But the greatest difference was in ideas of past and future. Roman religion presented its faithful with only the haziest conception of an afterlife. Its dreams of felicity were focused on a long-past Golden Age; perhaps this could be recaptured, but it was certain that the present was a descent from it. Christianity, however, had no powerful notions of lost happiness in an earlier life. What mattered most to the Christian was happiness or anguish after death, both eternal, both irrevocable. With the help of Jesus, the Christian soul had charge of its fate to a degree not imagined by classical religion. Hence the power of Christianity. Now the Christian and pagan paths were about to cross, with totally unforeseeable results.
A visit to the decaying remains of the Golden House of Nero can be disappointing, and there is unlikely to be much to see in another fifty or a hundred years. The most complete Roman building to survive from antiquity, however, was constructed somewhat later, during the reign of the Emperor Hadrian. This is the enormous concrete-and-stone masterpiece of engineering that is the Pantheon.
It was built to replace the original Pantheon, built and dedicated in 27 B.C.E., in the aftermath of Octavian’s victory at Actium by Marcus Agrippa during his third consulship. This building burned down, along with others next to it, in a huge fire in 80 C.E. Hadrian had it rebuilt, in its present form, in about 125 C.E. Rather confusingly, it bears on its pediment the legend M · AGRIPPA · L · F · COS · TERTIUM · FECIT, which stands for “Marcus Agrippa, Lucii filius, consul tertium fecit,” meaning, “Marcus Agrippa, son of Lucius, consul for the third time, made this.” But he did not make it—Hadrian did. Agrippa was long dead (12 B.C.E.) by the time the 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 béton 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.