections able to enlighten some geometrical aspects of this dome that still continued to be misunderstood. An original and practical geometric construction was aimed at linking together the drawing of plan (ichnographia) and section (orthographia) of this octagonal mixtilinear hall, showing a not common skill of the architect in taking advantage of geometrical properties. The result was a vault formed by a thin, continuous surface, as the one of a tent. Starting from a series of relevant sections obtained by the high resolution mesh model of the vault, then filtered and idealized after to a in depth metrological analysis, it was possible to achieve a new solution for this original artefact. In order to generate a continuous surface starting from contour lines of a partially preserved and highly deteriorated vault, whose central part fell down centuries ago, it was necessary to use subdivision surfaces rather than NURBS for the integration of missing parts. It seems probable that this daring solution was applied to other two buildings inside the Villa: the southern hall of the Piazza D'Oro and the access to the Accademia. Both these spaces present a common initial octagonal shape, modified in order to obtain a variety of striking halls. At Hadrian's Villa there are many domes, some of them are characterized by more traditional shapes (semi-spherical and basins), others are ribbed vaults, and some others are intermediate solutions. But the singular "tent" of the Small Thermal Bath, perhaps the most original, had no luck and did not become a formal archetype for the later periods, in contrast to what had happened to the domes of the vestibule of the Piazza d'Oro and of the Serapeum.