The XVth-century Cupola or dome to the cathedral of Florence, designed by and built under the supervision of Filippo Brunelleschi is still the largest brick dome on earth with an interior diameter of 45 meters (135 feet). The cupola is a feat of engineering by which Brunelleschi succeeded solving a number of difficulties which had prevented any attempt to raise a dome above the very high drum on which it was to rest, particularly the fact that, given its size, this dome could not be built using beams to support the structure as it rose.
Although having been projected in 1296 by Arnolfo di Cambio, Florence’s Cathedral – Il Duomo – remained domeless for centuries.
The foreseen dome (“cupola” in Italian) was greater than any other that had ever been built, leaving architects baffled as to how to construct di Cambio’s ambitious project.
Several designs were created, but eventually rejected for reasons including a lack of know-how and indeed materials to actually implement them.
In 1367, Neri di Fioravanti created the model for the dome that would eventually be constructed. It broke away from Medieval Gothic design and is said to have sparked a new Renaissance styling. With an opening similar to the Pantheon in Rome, Neri came up with the idea of having an inner and outer dome balanced together. All that was missing was a means to construct it without it collapsing due to “hoop stress”.
Alas, the dome remained unbuilt for decades, until along came Filippo Brunelleschi (1377 –1446).
Brunelleschi studied various dome structures, including that of the Pantheon, and came up with a unique solution to the problems of building Neri’s Cupola design.
The major problem he had to face was the proportions of the hole to fill. The octagonal dome base was so wide and so high, such as had never been built before. It would be difficult to construct a dome that could cover the hole, and not collapse on itself.
Not wanting to have anyone else be given the project of carrying out the building of the dome using his ideas, Brunelleschi created an incomplete model (that still exists today) using bricks to show what he could do.
This secrecy continued throughout construction of the Duomo’s dome. Although throughout the centuries, technology has allowed for an accumulation of knowledge, much of the construction remains a mystery.
Brunelleschi’s two domes structure has a delicate balance between the inner and outer shells, with the thickness of each and the spaces between perfectly calculated to provide mutual support.
He used a series of stone chains to balance the dome as it was constructed of iron, bricks (around 4 million of them!) and mortar – all without any internal scaffolding. The end result is slightly steeper than that foreseen by Neri.
Hoisting devices were invented by Brunelleschi to take up the near-40,000 tons of material using specially-created machinery that inspired much of the hoisting technology still in use today.
On top of the dome is a roof lantern, an ornate structure that serves as a skylight to allow natural light to enter into the dome. During construction, this was quite a contentious point and Brunelleschi had to bid for the right to build it against other esteemed local architects and engineers. Alas, it was not completed by the time of Brunelleschi’s death in 1446.
For the next 15 years, many others tried to complete the roof lantern, each making slight adaptations to the original design. Eventually it was completed in 1461 by Michelozzi, a friend of Brunelleschi, with a large copper ball placed atop.
In the year 1600, lightening struck the ball, knocking it down. It rolled down the side of the cupola. A round stone piece was placed on the ground where it struck to commemorate the event.
The ball was eventually replaced two years after, with an bigger ball that came from the workshop of sculptor Andrea del Verrocchio, whose apprentice at the time was none other than Leonardo da Vinci. This may have been what sparked his lifelong interest in studying, drawing and designing machines such as those invented by Brunelleschi.
A statue of Brunelleschi was placed just by the side of the Duomo, from where he glances eternally up at his amazing construction.