The Golden Age of French Stereotomy and its applications. The vault of the Hotel de Ville in Arles

The discipline of stereotomy, namely the application of descriptive geometry, is strictly connected to architecture and the methods of representation and, as such, it is able to shape complex and sometimes fascinating surfaces solely meant to be covering areas of particular architectural spaces (and not only that), while offering junction solutions connecting different types of surfaces. Stereotomy was originally viewed as a response to the need for a general theory capable of solving all the technical problems resulting from the invention of complex and even daring stone structures. Nowadays, stereotomy is particularly relevant, not only for its vital contribution to the restoration works on antique buildings, but also for a new kind of design that recovers and revitalizes natural qualities, especially as far as stone buildings are concerned, which are also less expensive thanks to modern technologies that enable renewed and original uses of these buildings. This paper will analyze some of the solutions featuring an extreme configurative complexity that were realized during the golden phase of the discipline, by investigating the relationship between geometry, architecture, and structure. In particular, the paper will analyze the roofing vault of the entrance hall of the Hôtel de ville in Arles (France), whose complex project together with its difficult static realization, with the huge vault being significantly lowered and featuring no central support, are to be essentially ascribed to its designer’s geometric wits, architect Jule Hardouin-Mansart, as well as to the practical experience of the mâitres maçon who were first consulted in relation to the structure plan and the final realization of the work. Mansart’s project modified the previously-approved one by Pilleporte and Peytret, over which he was in complete disagreement. He was then asked by consul Grille to study some possible changes without altering the existing foundation structures and maintaining the 16th-century Clock Tower. The main innovation introduced by Mansart, as far as the entrace hall is concerned, regards its roofing structure: it is a single huge vault with no central support (whereas the project by Pilleporte and Peytret counted four columns) and it is significantly lowered. On the 16th July 1673, the technical and aesthetic corrections made by Mansart were approved. However, the concerns caused by such a daring project led the consul to invite different maîtres maçon and adepts at stone cutter corporations to offer their skillful advice on the structure designed by Mansart and also on the structures previously realized (foundations and bearing walls). Moreover, three models, in both wood and stone, were realized for a better study of the behaviour of the vault. As a matter of fact, the complex geometric configuration of every single element composing the vault ensures an overall effect that is of great interest. It is the proof of the designer’s high skills in conceiving this wonderful “stone machine”, it is the proof of the ability of the workers of the time to turn such a project into reality, as well as of their complete acquisition of the rules and methods to cut stones during the golden phase of this discipline.