Florence Cathedral

• 1. Introduction

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  Florence Cathedral, known formally as the Cathedral of Saint Mary of the Flower, holds a title that no other building on earth can claim: its dome is still the largest masonry dome ever constructed. Finished in 1436 after 140 years of building, it stands 114.5 metres tall from the floor of the nave to the top of its lantern. It holds the fifth-tallest dome position in the world. Yet the story of how that dome came to exist is also a story of bitter rivalry, ingenious improvisation, and a structural gamble that engineers are still debating today. How do you build a dome larger than anything ever attempted, with no scaffolding to hold it up, and no formula for the concrete that ancient Rome used to do something similar? That question drove one of the most extraordinary engineering achievements of the Italian Renaissance, and the answers Filippo Brunelleschi invented are still holding the cathedral together, cracking and breathing, five and a half centuries later.

• 2. A Church Outgrown

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  On the 9th of September 1296, Cardinal Valeriana laid the first stone of what would become Santa Maria del Fiore. He was the first papal legate ever sent to Florence, which gives some sense of how significant the occasion was. The city already had a cathedral, Santa Reparata, but according to the 14th-century chronicle Nuova Cronica by Giovanni Villani, the ancient structure was crumbling and no longer large enough for Florence's growing population. The original bishop's seat had been the Basilica of San Lorenzo, consecrated in 393 by Saint Ambrose of Milan, so the city's religious history ran very deep. Florence was also watching its Tuscan rivals. Pisa and Siena had already launched ambitious cathedral reconstructions; Siena's proposed extensions were so enormous they were never completed. The city council approved architect Arnolfo di Cambio's design in 1294, two years before the first stone was laid. Di Cambio was not unknown: he had already designed the church of Santa Croce and the Palazzo Vecchio. His plan called for three wide naves ending under an octagonal dome, with the middle nave covering the footprint of the old Santa Reparata. When Arnolfo died in 1302, construction slowed for almost 50 years. The discovery of the relics of Saint Zenobius in 1330, inside the old Santa Reparata, gave the stalled project new momentum. The following year, the Arte della Lana, the powerful guild of wool merchants, stepped in as the new patron.

• 3. Giotto, the Guild, and the Black Death

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  In 1334 the Arte della Lana appointed Giotto to oversee construction, and the painter made his most lasting architectural mark by designing and beginning work on the campanile, the bell tower that still bears his name. He worked alongside Andrea Pisano. When Giotto died on the 8th of January 1337, Pisano continued the work until the Black Death brought everything to a halt in 1348. Building resumed in 1349 under Francesco Talenti, who finished the campanile and expanded the overall project to add the apse and side chapels. A sequence of architects followed: Giovanni di Lapo Ghini divided the central nave into four square bays between 1360 and 1369; Alberto Arnoldi, Giovanni d'Ambrogio, Neri di Fioravanti, and Andrea Orcagna all contributed at various stages. By 1375 the old church of Santa Reparata had been pulled down. By 1380 the nave was finished. Only the dome remained, and it had been waiting for a solution for decades. The critical architectural decision had actually been made in 1367, when Neri di Fioravanti's model for the dome was chosen over a competing one by Giovanni di Lapo Ghini. That choice to reject traditional Gothic flying buttresses, which Italian architects considered ugly makeshift solutions and which were politically charged because they were favored by Florence's enemies to the north, was one of the first acts of what historians now recognise as the Italian Renaissance.

• 4. Brunelleschi's Gamble

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  On the 19th of August 1418, the Arte della Lana announced a competition to solve the dome problem. The two main competitors were both master goldsmiths: Lorenzo Ghiberti, who had already won the commission for a pair of bronze doors for the Baptistery in 1401, and Filippo Brunelleschi, who was supported by Cosimo de' Medici. Brunelleschi won. Ghiberti was appointed coadjutor, drew a salary equal to Brunelleschi's, and was promised equal credit, even though neither man received the announced prize of 200 florins. Ghiberti spent most of his time on other projects. When Brunelleschi fell ill, or feigned illness, Ghiberti briefly took charge, but reportedly had to admit the project was beyond him. By 1423 Brunelleschi was back in sole control. The technical challenge he faced was stark. The dome Neri di Fioravanti had designed would start 52 metres above the floor and span 44 metres. There was not enough timber in all of Tuscany to build conventional scaffolding for a structure that size. Brunelleschi studied the Pantheon in Rome, but the Pantheon was a single shell of concrete whose formula had long since been lost. His solution was a double shell, a construction method developed under the Seljuk Empire, built from brick because of its light weight and ease of shaping, with the two shells tied together through an elaborate internal system. To illustrate the plan, he built a wooden and brick model with the help of Donatello and Nanni di Banco. That model is still on display in the Museo dell'Opera del Duomo, though Brunelleschi deliberately left it incomplete to keep the construction's secrets in his own hands.

