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[...] One of the most critical elements in the construction of the vault is the intersections of the ribs. In this vault all intersection points are resolved by a boss stone consisting of a central cylinder which hides the actual intersection of the ribs. Once again, the full scale drawing is fundamental to determine the volume of the boss stone and its carving. On the upper face of a block of stone, large enough to contain the piece, the horizontal projection drawing of the boss stone has been copied; then, the carving can begin in vertical (photo 10).
Then, the arms of the boss stone have to be cut with the right slope to connect them with the voussoirs of each arc. To do this, we use again the full scale elevation drawing in which, with the utmost precision, has been drawn each of the boss stones; from this design can be drawn the springing angle of each arm. To measure the angle, the horizontal plan of the upper or lower face of the central cylinder, as appropriate, has been taken as reference. Once the angle is transferred upon the arms of the boss stone, we can proceed to cut it (photo 11,12).
The tas-de-charges is composed of ashlars laid in horizontal layers which are, really, part of the wall. To determine its geometry, the full scale drawings were again used. First, in the elevation design, the horizontal layers of the ashlars have been defined and, then, these sections have been drawn onto the plan in order to make a series of frames. These frames will allow to draw the upper and lower face of each ashlar and to carve each piece of stone with the accurate shape (photo 13,14). [....]
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