Secrets of the Duomo

JOURNAL: ISSUE 2 - 2014

By Don Hunt, Training Director, Local 15 Missouri/Kansas/Nebraska

Florence’s Cathedral of Santa Maria del Fiore and its majestic Duomo. The base of the actual Duomo is 175 feet above the floor of the Cathedral, rising to a height of about 300 feet. The distance across the octagonal base is 147 feet.

Editor’s Note: In April 2013, a team of BAC bricklayers comprised of IMI National Director of Apprenticeship and Training Robert Arnold, IMI North Central Regional Safety and Training Director Dave Wysocki, Training Directors Don Hunt of Local 15 Missouri/Kansas/Nebraska and Tom Ward of Local 2 Michigan, and National Job Corps Director Jonas Elmore, traveled to Florence, Italy to help unravel the building secrets of the majestic Duomo of the city’s famed Cathedral of Santa Maria del Fiore. The BAC/IMI team’s travel to Italy and participation in this project was entirely financed by National Geographic in connection with its television special that aired in February on PBS’ NOVA series. The following first-person account of the project, written by team member Don Hunt, who also serves as Vice President of Local 15 MO/KS/NE, is reprinted with his and the Local’s permission and the Journal’s gratitude.

It was a real honor to be invited by the International Masonry Institute (IMI) to participate in this project for National Geographic and NOVA.

This article tells the tale of the magnificent masonry dome topping the great cathedral in Florence, Italy. It is the unparalleled achievement of the enigmatic Renaissance genius, Filippo Brunelleschi. We explored the mystery of how he was able to build the world’s largest brick and mortar dome when the level of technological know-how at the time should have made it impossible. The Duomo, or in English, the Dome, was constructed between 1418 and 1436 of over 4 million bricks, and Limestone weighing approximately 37,000 tons. It is the largest masonry dome ever constructed, and, was built without any “centering” or support system.

The octagonal dome is 4/5 pointed arch, the distance from opposing corners divided by five, using the fourth point as the center for the opposing side.

The “flower” is an arc between the 4/5 points. This arc creates a horizontal arch between each corner and is one of the two key reasons the Dome is self-supporting.

Massimo Ricci, a professor of Architecture at the University of Florence, has made it his life’s work to decode the mystery of how the Dome was constructed. Professor Ricci was granted permission by the city of Florence to build a 1/5th scale model in a city park. Since 1989 he and his students have used the model to prove his theories.

The vertical bricks in the herringbone pattern are called spines; they continue to rotate in a counter clockwise direction.

The vertical bricks in the herringbone pattern are called spines; they continue to rotate in a counter clockwise direction.

Until we arrived last April, no masons had worked on the model, only students. Our mission was to use the geometry and control measures, which, through Ricci’s research, were determined to be used and apply actual bricklaying techniques on the model to validate those theories. The pictures on this page help tell the story in greater detail. 

The angle and slope of these spines are determined by pulling a line from the spine through the center, which is established by the two opposing corner lines, to the flower. Once this angle is established, the line is pulled tightly and is lowered just to the point at which it touches the two opposing corner lines, which determines the height and slope.
A platform at the base of the model is where the flower was laid out, with lines or “ropes” attached to points along the flower that were pulled taunt.
This process is repeated for each of the spines, which are laid first. Once the spines are placed, the course of brick between the spines can be laid.

Because of the model’s 1/5 scale, bricks were not used and instead, we used readily available Quarry tile that measured 5/8” x 3”x 6”. When our team arrived on-site, the model’s height at that time featured roughly 2000 units per course, with each course laying to about 1”. It took a few days to develop a system that we believed had been used during the original construction of the Dome. After establishing that, four of us were able to complete about one course per day.

Once a segment is completed, the lines must be moved to the next segment and the process is repeated until all eight sides are completed. This is one course. To start the next course, the lines are raised at the corners by one course height and the whole process is repeated. The Dome consisted of two wythes, separated by about 4.5 feet; the interior wythe was 10 feet thick and the exterior was about 3.5 feet thick. It also featured “corner ribs” at each corner and two “interior ribs” between each corner of solid masonry that tied the two wythes together. These ribs supported the thinner exterior wythe.

At the height we began, the angle of slope from the inside to the outside at the corners was at about 30°; this angle increased with the height.

Aside from the ingenuity involved in designing the Duomo, Brunelleschi was faced with how to hoist tons of materials hundreds of feet. He designed hoisting devices powered by oxen. Because of the difficulty in convincing oxen to walk backwards, he designed the first clutch and reversible mechanism.

Once a segment is completed, the lines must be moved to the next segment and the process is repeated until all eight sides are completed. This is one course. To start the next course, the lines are raised at the corners by one course height and the whole process is repeated. The Dome consisted of two wythes, separated by about 4.5 feet; the interior wythe was 10 feet thick and the exterior was about 3.5 feet thick. It also featured “corner ribs” at each corner and two “interior ribs” between each corner of solid masonry that tied the two wythes together. These ribs supported the thinner exterior wythe.

It may seem mind-boggling to truly grasp the magnitude of this engineering miracle without seeing the actual Duomo in person, but it is truly an amazing feat. And even aside from the Dome, the Cathedral is remarkable in itself. Inside and out, the amount of marble quarried and brought to the site, hand carved and erected is incredible.

This truly was an experience of a lifetime.

To purchase a DVD of NOVA’s “The Great Cathedral Mystery”, go to www.pbs.org/wgbh/nova/ancient/great-cathedral-mystery.htm.

EDITOR’S NOTE: As illustrated in these photographs, Brother Hunt and his fellow BAC/IMI masons were granted extraordinary access to the interior of the actual Duomo to gain insights into its ingenious construction techniques.