• 5. Chains, Ribs, and Herringbone Bricks

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  Brunelleschi's solution to the dome's spreading force, known as hoop stress, was a set of four internal horizontal chains made of stone and iron, embedded within the inner dome like barrel hoops. A fifth chain, made of wood, was placed between the first and second stone chains. Each stone chain was built like an octagonal railroad track, with parallel rails and cross ties made of sandstone beams 43 centimetres in diameter and no more than 2.3 metres long. The rails were connected end-to-end with lead-glazed iron splices. A magnetic survey conducted in the 1970s failed to detect evidence of additional iron chains that were supposed to reinforce each stone chain, suggesting they may be too deeply embedded to find, or were never installed. Brunelleschi also set vertical ribs at the corners of the octagon, each 4 metres deep, supported by 16 concealed ribs radiating from the centre. These ribs had slits to take beams that held platforms, allowing the work to rise without conventional scaffolding. For the bricks themselves, he used a herringbone pattern that transferred the weight of freshly laid bricks to the nearest vertical rib before the mortar had time to cure. The outer dome presented its own challenge: at only 60 centimetres thick at its base and 30 centimetres at its top, it was too thin to embed horizontal stabilising rings. Brunelleschi solved this by thickening the outer dome at the inside of its corners at nine different elevations, creating nine masonry rings that can still be observed today from the space between the two shells. To raise the 37,000 tons of material, including over 4 million bricks, he invented new hoisting machines and lewissons for lifting large stones. These machines so impressed the young Leonardo da Vinci, who saw them used later when Verrocchio hoisted the gilt copper ball atop the lantern, that Leonardo made a series of sketches of them. A note in Leonardo's own handwriting, in what is called the G manuscript of Paris, reads: 'Remember the way we soldered the ball of Santa Maria del Fiore'.

• 6. Rivals, Lanterns, and a Lightning Strike

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  Even after winning the dome commission, Brunelleschi faced one more competition: the right to crown his dome with a lantern. His design called for an octagonal lantern with eight radiating buttresses and eight high arched windows. He beat out Lorenzo Ghiberti once more, along with Antonio Ciaccheri. Construction of the lantern began a few months before Brunelleschi's death in 1446. For 15 years afterward, progress stalled as various architects made alterations. Brunelleschi's friend Michelozzo finally completed the lantern in 1461. In 1469, the sculptor Andrea del Verrocchio crowned the conical roof with a gilt copper ball and cross containing holy relics, bringing the total height to 114.5 metres. That copper ball was struck by lightning on the 17th of July 1600 and fell to the ground. It was replaced by an even larger one two years later. The drum gallery below the lantern was decorated by Baccio d'Agnolo, but the work was never finished after Michelangelo disapproved of it. Ghiberti's contributions to the cathedral, often overshadowed by the Brunelleschi rivalry, were substantial: he worked on the project for eighteen years, designing stained-glass windows for the oculi in the drum, creating the bronze shrine of Saint Zenobius, and producing marble revetments on the cathedral's exterior. The dome's consecration, by Pope Eugene IV on the 25th of March 1436, was itself a historic occasion: during the ceremony, Guillaume Dufay's motet Nuper rosarum flores received its first performance.

• 7. The Cathedral's Inner Life

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  The Late Gothic interior of Santa Maria del Fiore is vast and deliberately spare. Its relative emptiness reflects the austerity championed by the mendicant religious orders of the 12th and 13th centuries. The cathedral was built with public funds, and this civic character shaped what went inside. Frescoed equestrian portraits honour military leaders who served Florence: Paolo Uccello painted the Funerary Monument to Sir John Hawkwood in 1436, in a near-monochrome pigment called terra verde, chosen to mimic the patina of bronze. Andrea del Castagno painted the Equestrian Statue of Niccolò da Tolentino in 1456, in a style that mimics marble instead. Both painters struggled to apply the new rules of perspective to foreshortening; rather than a single vanishing point, both used two separate points, one for the horse and one for the pedestal. Above the main door, a colossal clock face painted by Paolo Uccello in 1443 shows the 24 hours of the hora italica, a system that counted time ending with sunset at 24 hours, a practice that continued until the 18th century. This is one of the few clocks from that era still in working order. The cathedral's 44 stained-glass windows were the largest such undertaking in Italy during the 14th and 15th centuries, made after designs by Donatello, Ghiberti, Uccello, and Andrea del Castagno. The ceiling of the dome was left whitewashed after completion, until Grand Duke Cosimo I de' Medici ordered it painted. Giorgio Vasari began the enormous Last Judgment fresco, covering 3,600 square metres, in 1572 and completed the upper portion, representing the 24 Elders of the Apocalypse, before his death in 1574. Federico Zuccari and several assistants finished the rest of the cycle by 1579, and these frescoes are considered Zuccari's greatest work, even if the quality varies across the vast surface due to the different hands and techniques involved.

• 8. A Structure That Breathes

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  Cracking in the dome was noticed even before construction was complete, and a strong earthquake in 1453 may have caused the first major fissures. The first written record of cracks appears in a report by Gherardo Silvani dated the 18th of September 1639, which referred to them simply as 'peli', meaning 'hairs'. By 1694 the situation had progressed: Giambattista Nelli and Vincenzo Viviani surveyed the cracks and recorded two major ones with a maximum width of 29 millimetres. A commission headed by Viviani concluded in 1695 that the cracking was caused by the dead weight of the structure and proposed installing four large iron belts around the dome, similar to a repair done on the dome of Saint Peter's in Rome. After long debate, the decision was made to leave the dome as it was. The nature of the cracking became clearer in 1934 when Pier Luigi Nervi, leading a commission for the Opera del Duomo, observed that the cracks opened in winter and closed in summer, as the masonry contracted and expanded with temperature changes. The dome, in effect, developed its own expansion joints. A survey completed in 1984 counted 493 cracks of various sizes, sorted into four categories labelled A through D. The most serious, Type A cracks, are sub-vertical major fissures running through both shells of the dome, ranging from 55-60 millimetres wide in some webs and 25-30 millimetres in others. Analysis published in 1983 by Andrea Chiarugi, Michele Fanelli, and Giuseppetti concluded that the primary cause is the dead weight of the dome itself, estimated at 25,000 tons, pressing against the insufficient resistance of the ring beam. A digital monitoring system installed in 1987 by ISMES, using 166 instruments including 72 displacement transducers and 60 thermometers, records data every six hours. Linear regression analysis of that data suggests the major cracks are widening by approximately 3 millimetres per century, a rate that has not yet produced any catastrophic damage but ensures the dome will be watched for as long as it stands.