Rome

Pantheon was finished.

The Greek word pantheion means “temple of all the gods,” which makes the building polytheistic. Cassius Dio, a Greco-Roman senator writing some seventy-five years after the present Pantheon was built, opined, “It has this name … because its vaulted roof resembles the heavens.”

In the audacity and thoroughness of its engineering, in the grand harmony of its proportions, and in the eloquent weight of history with which it is imbued, the Pantheon is certainly the greatest of all surviving structures of ancient Rome. The Colosseum exceeds it in mass and size, but it is the form of the Pantheon that elicits one’s amazement: that huge dome, opened at the top by an oculus which seems not merely to show but to admit the sky, is a landmark in the history of construction and, one might add, of architectural metaphor. Even today, almost two millennia after it was finished, the alert visitor is likely to be struck less by its great age than by its inexhaustible newness. This is truly Roman architecture, not Greek. Greek building was a matter of straight posts and straight lintels. The Roman genius was to conceive and build three-dimensional curved structures, of which the Pantheon’s dome is the sublime archetype. This could not be done, at least not on a larger scale, in hewn stone. A plastic, moldable substance was needed, and the Romans found it in concrete, whose use was unknown to Greek architecture.

Roman concrete was a structural ceramic which set hard, not from the action of heat (as pottery does in a kiln) but from the chemical interaction of hydrated (slurried or water-saturated) ingredients. Concrete consists of an aggregate (small pieces of hard stone) mixed with a semi-liquid mortar of hydraulic cement, made from a mixture of water, lime, and a crushed volcanic-ash deposit known to the Romans as pozzolana. This thick liquid is then tamped into a mold, known as a “form.” It may be reinforced with metal rods to increase its tensile strength, although this was not the Roman practice. Chemical changes in the mass as it dries make it set into a hard, impervious block, whose shape is that of the negative space inside the form. The mold is dismantled and removed; the concrete block remains.

Ancient Roman builders would mix their ingredients, wet lime and volcanic ash, in a barrow, spread this over the rock fragments of the aggregate, and then pound it together well, using a hefty wooden compactor known in English as a “beetle.” The less the water, the better the amalgamation of mortar and aggregate, the stronger the result—and it is amazing what Roman builders could do by hand, without mechanical compactors, rotary mixers, or any of today’s motorized tools. Vitruvius’ Ten Books on Architecture (c. 25 B.C.E.) recommended a ratio of one part of lime to three of pozzolana for buildings, and one to two for underwater work. Pozzolana concrete behaved like Portland cement. It also had the extraordinarily useful merit of drying under seawater, which made it invaluable for maritime structures; the Emperor Claudius had a harbor mole built at Ostia by the simple expedient of using a whole large ship as a form, filling it with pozzolana, lime, and water, and then sinking it, so that it set into a (literally shipshape) block.

With concrete, the Romans could build aqueducts, arches, domes, and roads; it opened up means of rapid transport, storage, and defense that had not existed in earlier masonry cultures. Concrete built hundreds of bridges, which gave the Roman army swift access to the most remote parts of the Empire. The stuff of power and discipline, it was ugly and always would be—the brief mid-twentieth-century vogue for beton brut produced some of the most hideous, grime-attracting surfaces in all architecture, as a visit to London’s Festival Hall will confirm. But it could be rendered with stucco or faced with thin sheets of stone, and it was very strong and cheap, allowing the construction of very large structures. And size—raw, powerful size—had great appeal to the Romans in building their empire, as it would to the Americans in building theirs, two thousand years later.

The Pantheon is circular, and it rests on a ring beam of concrete four and a half meters deep and more than ten meters wide. The drum walls are six meters thick, and solid: the only light for the interior comes from the great oculus in the dome above. The dome of the Pantheon was constructed from concrete elements shaped by wooden formwork. It was not necessary to do this in one continuous pour. What was essential was a complete control of the dimensions of the formwork, which produced the stepped coffering within the dome; of the angling of the elements, which gave a perfect circular shape to the oculus (8.92 meters in diameter, the edge originally ringed with bronze); and of the varying density of the concrete mix. This last was crucial, because the dome, in the interest of structural stability, had to be lighter at the top than at the bottom; its thickness increases from 1.2 meters around the oculus to 6.4 meters at the bottom, where the base of the dome meets the drum. Moreover, the integrity of the structure depended on using a lighter aggregate at the top—pumice stone and tufa—than the brick and travertine at the base. The builders took care to add the concrete in small batches and tamp it very thoroughly to expel air bubbles and water before adding the next batch. In sum, the construction of this five-thousand-ton dome was a marvel of architectural forethought—what we would call “systems planning” today—and an architectural historian might well long for a sight of the wooden formwork, scaffolding, and shuttering that its construction must have entailed.

The statistics, bare as they are, remain. With a diameter of 43.3 meters, its dome is the world’s largest in unreinforced concrete, surpassing the diameter of Saint Peter’s cupola by just seventy-eight centimeters. There are larger domes on earth, but they are segmental and reinforced with steel, like the hundred-meter dome of the 1960 Palazzetto dello Sport in Rome. No modern architect would dare to attempt another Pantheon using the same structural principles—nobody would insure it. But the Pantheon has stood for nearly two thousand years and shows no prospect of collapse.

It has, however, endured some damage. Originally, the great dome was sheathed in gilded bronze, all of which was plundered by the Christian (Byzantine) Emperor Constans II in 655 C.E. Pope Gregory III replaced it, more prosaically, with lead sheets a century later. It had long been believed that the Barberini Pope Urban VIII, patron of Bernini, had the bronze beams of its portico removed, melted down, and recycled into the baldacchino of Saint Peter’s and cannons for the Castel Sant’Angelo. Alas, some historians now doubt this story.

Imperial Rome also gave birth to what must be, bar none, the greatest piece of narrative sculpture from the ancient world. It stands in the remains of the Forum Traiani, Trajan’s Forum, rising up as one of the chief landmarks of the city. Much of the sculpture of Trajan’s enormous forum was destroyed in Christian times, either burned for lime or taken away to embellish other buildings. Thus several reliefs on the Arch of Constantine (315–16 C.E.), showing battles between Romans and Dacians, were taken from the Forum, altered by inserting portrait heads of Constantine and Licinius, and installed on the arch.

Trajan’s Column, on the other hand, is virtually intact. However, it is still impossible to “read” as a continuous story, because of its form: a continuous stone frieze, seven hundred feet long, carved in low relief and wrapped in a spiral around a hundred-foot high vertical cylinder. It is a huge ancestor of the comic strip. Other monuments to this great emperor have disappeared; there was, for instance, in the middle of the Forum of Trajan, a magnificent equestrian statue of the emperor, offered to him by the Senate but later destroyed in one of the barbarian invasions of Rome (or perhaps by the Catholics).

But the column still stands, well preserved. Yet, although it is an astonishingly ambitious piece of propaganda, there is no vantage point from which it has ever been possible to see more than segments of the whole design, and of course (for a spectator at ground level) the detail toward the top tends to vanish in the recession of its perspective, although the artist or artists who designed it made an effort to counteract this by gradually increasing the height of the figures from 0.6 meters at the base to 0.9 at the top. It represents that frustration with narrative sequence which would not be conquered until the invention of the movie camera.