Of the buildings extant which were built by the ancient Romans, the Pantheon is the largest roofed building remaining. The Pantheon is also perhaps the most spectacular building left in something like the form in which it was intended to be seen. The dome of the Pantheon is one of the major contributors, if not the main source of its splendour. This paper will concentrate on that great dome.
The Pantheon was built on the Campus Martius by the Emperor Hadrian, who probably finished it in about A.D. 128. Although it was restored in Severan times, most of the building is Hadrianic. Probably the main reason the Pantheon still exists is that it was made into the church of Saint Maria ad Martyres in the early 7th Century, by Pope Boniface IV. The name of the building, which is Greek, means "all gods," because it is a temple to all of the Roman gods.
The dome which covers the main part of the Pantheon measures 142 feet in diameter, this is also the measurement of the distance from the floor to the top of the dome (see illus. 2). The inside of the dome is a hemisphere, which is mounted on a cylinder so that if the curve of the dome was extended, it would be tangent with the floor. The lower part of the dome is covered with five rows of 28 coffers each. Past the rings of coffers, towards the top of the dome, there is a bare space, then the central oculus, or opening, which measures 30 feet in diameter.
One of the most important features of the dome is the coffering, which lightens the structural load, and also lightens the dome perceptually. The dimensions of the coffers range from about 7.9 to about 12.5 feet. The top row of coffers has 3 recesses, the rest of the rows have 4 recesses. The space between the tops and bottoms of the coffers remains constant, while the spaces between the sides of each coffer diminish as they proceed up the dome, much like the lines of latitude and longitude on a globe (see illus 1).
All of the surfaces of the coffers which would face an observer standing in the center of the Pantheon are moulded as if part of spheres beyond the sphere of the dome. All of the other surfaces of the coffers, the receding surfaces, are flat planes. The coffers may have been stuccoed and/or gilded in antiquity.
The oculus, which is seperated from the coffered area by a bare space (which may have been gilded), encloses an area of over 700 square feet. Although it is quite a large area, it is less than 5 percent of the inside area of the floor, so it wasn't glazed to keep out rain, but was left open and drains in the floor below dispel what little rain that falls in. The original bronze flashing is still in the oculus, along with three rows of bricks, which form a compression ring to help hold the pressures of the dome. This ring of bricks around the oculus represent the only masonry actually used in the dome. There was originally a bronze cornice in the oculus, but it has since been removed.
The exterior of the dome is covered with lead plates, which replaced the original gilded bronze tiles which were taken to Constantinople in the seventh century. Under the lead plates there is a skin of light pozzolana cement, about 6 inches thick, placed over tiles set in the concrete of the dome.
On the ouside of the dome, extending up the curved surface, there are "steps" constructed of masonry (see illus. 2 and 3). These six "steps" are there to help balance the thrust of the dome on the walls, they are basically dead weight which helps stabilize the structure.
Part of the exterior cylinder actually forms the bottom part of the dome, so the exterior dome does not actually represent the profile of the interior dome. The outside dome would be part of a sphere of a larger diameter than the actual interior dome (see illus. 2). This helps facilitate the thinning of the dome towards the top to lighten the load. The outside vault actually springs from the cylinder higher than the beginning of the inside dome.
The engineering technology required to solve the technichal problems which the construction of the dome posed, was more advanced than anything which had been used before. The execution of the project and the speed of its erection surely indicate that the Pantheon was explicitely planned out in advance. The building materials were graded from the foundation up according to strength and weight, and in the dome, many successive grades of concrete were used, ending with the lightest pumice aggregate. The pozzolana mortar throughout was of the highest quality available.
The dome was probably constructed by erecting a huge wooden hemisphere with wooden negatives of the coffers placed appropriately around it. Successive rings of concrete were poured, interspersed with poured concrete ribs for more strength. The wooden hemisphere was probably supported by scaffolding, although medieval legend says that Hadrian filled the interior with a mound of dirt interspersed with gold to assure it would be hastily removed after the construction was complete.1 The "steps" on the ouside of the dome were probably added as there was room for them, by making terrace walls with wood or brick then filling them in.
The effect of the dome is spectacular. In antiquity, one could barely see any of the dome when approaching from the outside, it was obscured by the porch and the intermediate block, so it was a surprise when one entered the rotunda. Ones eyes are carried up toward the oculus, which lets in enough light to make the whole area seem light and airy. The coffers give the effect of producing a magnitude of lights and darks in the dome, constantly changing as the beam of light and its projected circle from the oculus move with the sun. This gives one the impression that the building is dynamic and not solid and weighty, but light, airy, and uplifting, like the sky outside on a clear day (see illus. 4). The dome seems to expand as one looks at it.
All of the great effects of the dome are magnified by the surprise of the viewer, stepping from the angular, traditional style of he porch and intermediate block into the fabulous uplifting rotunda. It is said that the true effects of the Pantheon cannot be described, they must be experienced.
The dome can be said to symbolize both the heavens of the gods honoured there, and the empire of the Emperor who built this great building. The light and airy dome of the Pantheon, was easily associated with the dome of the firmament of heaven. About the Pantheon, approximately one hundred years after its completion, Dio Cassus wrote, "thanks to its dome, it resembles the vault of heaven itself." One could easily also associate the dome with the all encompassing rule (perhaps "sphere of influence") of the great Roman Empire.
This was he first time a dome was used to roof a religious building in Rome. It was as if the architect lifted the dome out of a bath or palace and used it to create a temple.2 The architect, probably Hadrian himself, was surely influenced by the Domus Aurea. Where the Domus Aurea was the first real playing with space, the Pantheon was the first official public playing with space. One can definitely see that a similarity exists between the octagon room of the Domus Aurea, a dome supported by eight walls, and the Pantheon, a dome supported by eight piers. The Temple of Venus and others at Baiae, which were smaller domed buildings, may also have been influential. Although the architect may have been influenced by many things extant at the time of the designing of the Pantheon, there was no precedent of a domed building of this kind or scale anywhere. The Pantheon set a precedent which has been influential for ages, and will probably continue to be so for ages more.
The dome of the Pantheon was a very impressive marvel of technology and imagination which had not been surpassed until recent times, and even then only with the help of metal reinforcement. The dome and porch combination of the Pantheon has been imitated and used from right after it was built until the present. About 35 years after its construction the Pantheon was used as a model for the domed temple of Asklepios Zeus Soter in Pergamon, and ever since, the Pantheon has been studied, imitated and used as a model for buildings too numerous to name, but including temples, churches, tombs, capitol buildings, and even homes. Brunelleschi studied it to complete the dome of the cathedral of Florence. Thomas Jefferson was influenced by its design when he built Monticello. It's influence can be seen in almost every American State Capitol building. The Pantheon was a major breakthrough in the architectural use of space, and this can be proved by tracing its influence through the centuries.
One can wonder why the architect used a dome. Was it just because it is the best way to make a lasting roof on a cylinder without supporting columns? Did the architect decide to use a dome in a building and then design the Pantheon around it? Did the architect decide to build a spectacular building and think a dome-porch combination would be best? The most likely thing is that the architect wanted to design a building to enclose a space, do it with magnificence, and break from the traditional, so he choose a dome over a cylinder behind a traditional facade, showing his great innovation and creativity. The dome itself is probably the means, not the end, of this architect, to create a spectacular enclosed space.
The dome of the Pantheon was an imporant and very influential innovation, with little to predict it, much to imitate it, but perhaps nothing to equal it.