di giro in giro
“Circle by circle.” This expression no doubt alludes to the technique of bricklaying at Santa Maria del Fiore: the process of waiting for the mortar of one course of masonry to dry before laying another. But even allowing for metaphor and poetic license, it is still a slightly odd description if we consider that the dome is octagonal and not circular, a fact apparent to anyone who sees it. The difficulty in raising the dome lay in the fact that it was not circular. So what does Alberti mean when he speaks of a circular dome contained within the thickness of the polygonal one, or Strozzi when he says that the dome was built “circle by circle”? The engineering, ingenious as it was, would not alone have been enough to stop the dome collapsing inward. Filippo’s real stroke of genius was in creating a kind of circular skeleton over which the external octagonal structure of the dome took shape. That is, the dome was constructed so that it contained within the thicknesses of its two shells a series of continuous circular rings. The inner shell of the cupola, as we have seen, is the thicker of the two, measuring between seven feet at its widest and five feet at its narrowest. With these dimensions it was large enough to incorporate into its center a complete circular vault roughly two and a half feet thick. Rowland Mainstone, the English structural engineer who determined this form following a survey in the mid-1970s, explains that the inner dome was constructed “as if it were a circular dome… but with parts cut away from both the inside and outside to leave the octagonal cloister-vault form.” The herringbone bond was then used to secure the bricks that protruded forward of this circular ring — that is, those bricks on the inner shell that were not part of the horizontal ring (King, 2000). (King, 2000)