We head up via one of the crossing pillars and walk towards the lantern in between the two shells of the dome. Along a staircase situated on the inner dome wall, we arrive at the lantern where we are given a beautiful view over the city of Florence with its many red roof tiles.
Arnolfo di Cambio had always intended to cover the enormous space of some 45,50 metres with a dome. The problem was that no one really knew how to place a dome over such a wide space.
In 1366, the board of the Opera del Duomo orders the construction of two dome models. One is a quite traditional design by master building Giovanni di Lapo Ghini. The other model, which would end up winning, was by a group surrounding Neri di Fioravanti. Di Lapo Ghini’s model was based on the aerial arches and buttresses that were commonplace in Gothicism. This wasn’t particularly popular in Italy. Quite a number of Italian architects saw aerial arches as a rather ugly aid. What was remarkable about the winning model is that it was not being supported by aerial arches. And, quite unusually so, the dome would become double-walled. Fioravanti’s model was deemed the best during a referendum. The model was placed in the aisle of the Duomo for quite some time and after Brunelleschi had built his dome it became a urinal. Despite Neri di Fioravanti’s model, no one really knew how to build that kind of dome.
On 19th August 1418, the ‘construction firm’ wrote down the following problem:
‘Whoever desires to make any model or design for the vaulting of the main dome of the Cathedral under construction by the Opera del Duomo for armature, scaffold or other thing, or any lifting device pertaining to the construction and perfection of said dome or vault — shall do so before the end of the month of September. If the model be used he shall be entitled to a payment of 200 gold Florins’
Contenders included Filippo Brunelleschi and Lorenzo Ghiberti. These two artists had been competitors before when trying to land the assignment for the doors of the Baptistery. Vasari, but also Brunelleschi’s biographer, Antonio Manetti, delve into the affairs of these rivals and the construction of the dome. The design of Brunelleschi was revolutionary as he wanted to construct the double-walled dome without centring.
Centring is a type of falsework: the temporary structure upon which the stones of an arch or vault are laid during construction. After the cement dried, a process that often took many months, the wooden construction was removed. The shape of the dome would ensure the stones would support themselves. Ghiberti, who lacked experience and knowledge of complicated constructions, insisted on using centring. No one had ever constructed a vault without wooden falsework. Let alone a dome that spanned some 45,5 metres. Another issue is that Fioravanti’s old model of some 50 years prior lacked the semi-dome shape that you would see at the Pantheon, for instance. The arch didn’t have to be semi-round, but a bit pointed: the quinto acuto arch (pointed fifth). This was an extra handicap. How would they ever construct it without centring (the tholobate alone was nine metres high) at a height of fifty-two metres?
Vasari gives a detailed account of the meeting where the commission wanted to reach a decision on what model to construct:
Who, no matter how proud or jealous, would not praise Flip [Filippo] at seeing this enormous construction that rises to the heavens, wide enough to cast a shadow over all citizens of Tuscany and built without buttresses or wood supports [scaffolding from the ground up], a work of art that, if I recall, was deemed impossible in our time, while the ancients might not even have been able to conceive of it?” Translated from: Giorgio Vasari ‘De Levens van de grootste schilders, beeldhouwers en architecten’ Deel I blz. 166-167
The commission responded in disbelief. They immediately asked how Filippo Brunelleschi intended to do this. Brunelleschi, who always feared someone would copy his designs, refused further explanation, but according to Vasari he said: ‘[…] Dear sirs, please understand that the dome cannot possibly be constructed in any other way, and while you may mock me, you will understand that this is the only way, should you refrain from stubbornness, that this dome will be built.’ Translated from: Giorgio Vasari ‘De Levens van de grootste schilders, beeldhouwers en architecten’ Deel I blz. 167
Brunelleschi only received more mocking laughter and was forcefully removed from the hall. According to his biographer Manetti: ‘He had a feeling they said behind his back: ‘Look at that crazy man, talking such rubbish.’ Manetti, A., The life of Brunelleschi, Pennsylvania State University Press, University Park and London 1970 641-642
After some laughter of those present, some of them gave it a shot but of course failed miserably. Until Brunelleschi walked forward with confidence, picked up an egg, tapped it onto the plate and whilst it was broken on the bottom, it none the less stood upright. Those present exclaimed that if they knew that was allowed, they’d have done the same thing. Filippo answered cleverly: “if you looked at my model and design, you would also be able to build the dome.’ Translated from: Giorgio Vasari ‘De Levens van de grootste schilders, beeldhouwers en architecten’ Deel I blz. 169
The commission finally yielded, according to Vasari, and they granted him the assignment for the dome. It’s quite likely that this egg story isn’t completely factual. Manetti makes no reference to this story in his biography about his friend Brunelleschi. What is for certain, however, is that Filippo Brunelleschi’s model would be carried out.
