Brunelleschis Dome by Ross King Read Free Book Online Page B

for the required timber. As the competition was being proclaimed, 32 large tree trunks were delivered to the Opera and cut into 900 feet of planks and 135 stripped beams for use in the scaffolding, centering, and loading platform of the south tribune of the cathedral, now ready for vaulting. The cupola, however, was to be much larger than the tribune and would therefore have required, in one estimate, twenty times as much wood, or as many as 700 trees. 2 The Opera owned a number of forests on the slopes of the Apennines, but timber was rivaled only by marble for its expense and the logistical difficulties of its acquisition, being in short supply and, in the absence of hydraulic saws, extremely labor intensive. It was perhaps an omen that the capomaestro Antonio di Banco died while on a trip in search of timber supplies with which to build the centering for the dome.
    Even if sufficient numbers of good-quality trees could be found, and even if the expense of sawing the wood and assembling the vast structure could be absorbed, other problems would have confronted the wardens. The act of decentering — the removal of the wood from beneath the finished vault — was one of the most hazardous operations in the entire building process. During the Middle Ages the most usual method of decentering was to set the supporting poles of the center’s scaffolding in sand-filled kegs and then, at the time of striking, to unplug the kegs and allow the sand to escape, thereby slowly lowering the level of the wooden framework. This operation may seem simple, but timing was a major problem. Medieval mortars remained “green” for up to a year or even eighteen months, until the water necessary for crystallization had completely evaporated. The centering for the vaults of the south tribune, for example, remained in place for thirteen months, from June 1420 until July 1421, thus tying up a large amount of timber that could have been reused elsewhere — for example, in the loading platform for the cupola. If centering was struck too early, the mortar would still be plastic and its strength insufficient. On the other hand, long-term loadings create a deformation of wood known to engineers as “creep”: if the centering was left in place for too long, the timber would warp beneath the weight of the vault it supported, causing the masonry to shift. This phenomenon was known to the ancient Greeks, who would remove the wheels of their chariots at night, or else prop the chariots vertically against a wall (as Telemachus does in Book IV of the Odyssey ) in order to prevent the wheels from warping under the weight of the stationary vehicles.
    A final difficulty was that the centering for such a massive dome would have been awkward and obtrusive, even when erected in an area as large as the cathedral’s central octagon. Vast in scale, running from the ground to the oculus — the open window at the top of the dome — it would have crowded the octagon and left little room for the masons to maneuver.
    One design for the dome’s centering existed already, a legacy of Giovanni di Lapo Ghini, the capomaestro whose plan for the cupola had lost out to Neri di Fioravanti’s. His wooden model of the centering, executed in 1371, sat inside Neri’s 1367 model. But evidently this model, like that of Giovanni d’Ambrogio, was inadequate to the task.
    By the end of August, barely two weeks into the competition, Filippo had already begun building a brick model of the cupola. The wardens of the Opera appointed four master masons to assist with its construction. They must have been taken aback by what they saw, perhaps suspecting Filippo of preparing a clever illusion like his painting of the Baptistery, one that would deceive the senses and defy the laws of reason. As with his panel, Filippo set about his task with meticulous craftsmanship. For the woodwork he had hired two of Florence’s most gifted sculptors, his friend Donatello and also Nanni di Banco, the