Keystone In Time

Ancient Rome was built out of concrete, brick, and quarried rock. Today, you can find elegantly carved rock everywhere in the Forum, most obviously in the grand ruins of the site's many temples and triumphal arches. But chunks of lovely fluted column and fragments of carved plinths, capitals, entablatures, and lintels are also strewn everywhere, often half buried in the dirt and heaped in rough piles.

Few visitors pause to think about the enormous labor it took to cut these rocks out of the ground, carve them, and carry them to Rome from distant quarries, or about the ingenuity needed to accomplish such a feat. Yet these scattered, broken stones often have fascinating stories to tell.

So it was with one piece of rock that caught my attention. It wasn't in the Forum, but a five-minute walk away, to the southeast, in the Colosseum—the greatest of all Roman amphitheaters, built to gratify the Roman lust for gladiatorial combat and wild beast shows. I spied this piece of rock as I passed underneath an arch to climb from the Colosseum's street-level entrance to the main promenade inside. I looked up briefly and noticed the arch's keystone. For some reason—I don't really know why; maybe it was something about the keystone's sheer unremarkableness—I stopped to look more closely. I saw that the arch itself was huge—over seven meters high and four meters wide—but in the context of the amphitheater's great structure it didn't stand out. It was only one of eighty of the same size and construction that formed the bottom tier of the building's immense first interior wall. But looking

The keystone in the Colosseum more closely I realized that while seemingly insignificant, the arch and its keystone were actually a brilliant feat of engineering.

The Roman arch is a type of structure that engineers designate a "voussoir arch" because its wedge-shaped stones are called voussoirs. It was an engineering breakthrough. Prior to its invention, someone building a bridge across a river, say, would usually span the horizontal distance using a long length of rigid material, like rock or timber. But this type of structure is vulnerable to collapse in the middle of its span. Worse, it's bedeviled by a basic contradiction: if the span's substance is thickened to make the bridge stronger, the span itself becomes heavier. Eventually, it becomes so heavy that the bridge is weakened, not strengthened. The voussoir arch circumvents this contradiction. It gains its strength not just from its constituent materials but also from its design: because it's an arch, the structure's load is distributed downward into the pillars on each side, and because its voussoirs are wedge shaped, up to a point their weight and that of any material they carry actually strengthens the structure by driving the voussoirs together more tightly.

The design of the voussoir arch gives it strength.

The Romans didn't discover this idea; they probably adopted it from the Greeks and Etruscans.1 They did master the technology and worked out the surprisingly intricate engineering principles that govern the voussoir arch's proper construction. Then they spread the technology far and wide, using it to build bridges, aqueducts, temples, domes, and amphitheaters throughout their empire. Today, a large proportion of the residue of ancient Rome—the most durable bits, in fact, like the occasional segments of aqueduct that can be spied in the French, Italian, or Spanish countryside—incorporate voussoir arches.

The keystone lodged above my head was massive: about two meters deep, one meter high, one and a half meters wide at the top of its wedge, and three-quarters of a meter wide at the bottom. It was made of travertine, a kind of limestone that has been quarried for millennia near the present-day town of Tivoli (known to the Romans as Tibur), about thirty kilometers to the east. Tivoli travertine is a cream color, streaked with darker indentations and small holes that make it look a bit like white cork. But here, over time, age, pollution, and years of inattention had allowed a thick layer of grime to coat the rock's white surface, turning it a muddy brown.

Scruffy though this keystone was, I was struck by how precisely it had been cut: one couldn't insert a razor blade between it and the adjacent stones. Romans didn't have high-speed diamond-edged rock-cutting saws—they cut their stone by hand, with iron picks and chisels, one hammer blow at a time. The labor involved, and the sheer determination and patience, boggled my mind. I was struck most, though, by something more essential: stone is heavy stuff, and I estimated this particular keystone weighed about 5.7 metric tons.21 started to think about how Roman engineers and laborers had put it there, imagining the giant crane they would have needed—constructed in an A-frame, festooned with wooden pulleys—with dozens of men straining on ropes at its base.

