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[....] The vault's central keystone, approximately 80 cm of real diameter, constitutes another part of interest. The total station, once again, allowed us to obtain its exact shape, which shows peculiar characteristics (Fig. 13). The central cylinder lacks outward arms on which to receive the arches; besides, its side walls are strictly vertical. To hold the keystone and guarantee its contact with the arch there are small parts which, wedge - shaped, connect the last testa face of the arch to the boss stone's vertical face (Fig. 14).
The most complicated part of a cross ribbed vault is, undoubtedly, its springing (tas-de-charges). The outward cluster of ribs is done thanks to a magnificent part in which the arches start to separate, from the lowest level, where they are tighter, up to the upper level situated where the arches are completely separated.
This part of the vault can have considerable dimensions, and may reach a height of more than 1/3 of the total height, as it is the case of this vault whose tas-de-charges are 1,80 m high. It is a massive part which assures the perfect embedment of the vault to its support plans and the side walls. It is carved in horizontal plans, that is to say it does not belong to the voussoir area of the vault.
In Santa María de Huerta the tas-de-charges are done by seven horizontal courses and their carving is of an extraordinary quality. Again, a rigorous data collection permitted to obtain the profiles of each of the horizontal plans (Fig. 15) and, with them, elaborate the models with which to carve them (Figs. 26 y 27). In real practice, the stone masonry masters should get those profiles from the layouts, by breaking the vault's section in horizontal plans that, when cutting the different arches, allow to calculate in plan their projection so as to be able to draw the profiles of each horizontal plan (Figs. 24 y 25). In each section there are two types of tas-de-charges, the corner ones, where five arches get together, and the central ones where three arches charge. The intricate moulding of the arches, not only the main ones but also the formeret arches, result in highly complicated tas-de-charges. The carving of each course must be carried out just with the help of the lower and upper testa faces (Figs. 28 y 29). The upper level of the tas-de-charges is carved with its faces sloped to make the support of the arches' first voussoir easier (Fig. 30). [....]
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