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 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.
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?
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.
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.
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.
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.
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 that 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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Back to our topic.
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.
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.
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.
I have a new book, titled Insane Emperors, Sunken Cities, and Earthquake Machines.
It’s a sequel to Naked Statues, Fat Gladiators, and War Elephants; and like Naked Statues,
it answers frequently-asked questions about the ancient Greeks and Romans with short and
hopefully humorous essays.
It’s available for pre-order now on Amazon, Barnes & Noble, and through your local bookstore.
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