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[....] A more detailed analysis of one of the vaults lets us see that it has been built with an oval arch, therefore it is a lowered vault. After measurements of the real vault, we can state that the diagonal arch is a three-centered arch, that is to say an oval arch in which the centre of the central arch is located exactly at the ground level of the sacristy (photo 10B). Also, we can ascertain that the height of the tas-de-charges’ level lies in the tangency point between the lower and upper circle of the oval arch (photo 7).
Moreover, in these vaults Joao del Castilho uses a new technique which he often employed and was largely developed in the 16th century throughout the Iberian Peninsula: tilted bosstones. As it can be seen in photo 4, the vaults are built with two types of bosses, the vertical ones, only used in important intersections, such as the central keystone, the tiercerons and over the perimeter arches (photo 8), and the tilted bosstones only used in decorative ribs (photo 9, 10A).
Building the arches
On taking the vault’s measures, we find out that the height reached by the tiercerons coincides exactly with the height of the diagonal arch, so both arches are identical; it is exactly the same oval form (photo 11). Nevertheless, since the tierceron has a slightly smaller section than the diagonal arch, its springing has to be a bit further up. When watching the ribs’ springing, the tiercerons do appear at an upper level (photo 7) [....]
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