The powerful emperors Augustus, Caligula, and Nero had all considered erecting a huge amphitheater in the middle of Rome. But Nero's death in 68 CE—and the brutal civil war that followed—spurred his successor, the emperor Vespasian, to build the Colosseum as a gift to the city's people to show that his rule would be different from Nero's vainglorious and cruel reign. Vespasian began construction between 72 and 75 CE, likely financing the scheme using spoils from his crushing victory over the Jews and the sacking of Jerusalem a few years before. His son Titus inaugurated the building in 80 CE with one hundred days of games in which some ten thousand beasts were killed.

In as little as five years, the Colosseum's architects, engineers, and laborers had erected a building with the dimensions and seating capacity of New York's Yankee Stadium. It was, the architectural historian Rabun Taylor writes, "the most complex structure ever successfully completed in antiquity." Not only could it support the combined weight and simultaneous swaying and stomping of over fifty thousand spectators, but its architecture also provided for the smooth circulation of these spectators (the stadium could be emptied in as little as fifteen minutes), while an elaborate arrangement of pipes brought perfumed water to fountains throughout the building and carried away waste from lavatories. "It is surpassed in daring, originality, and beauty by other buildings," Taylor continues, "but as a monument to architectural process it stands alone."3

Public buildings like the Colosseum were designed to astonish, awe, and even humble Rome's citizens with the sheer audacity of their engineering. They were the physical expressions of an ideology of power. "They spoke of strength, control, and stability," says Taylor. "The intent was to induce participatory pride and willing submission and allegiance to the emperor." In fact, the Colosseum's most spectacular views, available only from the uppermost seating farthest from the arena, were reserved for the common person, whose allegiance was most uncertain and most in need of reinforcement.4

The Romans began this monument to their power by draining and excavating a lake that had been on Nero's palace grounds. They then laid an elliptical ring of cement and crushed stone—thirteen meters deep, fifty meters wide, and over five hundred meters in circumfer-ence—to serve as a foundation, reinforcing each side with brick walls three meters thick. On top of this foundation they placed a floor of travertine almost a meter thick, and then anchored the base blocks for the building's main pillars to the floor with molten metal.5 On these blocks, the Romans erected the columns and arches of the amphitheater's eighty radial walls that fanned outward from the central field like the spokes of the wheel—each intersecting the three immense outer walls that girdled the building.

The final building's total mass was simply staggering. The builders extracted, moved, shaped, mixed (in the case of concrete), and assembled

The Colosseum and its foundation originally contained about a million metric tons of rock, concrete, and brick.

about a million metric tons of raw material, including 295,000 tons of travertine, 653,000 tons of concrete, 54,000 tons of tufa (a soft volcanic rock abundant along the west coast of Italy), 58,000 tons of clay brick, 6,000 tons of marble, and 300 tons of metal to connect the major stones.6

That scruffy keystone in the Colosseum could tell a story, I thought, about Rome's huge energy requirements and about how those requirements shaped the empire's evolution. Empires run on energy. A central task of any empire is to produce, transport, and focus enough energy to maintain and extend its economic and political power. Acquiring and protecting the sources of this energy, the routes along which it's carried, and the people and organizations responsible for generating and transporting it becomes a key job of an empire's security and military forces.

Like any ancient society, Rome was powered in essence by the sun's energy—absorbed by plants in Roman fields and converted into food.7 As I inspected the keystone, I asked myself what area of farmland was needed to grow the calories that powered the muscles that put just this stone in place? And how much was needed to power the construction of the whole Colosseum? This was not an idle question, I believed, but tied to Rome's rise and fall.

The stone could also tell us something, I suspected, about the similarities and differences between Rome's empire and our own of more recent times—including today's American imperium.

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