Unfortunately for Filippo, the assignment would not be without obstacles, revealing that the clients were not entirely reassured:
1. Much to his shock, his arch rival Lorenzo Ghiberti was named capomaestro alongside him.
2. Not using the centring would apply only for the first thirty braccia, or 17.5 metres (1 bracchio is 58.36 centimetres; derived from an arm length).
3. After the first thirty braccia of the pointed dome, the curvature became sharper: sixty instead of thirty degrees at the start of the dome. At reaching a height of 17.50 metres, they would re-examine how to mason the remaining 14.40 metres.
One of the biggest issues faced by Brunelleschi was how to get the massive marble bricks and sandstone up to a height of fifty-two metres. Careful estimations would place the weight at some thirty million kilos. For this purpose, Brunelleschi developed a clever hoist driven by oxen. What was unique about it is that it enabled a swift transition from upward movement to downward movement, while the oxen simply continued their path in one direction, clock-wise. In addition, weights up to seven-hundred kilos were hoisted up effortlessly. Later, Brunelleschi developed another hoist that could lift large and heavy beams in all directions and accurately place them, too: the castello.
A twelve-page document, likely by Filippo Brunelleschi himself, accurately describes how the dome was to be constructed. There was consensus about two things ever since the model of 1366 by Neri di Fioravanti:
1. The dome had to be double-walled.
2. The forces were not to be absorbed by gothic-style support arches, but by rings and chains in the dome walls.
The three exedras did function as buttresses, but they looked much nicer than the usual gothic cross brace system. The latter (point 2) means that the required support to stop the outward force was kept from view.
The two walls of the dome were raised simultaneously. There were eight masonry shifts; one working at each side of the octagon. Heavy marble ribs were placed on the corners of each side: eight in total. Another two ribs were placed between the two walls on each plane, creating a total of 24. The inner wall started at the tholobate, with a diameter of 2 metres, but down to 1.5 metres near the top. The outer wall began sixty and ended at thirty centimetres. Like in the Pantheon, heavy materials were used at the base of the dome wands and lighter materials including pumice at the top. This was done to reduce the pressure put on the dome.
By implementing a chain of heavy sandstone, but some made of wood as well, into the wall surface, the dome was kept in place and the gigantic horizontal forces were kept in check. The two rows of chains were supported by hardstone beams in the same way sleepers carry rails.
This can still be seen on the outside of the dome. The nine rings that move horizontally have a similar function as the sandstone chains. The corner ribs, the ribs between the shells and the many connecting arches keep the two walls firmly together.
The brickwork was proceeding nicely. One problem was that after each walled layer, the builders had to wait for the cement to harden; after all, they did not use scaffolding and each horizontal layer had to support itself.
Vasari, but Manetti too, describe how Brunelleschi eliminated his rival Lorenzo Ghiberti as fellow capomaestro (Manetti, di, A., The life of Brunelleschi, Pennsylvania State University Press, University Park and London 1970 880 e.v.; Vasari, G., ‘De Levens van de grootste schilders, beeldhouwers en architecten’ Van Cimabue tot Giorgion, Contact, Amsterdam 1990 deel I 173 e.v.).
One day, Brunelleschi reported in sick and stayed in bed. Work was halted. Ghiberti had no idea what to do and provided vague answers to masons who wanted to know how to continue. The board went towards Brunelleschi’s home and pleaded for him to continue work as the masons were all stuck. That is how it became known who was the expert and who wasn’t. Later on, it became evident that Ghiberti does a sloppy job at constructing the stone chain. His rival, needless to say, endeavoured to make this fact public. Ghiberti played his part as well. Brunelleschi was arrested for not being a member of the masonry guild, which was in violation with the laws of that time. Presumably, Ghiberti or one of his friends was behind this. By interference of the board, Brunelleschi was soon released. Lorenzo Ghiberti wrote an angry letter to the board of the Opera del Duomo, blaming Brunelleschi for not adhering to the old model by Neri di Fioravanti. Ghiberti accused Brunelleschi of being incompetence and hubris of those responsible for its construction. He called him a dreamer.
But the board did trust the dreamer: Filippo. His miraculous hoists left a great impression. In fact, Filippo Brunelleschi ultimately received the responsibility for the project and Ghiberti had to settle for a far lesser role.