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Bosstones
The bosstones of all grid crossing vaults studied by Rives are radial. The radial or inclined keystones appear in Spanish and Portuguese vaults in the 16th century, and this is perhaps a specific characteristic of the Gothic style of the Iberian Peninsula. It appears, most likely, due to economic reasons since the solid stone material needed to carve an inclined bosstone is significantly lower than that needed to carve an upright one. However, the carving of an inclined bosstone, with its corresponding joining arms, implies going from the vertical projection of traditional bosstones to the projection on the tangent plane, this being a great geometric achievement of Gothic masonry (Fig. 27). The inclined bosstone, unlike the vertical bosstone which is carved with its horizontal projection, is carved from its projection on the tangent plane (Fig. 28).
The centering
As on previous occasions, the centering has been built according to the guidelines suggested by Rodrigo Gil de Hontañón, who recommended in the first place the construction of a wooden platform at the height of the springing-stones (Fig. 29). On it, we must draw again the life size horizontal montea of the vault. In the intersections of the lines, that is, at the points in which the keystones are located, we would place studs with the suitable height (Fig. 30); this information is once again extracted from the montea. Between these studs we place the arch centerings, bonding them together and stabilizing the whole. In order to remove the centering more easily, the studs of the four keystones placed in the center of the vault are put, in turn, on a smaller platform lifted with wedges; when removing the wedges, all the keystones will go down simultaneously, so that the whole vault will be leveled at the same time.[....]
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