Picture this square with something missing. Same marble, same crowd — but where the dome is, there is nothing. An octagonal wall forty-four metres across, open to the sky, rain falling into the middle of the church. That is what Florence looked like for about forty years, because the city began a cathedral it did not know how to finish and then swore an oath binding its successors to finish it a particular way.
The order things happened in is the whole story
Ground was broken on 9 September 1296, to a design by Arnolfo di Cambio. The building he laid out is 153 metres long and 38 wide, widening to about 90 across the transepts — roughly 8,300 square metres of floor. Pisa had its cathedral; Siena was planning one that would have dwarfed this. Civic pride in fourteenth-century Tuscany was a construction competition.
Then it stalled. Arnolfo died in 1302 and work all but stopped for a generation. What restarted it was a relic: the remains of Saint Zenobius, the first bishop of Florence, found in 1330. The following year the Arte della Lana, the wool guild, formally took over the works through a body called the Opera del Duomo — which still exists, and still maintains this building. Seven hundred years of continuous institutional responsibility.
Giotto was appointed master of the works in 1334 and began the campanile. He died in 1337; Andrea Pisano carried on. In 1348 the Black Death stopped everything.
Work resumed in 1349 under Francesco Talenti, who enlarged the design — which made the space at the crossing wider, and made the eventual problem worse. By 1380 the nave was finished. The old cathedral of Santa Reparata, which had stood inside the shell of the new one for decades like a smaller church wearing a larger one, had been demolished in 1375. You can visit its excavated remains below the nave floor.
And above the crossing: nothing.
1367: the oath
In 1367 a review approved a model by Neri di Fioravanti over a rival design. Fioravanti's dome had no external buttresses — none of the flying buttresses that held up every great Gothic cathedral north of the Alps. Partly this was taste: a buttressed dome looked northern, and Florence wanted to look Mediterranean and classical. Partly, it seems, it was politics.
Then the masters of the works swore an oath to build to that model and not deviate from it.
They bound themselves, and everyone who came after them, to a design nobody on earth knew how to execute.
1418: the competition
On 19 August 1418 the wool guild announced a competition to solve it. The serious contest came down to two goldsmiths: Lorenzo Ghiberti, already famous for winning the Baptistery doors seventeen years earlier, and Filippo Brunelleschi, who had lost that competition and spent the intervening years in Rome measuring ruins.
Brunelleschi won. The guild, being a committee, hedged — appointing Ghiberti alongside him at equal salary. By 1423 Brunelleschi had sole charge, Ghiberti having conceded that the problem was beyond him. The stories about the rivalry are marvellous and most of them come from Vasari a century later, so enjoy them and do not rely on them.
How the dome stands up
Four ideas, working together.
It is two shells, not one. An inner structural dome and a thinner outer weather shell, with a cavity between. Two thin shells weigh dramatically less than one thick one over the same span — and the cavity is where the staircase runs. The 463 steps visitors climb go up the inside of the solution.
It is pointed, not hemispherical. A pointed profile sends more of the load straight down and less of it outward, and outward is the direction that destroys domes.
There are chains inside it. Four horizontal chains of stone and iron are built into the inner dome, working like the hoops on a barrel, plus a fifth chain of timber between the first two. You cannot see any of them.
The brick is laid in herringbone. Runs of flat bricks with vertical bricks set in at intervals, locking each course to the one below and beside it. As the dome curves inward and the wall begins leaning over the mason's head, ordinary brickwork would peel away and fall. Herringbone does not: each completed ring becomes a self-supporting compression hoop the moment it closes.
Which is how it was built without centering.
"Built without scaffolding" is the version you will usually hear, and it is not quite right. Centering is the temporary wooden structure built underneath an arch or dome, in the shape you want, that carries the masonry until it can carry itself. For a 44-metre span starting 55 metres above the floor, the centering would have needed more large timber than Tuscany could supply and would itself have been an unsolved problem. There was still scaffolding — platforms, hoists, somewhere for the men to stand. There was no wooden dome underneath.
Brunelleschi also had to invent the machinery: ox-driven reversible hoists, so the animals did not have to be unhitched to lower a load, and devices for gripping stone. Over four million bricks went up, and the total mass lifted was on the order of 37,000 tonnes, over sixteen years.
On 25 March 1436, Pope Eugene IV consecrated the cathedral. The date was not random: 25 March was the first day of the Florentine year. A motet by Guillaume Dufay, Nuper rosarum flores, was sung in this space under a roof that had not existed when most of the congregation was born.
