The corde blande, the curved laying of brick courses, progressively increasing from the base to the top of the dome, are well documented by the survey of the dome extrados. This special laying system was caused by works needs, as it was necessary to give continuity to the differently inclined brick beds in the two corner ribs and in the central part of a single face. The consequent curved laying of the bricks was got and made progressive not only with varied thickness of the mortar joints, but introducing discontinuities in the
herring-bone row and in the sub-horizontal masonry between them as well as.
A further aspect has to be kept in mind. At the intrados the dome corner, measured from the gallery to the base of the lantern oculus, is only a little more than 39 meters, while at the middle of a face the corresponding length is 3 meters shorter. In the works, this different length (nearly 10%) required a greater number of brick courses in the corners, and this necessarily caused further irregularities.
During the construction these work adjustments were made easier by the masonry discontinuity given by the herring-bone bonds, that caused the included parts of masonry to be independent the one from the other. For this reason such adaptations are nowadays nearly invisible when examining the wall.
The marble mouldings at the external base of the lantern are apparently curved, and so they seem to reply the curved line of the corde blande. In fact, the masonry which supports them is straight and the curved line of the mouldings is caused by the brick masonry settling, which was not followed by a correspondent subsidence of the marble covering of the corner ribs. Here the joints between the elements are extremely reduced and so subject to only limited dimensional reductions for plastic deformation or shrinkage. On the contrary, these phenomena are relevant in all the walls of the dome, built with thick joint masonry (Petrini 1989).
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