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Transcript [pt 1]

Introduction

The Pantheon remains elusive. Despite its exceptional preservation, its original function is uncertain. And despite centuries of research, there are unanswered questions about how this structure, crowned by the largest unreinforced concrete dome in history, was built.

The Purpose of the Pantheon

The original Pantheon was constructed by Augustus’ friend and advisor Agrippa. Its dedicatory inscription can still be read on the façade of the present temple, but few details are known about the building itself, which was destroyed by fire during the reign of Trajan. Its replacement, the current structure, was completed under Hadrian. On the basis of scattered textual references, most scholars believe that the Pantheon was not, or not just, a temple for all the gods — as its name might suggest — but a place where emperors past and present were revered alongside the gods. At least occasionally, the Pantheon seems to have also functioned as an audience hall where emperors heard cases and imperial decrees were read.

The Design

Whatever its original function, the Pantheon’s design is unique. It consists of a rotunda — a huge circular room crowned by the famous dome — fronted by a colonnaded portico, with a transitional block between. The symmetry of the rotunda, whose interior diameter and interior height are both exactly 150 Roman feet, has impressed architects since the Renaissance. But the awesome impression made by that space, where the checkerboard floor seems to mirror the spiraling coffers of the dome, and the golden seal of the oculus sweeps in majesty over marble walls, is beyond purely mathematical analysis. How did the Romans create this masterpiece?

Roman Concrete

Any answer has to begin with concrete. As I describe in a few of my other videos, Roman concrete was not poured like its modern counterpart, but laid down, almost dry, in thin layers over beds of aggregate. It cured incredibly hard, even underwater — seawater, in fact, made it stronger. It built walls and foundations that were rock-solid and dirt-cheap. And it enabled the creation of vaults and domes that revolutionized architecture.

Quarries, Contractors, and Workforce

By the reign of Hadrian, Roman building technology had reached its peak. Besides concrete of consistently high quality, the builders of the Pantheon had access to bricks of standard sizes, churned out on a semi-industrial scale in Rome’s suburbs. They could also rely on pre-cut columns and shaped blocks of exotic marble, which were shipped to Rome in huge barges from quarries owned by the emperors. The architect of the Pantheon is unknown. Many scholars believe that Apollodorus of Damascus, Trajan’s favorite architect, was responsible. Although this cannot be proven, correspondences between the Pantheon and buildings that Apollodorus is known to have designed are suggestive.

Large structures like the Pantheon were built by experienced contractors. These firms seem to have usually had a core staff of master craftsmen, who directed a shifting cast of day-laborers and seasonal workers over the course of a project. Contrary to what you might assume, most of the laborers were free men, while some of the master craftsmen were slaves. At any given time, there were perhaps two or three hundred men at work on the Pantheon — a considerable number, but a far cry from the thousands involved in building the Colosseum or the Baths of Caracalla. With the usual breaks for winter weather and allowing time for all that concrete to cure, the Pantheon could have finished in six or seven years. The Pantheon’s foundations were made of concrete.

Foundations

Beneath the rotunda, they formed a ring 7.3 meters wide and probably at least 7 meters deep (no sounding has ever reached the footings, so we don’t know for sure). On this mighty mass of concrete, the walls of the rotunda rose, faced with brick. Roman bricks looked more like thick tiles than modern, rectangular bricks. Most of those used in the Pantheon were bipedales or sesquipedales — that is, two feet or one and a half feet square. These were often cut in half to make triangles, whose angles extended into the concrete behind. The concrete fill was set down in the usual way, layer by layer over fist-sized chunks of travertine.

Building the Walls

At intervals of about 1.2 meters, bonding courses of bipedales (the two-foot square bricks) were laid across the thickness of the wall. Notionally, the walls of the rotunda were 6.2 meters thick; but only a few sections were solid all the way through. A series of eight deep recesses — the entrance and seven niches, alternately rectangular and semicircular — indented the interior at floor level, and chambers punctuated the fill above. These had a practical purpose. Although Roman concrete can cure without exposure to air, air facilitates the process. And as Roman concrete sets, it generates heat. The recesses and chambers in the Pantheon’s walls released heat and admitted air, speeding the drying process.

Relieving arches and buttresses

The relieving arches — the parabolas of brick visible on the rotunda’s exterior — seem to have also had a dual function. They helped to direct the gargantuan weight of the rotunda toward the masonry “piers” between the recesses. And since the arches, made almost entirely of brick, set much more quickly than the concrete fill in which they were embedded, they stiffened the structure as it rose. Despite these innovations, the Pantheon’s rotunda settled during construction — you can still see this in the floor, which sags almost half a meter around the edges. As the concrete set, huge cracks appeared in the walls, including one opposite the entrance that was up to 7 centimeters wide. It was likely in response that a buttress was built from the neighboring Basilica of Neptune to brace the structure before the builders embarked on the gigantic challenge of the dome.

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