Among the stonecutters at work on the dome of Florence Cathedral, Filippo Brunelleschi took a knife to a turnip, carved the shape of a stone that did not exist yet, and handed it to the man who would cut the real one. Giorgio Vasari, writing more than a century later, says Brunelleschi made such models in wood or wax or turnip "hastily cut on the spot," so each stonecutter could see how his block would meet the next.
The turnips settled small questions. The large one had hung over the city for decades: a hole in the cathedral roof about 45 meters across, which Florence's plans required to be closed without the flying buttresses of northern rivals such as Milan. In 1418 the cathedral's overseers announced a competition. Nobody could say whether a dome that heavy would hold itself up, or whether Tuscany had enough timber for the wooden centering that carries a vault until its mortar sets.
Brunelleschi, a goldsmith by training, answered with bricks. With the sculptors Donatello and Nanni di Banco he built a large brick model of the dome, and he told the overseers the full-size version could rise with no centering at all. At one meeting, by Vasari's account, they had him carried out as a madman.
In 1420 they approved his plan anyway, under a written program that stopped short of a guarantee. The masons would raise the shells without centering to a fixed height, and past that point, the program said, "in the building, practice will teach what is to follow." Sixteen years later, with more than four million bricks laid and an ox-driven hoist lifting the loads, Pope Eugene IV consecrated the cathedral under what is still the largest masonry dome in the world.
The turnip and the brick model did one job at two scales. Each let a stonecutter or a committee look at an answer while being wrong still cost almost nothing.
