The foundation stone for Santa Maria del Fiore was laid in 1296. Work ebbed and flowed. Francesco Talenti took over about 1343 and instituted the basic plan. The nave had a ribbed groin vault resting on the nave arcade, providing an emphasis on width instead of height. By 1436 the nave vaulting was completed and work on the east end supporting the dome began. A committee of leading painters and sculptors decided the basic characteristics of the dome in 1367. With a diameter of 143 feet and six inches, this was the largest dome built since the Pantheon in Rome over 900 years prior.
Immediately the task of constructing the dome faced significant challenges. Centering that was typically used for erecting arches and domes was impractical here. The height for the centering was too great for any wood that could be sourced, and, even if the wood could be found it would require so much that it most likely would collapse under its own weight. Another option proposed was filling the nave and transept with earth. This would have provided the support though also have been a massive undertaking.
What emerged instead were mechanical inventions of Filippo Brunelleschi that allowed the dome to be constructed without centering. On August 7, 1420, a celebration was held at 140 feet in the air on the tambour from which the dome would rise. Stone cutters, masons, and laborers at the site ate breakfast to mark the start of construction of the dome. Shortly thereafter construction of a ox-powered hoist began that would raise building materials from the floor of the crossing to the top of the future dome. Over 15 years of service the hoist would help to raise 70 million pounds of materials. A second machine called a castello helped to position materials once they were raised.
The Inner Dome
Four sandstone chains at 35 foot intervals were placed. They were superimposed by iron chains. This provided resistance to horizontal thrust from the weight above. A fifth chain made of wood was installed in wood 25 feet above the first sandstone chain. Originally four additional wood chains were called for though later omitted once construction began. The masonry chains also had to tilt inward to radiate from the vertical center of the dome. Needless to say this required very precise measurement.
By 1426 the second sandstone chain had been completed and the dome had risen to 70 feet above the drum. At this point there was a reappraisal about whether to use centering. Part of the solution as the slope shifted from 30 degrees to sixty degrees was by using a series of uniquely shaped bricks laid in a fishbone bond. Upright bricks interrupt the horizontal course, dividing each layer into segments five bricks long. These were like bookends keying new bricks to the self-sustaining ones beneath as they cured. This bonding method also eliminated the need for centering. Alberti later described how this bonding technique was essential connecting weaker components to stronger ones.
The Outer Shell
The purpose of the outer shell was to provide protection from the elements. The width diminished from two feet at its base to one foot at its apex. A horizontal arch was built on the inside of the domes outer shell, allowing the dome to be brought to completion with greater safety. This amendment was made in 1426 to the cupola project plan. These rings are roughly 3 feet wide and 2 feet high and encircle the dome at 8 foot intervals, the first one above the second sandstone chain. Visible from the walkways between the two domes, they project at right angles from the inside of the outer shell, connecting the corner spurs to the intermediate ribs. Designed as a temporary measure, they likely transferred weight from the outer shell to the inner dome.
By 1434 the walls reached 280 feet above the ground. A year later masons laid the fourth and final stone chain that served as the closing ring at the top of the dome. It would take two more years to clad the dome in terra cotta tile.
The lantern is a distinctive feature of Renaissance domes. The design at Santa Maria del Fiore was octagonal in shape, sitting on a marble platform supported by the sandstone chain. Eight buttresses rise in line with the eight ribs of the dome. These support 30 foot pilasters with Corinthian capitals. Between the pilasters are eight windows, also 30 feet in height. The interior has a small dome above which rises a spire 23 feet high topped by a bronze ball and cross. Inside one of the buttresses a stairway leads to a series of ladders through the spire where it is possible to climb and look out the bronze ball itself at a level 350 feet above the street.
The lantern served as a key strengthening the ribs of the dome. It also allowed for light and circulation to the area below. As an aside Paolo Toscanelli, a well regarded astronomer and mathematician, was given permission to place a bronze plate at the base of the lantern. Rays of the sun would pass through an aperture in its center and fall 300 feet to a special gauge on the floor of the cathedral, a stone in the Chapel of the Cross. By transforming the cathedral into a giant sundial, Toscanelli was able to make detailed calculations. Among other things they helped to make better navigational charts for sailors.
The first stone of the lantern was consecrated by Cardinal Antoninus (later St. Antoninus), the new archbishop of Florence, in March 1446. Brunelleschi would die a month later on April 15 after a short illness. He was given a great funeral in the cathedral and his body laid in state a month at the Baptistery before finally being buried beneath the nave among the ruins of Saint Reparata.
A lighting strike on April 5, 1492, sent several tons of marble cascading into the streets on the north side of the cupola. This was enough to convince Lorenzo de’Medici his death was near, and sure enough he died 3 days later. A series of cracks appeared on the inner shell beginning in 1639. They run vertically from the drum to the oculus cutting through Vasari’s fresco. In the 1970s a large piece of the fresco fell. This caused local authorities to close the area surrounding the duomo to heavy vehicle traffic.
Argan, Giulio C. The Renaissance City. New York, NY: George Braziler, Inc., 1969.
King, Ross. Brunelleschi’s Dome: How a Renaissance Genius Reinvented Architecture. London: Chatto & Windus, 2000.