Geometry and traces of Gothic stairs. The squares as instrumental abacusesin construction spirals stairs of Tortosa Cathedral

Identifier:  PC:1469

Handle: https://hdl.handle.net/20.500.11797/PC1469

Authors:  J. Lluís i Ginovart

Abstract:
In the Gothic era, spiral staircases were one of the greatest showpieces of the magister operis knowledge. They were the focus of arithmetic and geometric medieval constructor knowledge. Their construction involved the use of templates known as gabarits, which required the polygonal division of the circumference. These regular 11-, 13-, 14-, 17- and 19-sided polygons do not appear in the Euclid¿s Elementa or the Tolomeo¿s Almagest, or in the few Practica Geometriae of the time. These polygons can be constructed by using a square with two arms, supporting repeatedly the legs on the hypotenuse, a rotation of the instrument. Squares constructed using arithmetic or geometric methods must always be calibrated and can therefore be used as an abacus, which enables the circumference to be divided with sufficient accuracy. San Pedro (1383) and Santa Catalina staircases (1424), of Tortosa¿s cathedral, have been drawn by the instruments explained previously.