Transcript [pt 2]
Building the Dome
Building the walls of the Pantheon’s rotunda was a relatively straightforward task. There was nothing straightforward about building the dome. As far as we know, the Romans had never attempted to build a dome of remotely similar scale. Yet, although they lacked a mathematical understanding of the static forces involved, they understood that the dome of the Pantheon, like all domes, was subject to both downward and horizontal thrust — that, in other words, it would tend to fall in and push out. The architect of the Pantheon managed horizontal thrust (that is, prevented the dome from spreading or pushing out the building beneath it) by making the walls of the rotunda extremely thick and embedding the lower third of the dome in their mass. Rings of concrete were laid on top of the dome to further counteract hoop stress. Downward thrust — the dome’s weight — was carefully minimized. As it rose, the thickness of the dome was tapered from 5.9 to only 1.5 meters. The concrete was mixed with progressively lighter aggregates, with only volcanic scoria — so porous and light that it floats on water — used around the oculus. The oculus itself strengthened the dome, both by obviating the need for a structurally dangerous crown and through its masonry rim, which functioned like the keystone of an arch. The lower reaches of the dome could have been built without centering. Scaffolding, however, would have been required to support the rest during construction. We should probably imagine a wooden structure with trusses like those used to support bridges and basilica roofs.
The Portico
Once the rotunda was completed, it was time to construct the portico. This was a traditional colonnaded structure, of a type that the Greeks and Romans had been building for centuries. It was octostyle — that is, had eight columns across the front — with another eight columns behind. As in most Roman buildings, the capitals were Corinthian. Unlike the Greeks, the Romans preferred monolithic columns, cut from a single piece of stone. The Pantheon’s columns are made of Egyptian granite, shipped down the Nile and across the Mediterranean from the quarries of Aswan and Mons Claudianus. The Pantheon’s columns are 40 Roman feet tall. Though impressive enough from the ground, these columns seem to be too short for the building. This fact, together with the existence of a secondary cornice line above the roof of the portico, has led some scholars to speculate that the Pantheon was designed for columns 50 Roman feet tall, which were either lost in transit or diverted to another building project.
Decoration
Once cranes had raised the columns and the Greek marble capitals and pediment blocks into place, the process of decoration began. The portico was roofed with trusses and sheets of cast bronze, from which a vaulted bronze ceiling was suspended, and gilded bronze tiles were laid over the dome. A bronze eagle, carrying a wreath, seems to have been emblazoned on the pediment. The exterior of the rotunda was covered with white marble and stucco, and the interior encrusted with a dazzling array of exotic stones, their rich colors and variety proclaiming the wealth and vastness of the empire. Colossal statues of Augustus and Agrippa were likely displayed in the niches on either side of the entrance. More statues, presumably of gods and emperors, must have stood in the alcoves and aediculae of the interior. The inner face of the dome was patterned with 28 vertical rows of coffers — possibly significant, since 28 was an Archimedean perfect number. The coffers’ brick faces were coated in plaster, and likely decorated with rosettes of gilded bronze, stars for the artificial heaven of the dome. Over the past 19 centuries, the Pantheon has been despoiled, neglected, redecorated, and occasionally even repaired. But alone among the great monuments of the emperors, it remains substantially intact, the most compelling extant witness to the building technology of ancient Rome.
Now you did it! Sunday morning following references in this fine article. Down the rathole I went...20 minutes learning about the quarries of Aswan and Mons Claudianus, who lived and worked there, how stones were quarried, shaped and transported, the diet of the workers, trade as far away as India to bring foodstuffs to the quarries, how they got sufficient protein, the high lifestyle of people at the quarries.
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Thx....enjoyed it.
A very impressive structure. There are photos on Google Earth, tho not as many of the dome.
Concrete does not dry - it cures. The water molecules in the mix form the chemical bond. Evaporation actually weakens the hardening concrete.