Brunelleschis Dome by Ross King Read Free Book Online Page A

of the actual sky. Finally, he drilled a small hole the size of a lentil bean into the vanishing point of the painting, or that central point on the horizon where the receding parallel lines appear to converge.

    Diagram of the optical instrument used by Brunelleschi to render the Baptistery in perspective. The painting is on the left, the mirror on the right.
    The panel was then ready for demonstration. Standing six feet inside the doorway of Santa Maria del Fiore — on the exact spot, in other words, where Filippo had executed the panel — the observer was to turn the painted side of the panel away from himself and peer through the small aperture. In his other hand he was to hold a mirror, the reflection of which, when the glass was held at arm’s length, showed (in reverse) the painted image of the Baptistery and the Piazza San Giovanni. So lifelike was this reflection that the observer was unable to tell whether the peephole revealed the actual scene that should have been before him — the “real scene” lying beyond the panel — or only a perfect illusion of that reality.
    When the competition for the model of the dome was announced in August 1418, Filippo must have jumped at the chance. In June the aged and infirm capomaestro Giovanni d’Ambrogio, who had been called back into service from retirement in 1415 following the premature death of his successor, Antonio di Banco, had built a model for the cupola’s scaffolding. But this model cannot have been especially inspiring, as the Opera saw fit to invite other attempts only two months later. With the prize of 200 florins at stake, Filippo and eleven other competitors hopefully submitted their models. The 1367 model was still sacrosanct, of course: the problem at hand was its practical execution.
    How to build the invisible supports demanded by the model — the circumferential chains that had been the subject of such debate in 1366-67 — was still a vexed question. Also essential to the project was the resolution of a difficulty not fully considered by Neri and his group: the temporary wooden framework, or “centering,” needed to support the masonry of the dome while the mortar cured. Except in the Near East, where there was a shortage of strong timber, all masonry vaults were (and still are) constructed on wooden frameworks that are supported either by scaffolding or from the ground. In the cases of most small-span arches the process is relatively simple. A timber center is built to the desired profile in order to support the stones composing the arch. This structure has to be both strong enough to bear the weight of the masonry and rigid enough to resist bending under the incremental loading of the blocks of stone. It also has to be easy to remove when the time comes.
    It is sometimes possible to build perfectly spherical domes without this sort of centering because each circular layer of masonry forms a self-sustaining horizontal arch. As one of Filippo’s friends, Leon Battista Alberti, explained in his treatise on architecture, “The spherical vault, unique among vaults, does not require centering because it is composed not only of arches but of superimposed rings.” Each stone or brick, that is, forms part of a horizontal as well as a vertical arch and is therefore held in place by the pressures of the surrounding masonry. But the shape of the cupola in Florence, dictated by the 1367 model, was not circular but octagonal and pointed, meaning that the horizontal courses of masonry would not be continuous, as in a circular dome, but broken at each of the eight corners.

    Wooden centering supporting an arch.
    The construction of a wooden centering for the dome of Santa Maria del Fiore therefore appeared essential. Yet its design presented the wardens with major difficulties, both technical and financial, first and foremost because the centering, like the dome itself, would have to be a structure unprecedented in scale. Innumerable trees had to be found