The Impossible Dome of the Cathedral of Santa Maria del Fiore


By William Landon, Northern Kentucky University

Florence’s grandest monument is the Cathedral of Santa Maria del Fiore, better known as the Duomo. Back in 1296, Arnolfo di Cambio, the Duomo’s original architect, had designed a building so large that architectural technology was unable to catch up with it. Erecting a dome over Duomo’s gaping hole—145 feet wide and 180 feet high—was impossible.

Image of the Cathedral of Santa Maria del Fiore.
Florence’s grandest monument is the Cathedral of Santa Maria del Fiore, better known as the Duomo. (Image: Artem Kavalerov/Public domain)

The Search for a Genius

Over a hundred years later, the Duomo still stood unfinished in the center of Florence. Its façade remained rough red brick, and the octagonal frame upon which its massive dome now rests was open to the sky. In poor weather, the cathedral’s high altar was exposed to it.

But in 1418, backed by civic confidence, the Opera del Duomo, the workshop that oversaw the century-long construction of the Duomo, was confident that if a competition was announced, somewhere in Europe, an architect could be found to crown their cathedral.

Backed financially by the Wool Merchants Guild, whose members included Giovanni and Cosimo de’ Medici, the Opera del Duomo publicized its competition. Architects arrived in Florence from the German states, France, and England and from across Italy. In March of 1419, they were invited to present their solutions to the Opera del Duomo and to representatives from the Wool Merchants Guild.

This article comes directly from content in the video series How the Medici Shaped the Renaissance. Watch it now, on Wondrium.

Absurd Ideas All Around

One architect suggested that the entire cathedral could be filled with soil and coins so that stonemasons could work on the ground (180 feet above the city streets) as they built the dome. When the dome was finished, Florence’s citizens could remove the soil from the cathedral and keep the treasure they sifted from it—a truly ridiculous idea, which didn’t even specify how the dome itself would be built. 

The other designs were unremarkable, and a number of them relied on building scaffolding from the street level to the dome’s opening—another impossibility.

All of this international brainpower had failed. So the judges turned to someone closer to home. Filippo Brunelleschi had already been ejected from an earlier consultation after he lost his temper and exploded at the committee for not taking him seriously. Now they decided to give him a second chance.

The Egg Incident

Producing an egg and a thin piece of marble, Brunelleschi asked those in attendance if any of them might be able to stand the egg on its end on the marble slab. Apparently, many of them tried, but none of them were successful. Then Brunelleschi cracked the egg and stood it upright so that it approximated the shape of the dome that he proposed to build.

Statue of Filippo Brunelleschi
By successfully building the Duomo’s dome, Filippo Brunelleschi became the most successful, self-trained architect of the early Renaissance. (Image: Richardfabi/Public domain)

The 16th-century artist Giorgio Vasari, the first great biographer of Renaissance artists, described the aftermath of the egg incident:

The craftsmen protested that they could have done the same; but Filippo answered laughing, that they could also have raised the cupola (the dome) if they had seen the model or the design. And so it was resolved that he should be commissioned to carry out this work, and he was told that he must give fuller information it to the Consuls (representatives of the Wool Merchants Guild) and the Wardens of Works.

Shortly thereafter, Brunelleschi did as he was instructed, providing the overseers of the dome’s construction with sketches and a detailed written description of the work that he was hired to undertake. Vasari tells us that no one on the selection committee was capable of fully grasping the genius of Brunelleschi’s plans, but that they trusted his judgment and his abilities to execute the construction of the dome.

Cosimo de’ Medici’s Contribution

What Vasari doesn’t mention, but what historians have since discovered, is that Cosimo de’ Medici, as a senior member of the Wool Merchants Guild, was a driving force behind the selection of Brunelleschi.

Work on the dome began in 1420. As a public works project, it employed thousands of people: from quarry workers to stone transporters, to brick makers (who produced 4 million bricks), to stonemasons who constructed the dome itself. 

In order that the work might be completed to his specifications, Brunelleschi mounted the dome daily to personally illustrate how the bricks were to be laid. His dome remains the largest masonry structure in the world; it weighs an estimated 40,000 tons and towers nearly 400 feet above street level. 

Brunelleschi, a man who trained as a goldsmith and who considered himself a failed sculptor, became the most successful, self-trained architect of the early Renaissance, if not ever. The dome he designed, together with the dozens of other projects that he completed in Florence, is a legacy to his genius.

Common Questions about the Impossible Dome of the Cathedral of Santa Maria del Fiore

Q: Why did the Duomo remain unfinished for over a century?

The Duomo was built in 1296 but remained unfinished because the original architect, Arnolfo di Cambio, planned a building that was too massive for the time. The technology was not advanced enough to build a dome for such a large building.

Q: Why was the international competition to suggest ideas for finishing the Duomo a failure?

It was a failure because none of the architects proved capable of planning a practical solution for building the dome of the Duomo. Some had ridiculous ideas, while others relied on techniques that were impossible to implement.

Q: How was the Duomo finally completed?

After the international search failed, Filippo Brunelleschi was hired to complete the construction of the Duomo’s dome. He successfully constructed the dome, and his dome remains the largest masonry structure in the world.

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