The 'Saint-Gilles screw'

The ‘Saint-Gilles screw’ is a rampant annular vault supporting a spiral staircase. It was named after the priory of Saint-Gilles near Nîmes, where it is believed to have been first built. Its structure is extremely complex because each of the stone’s sides and edges have double curves. ‘Stereotomy’ or ‘descriptive geometry’ is the art of drawing these stones for construction, but it can also be used in joinery and timber roof structures. Numerous plaster and wooden stereotomic models were used to teach this discipline in architecture and engineering schools. This ‘Saint-Gilles screw’ belongs to a collection of forty-four models donated to the Conservatoire in 1861. Their maker, F. Sevestre, exhibited seventy-two ‘stereotomy works’ in the Louvre in 1823.