What happened after he died
Brunelleschi won the competition for the lantern too, began it, and died in 1446. It crawled along for fifteen years under other hands until Michelozzo completed it in 1461. Verrocchio set a gilt copper ball and cross on top around 1470, bringing the total height to 114.5 metres. Lightning brought it down on 17 July 1600; a larger one replaced it two years later.
A teenage apprentice in Verrocchio's workshop watched the machines that raised the first ball and sketched them. His name was Leonardo. A note in one of his Paris manuscripts suggests he may have had some hand in the work itself.
Inside, which surprises people
After that façade the interior is grey stone, white plaster and a great deal of empty air. Three things to find.
The clock, 1443. Painted by Paolo Uccello, above the door you came in. The hand runs anticlockwise, the face has twenty-four hours on it, and they are counted from sunset. It is called ora italica and it was the normal way of telling the time in Italy until the eighteenth century. The clock is not broken; it keeps a kind of time we stopped using.
Two horsemen that are paintings. High on the left wall, Uccello's memorial to the English mercenary John Hawkwood (1436) and Andrea del Castagno's to Niccolò da Tolentino (1456). They are flat paintings made to imitate carved equestrian monuments. Florence wanted to honour its hired soldiers. Florence did not want to pay for bronze.
The Last Judgement overhead. About 3,600 square metres, begun by Vasari in 1572. He died two years in, having finished the upper section; Federico Zuccari completed the rest. Vasari worked in true fresco, into wet plaster; Zuccari worked dry, on top. The two halves have aged differently, and you can see the join if you know to look.
The cracks, which are real and counted
Cracks appeared during construction, and possibly after an earthquake in 1453. The first written report is from 18 September 1639. By 1694 Nelli and Viviani had measured cracks 29 mm wide and correctly blamed the dome's own weight. A proposal in 1695 to wrap four iron belts around it was rejected.
In 1934 the engineer Pier Luigi Nervi noticed that the cracks open in winter and close in summer. The dome has no expansion joints, because nobody built expansion joints in the fifteenth century. So it made its own.
A survey in 1984 counted 493 cracks in four categories, the largest about 60 mm wide in two of the eight webs. The rate of widening works out at roughly 3 mm per century. Since 1987 an automated system of 166 instruments — thermometers, displacement transducers, plumb lines, piezometers watching the groundwater — has read the structure every six hours.
Somebody knows, right now, exactly what it is doing.
And the façade is Victorian
Arnolfo's front was never finished — only ever completed at the lower level — and in 1587–88 Bernardo Buontalenti pulled it down for Grand Duke Francesco I, on the grounds that it looked outdated. The cathedral then stood with a bare brick face for nearly three hundred years.
A competition in 1864 was won by Emilio De Fabris in 1871, and the polychrome neo-Gothic façade you photographed was built between 1876 and 1887. It was designed to match the Baptistery and the campanile on purpose, and it does the job so well that nearly everyone assumes it is medieval.
The oldest-looking thing in the square is the newest, by about five hundred years.
Seven things almost everyone believes that aren't true
| Claim | Verdict | What is actually the case |
|---|---|---|
| The dome was built with no scaffolding at all | False | It was built without centering — a temporary wooden dome beneath it. There were platforms, hoists and working scaffolds |
| Giotto designed the cathedral façade | False | Giotto designed the campanile. The original façade was begun about twenty years after his death, and the present one dates from 1876–87 |
| The façade is medieval | False | Neo-Gothic, by Emilio De Fabris, built 1876–87 |
| Brunelleschi wrote down how he did it | False | No complete account by him survives. He was famously secretive, and the method was reconstructed later, partly from the brickwork itself |
| The cracks mean the dome is in danger | Misleading | They are counted, categorised and monitored continuously. They open in winter, close in summer, and widen a few millimetres a century |
| Brunelleschi won the 1401 Baptistery competition too | False | Ghiberti won that one. Losing it is why Brunelleschi was an architect at all |
| It is the largest dome in the world | Partly | It is the largest masonry dome. Larger domes exist in other materials |
One last thing
Brunelleschi is buried in this building — not in a chapel, but down in the excavations under the floor, where his tomb was identified in 1972. Very few architects have ever been allowed to lie inside the thing they made. He is about ninety metres below the middle of his own dome.
Hear this one standing in front of it
Wanderlight's Florence guide covers 4 landmarks and runs 1 hr 13 min in total. It plays offline, keeps going with the screen off, and doesn't ask you to make an account.
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Written from Wanderlight's research dossier for Florence Cathedral — the same one the narration is written from. Where the evidence is thin, this page says so rather than rounding the uncertainty up.