Stories in Stone

by David B. Williams

  The Theater of Marcellus, built in 13 BCE, exemplifies the building techniques described by Vitruvius, according to Marie Jackson, a geologist who has written extensively on the technical expertise of Roman builders.3 For the three-story structure, the Romans selected travertine for exterior arches and carved columns. The stone is scarred and cracked but still retains a sturdy grandeur. For the interior wall, the builders used brown tuff for the arch shafts, but in locations that had to withstand the greatest stresses, such as imposts and keystones, they chose travertine. To further protect the tuff, builders most likely applied thick stucco, which has long since eroded away. Less famous than the more centrally located and more imposing Colosseum, the Theater of Marcellus epitomizes a sense of the majesty of ancient Roman construction, the technical mastery of the engineers, and the beauty of its building stones.

  Travertine building blocks, though, have a weakness. “They cannot be safeguarded against fire. As soon as they make contact with it, they crack apart and fall to pieces,” wrote Vitruvius. (Like the Greek philosophers Aristotle and Plato, Vitruvius considered that matter consisted of unique combinations of the elements water, earth, air, and fire, which gave an object, such as a stone, unique properties. Too much air and fire, according to Vitruvius, made travertine susceptible to breaking at high temperatures.) Modern scientists point to the unequal amounts of extension and contraction along internal crystallographic axes in calcite for travertine’s poor performance in fires. Jackson found, however, that tuff survives fire better than travertine because of its porous texture, which allows tuff to expand when heated with far less fracturing than travertine.

  Fire often plagued ancient Rome. When the Gauls conquered it in 390 BCE, they burned Rome. Major and minor conflagrations hit once or twice a decade for the last two hundred years of the millennium. To combat fire at the Forum of Augustus, builders used an olive gray tuff called Lapis Gabinus, for a one-hundred-foot-high boundary wall. The wall still stands although Rome burned in 64 CE (Nero’s great inferno), followed by five large fires over the next two centuries.

  Recent geological work by Jackson and her colleagues has confirmed that Vitruvius made astute observations of the strengths and weaknesses of Rome’s local building stone. She compared the durability of tuff and travertine in dry, humid (foggy), and wet (rainy) conditions, as well as the two stones’ ability to withstand carrying a compressive load. The tuffs acted more like sponges and absorbed varying amounts of water, some to the point of crumbling, whereas travertine was more or less impermeable and retained its strength in wet conditions. Because of these qualities, travertine played a central role to Roman builders both as a structural reinforcement to tuff construction and as a decorative and protective facing on tuff walls.

  Nowhere are travertine’s attributes better seen than in arguably the most impressive building of ancient Rome, the Amphitheater of Flavium, or the Colosseum, as it came to be known in medieval times. Started in 70 CE by Vespasian and finished a decade later by his son Titus, the Colosseum contains 3.5 million cubic feet of travertine, or eighteen times the amount of Salem Limestone that covers the Empire State Building. Archeologist Janet DeLaine has estimated that in order for that much travertine to be transported the twenty miles by oxcart along the Via Tiburtina from Tivoli to Rome, one cart carrying half a ton of rock had to leave every four minutes. She concluded, “I leave it to the reader to work out the implications for keeping the roads clean.”4

  Despite offering such potentially pungent numbers, DeLaine suggested that most travertine reached Rome by the Anio River (now known as the Aniene). Barges would have been able to carry large quantities of stone down the calm, meandering river from the quarries at Tivoli without the need to acquire, feed, and take care of hundreds of oxen.

  Few, if any, barges would be able to descend the Aniene now. For a recent event focusing on river pollution, an Italian environmental organization highlighted a green space on the Aniene about ten miles from Rome. The group had brought in a backhoe to remove trash, mostly plastic debris, but also refrigerators, wood paneling, and metal file cabinets, that covered the Aniene in a fifty-foot-long raft from bank to bank. Not that the modern Via Tiburtina is much more appealing; the road runs for miles past car dealerships, fast-food restaurants, and stolid apartment buildings.

  Modern travelers can speed from the Tivoli quarries to the Colosseum via bus and subway, which drops you on a low rise with a fine view of the long and more intact north side of the amphitheater. From the vantage point on the hill, you can see how the massive stones play a central role in providing structural support. Three levels of travertine arches rise to support a final, unarched story of squared blocks of travertine. There are no false walls here; the building’s exterior skeleton stands because of the interplay of stone and arch. You can also observe how traffic has blackened the stone with pollution, a problem found in travertine-clad structures around the world.

  To reach the travertine, descend the hill and cross Via dei Fori Imperiali. Once across you can touch the stone. The walls feel pitted and eroded from centuries of weathering; the steps and walkways feel smooth from millions of feet. For nearly two thousand years, this awesome building has dominated and graced Rome.

  A clockwise circumnavigation takes you around to the entrance and a quick security check. Travertine building blocks make up most of the outer corridors. In the open amphitheater,however, travertine gives way to brick and tuff, particularly in the walls of the substructure and the cavea, or seating area. The brick and tuff look refined and well chosen, but the travertine arches of the outer walls are the glory of the Colosseum. Masons cut each block on site so that keystones, voussoirs, and imposts fit together as an intricate puzzle, where each piece balances another. In many blocks, you can still see the holes where builders used forceps and levers to lift the blocks into position. (Other holes in blocks indicate where thieves excavated iron dowels and clamps that once held blocks together.)5 On many arches the builders placed the blocks with their bedding planes aligned with the curve of the arch so that the beds look like rays shooting outward. Each arch is a small-scale illustration of the wonderful marriage of Roman geology, engineering, and art.

  Not everyone has appreciated the Colosseum. Starting in the 1300s and continuing for nearly five hundred years, the amphitheater served Rome as a quarry. One Giovanni Foglia of Como received permission in 1542 to remove 2,522 cartloads of travertine. Particularly egregious was the stealing of stone by the great families of Rome, who used the travertine for the Barbarini, Venezia, and Farnese palaces, each of which still stands. In 1540 Cardinal Farnese’s uncle, Pope Paul III, gave his nephew permission to ransack the Colosseum for twelve hours. Farnese brought four thousand men to help him. Although no direct evidence exists, one who most likely benefited from the cardinal’s travertine transgression was Michelangelo, who took over the design of the Farnese palace in 1546 and completed the building in travertine.

  Arches at the Colosseum, Rome.

  No other building may be better known in Rome for its travertine than the Colosseum, but you cannot travel more than a few minutes without stubbing your toe on the stone. You can walk up the travertine Spanish Steps, stand with hundreds of tourists at the travertine Trevi Fountain, wait to cross streets on travertine sidewalks and curbs, admire Bernini’s oval, travertine Sant' Andrea al Quirinale, and drink water out of a splendid travertine fountain carved in the shape of stacked books. Travertine is the most common building stone in Rome. After two thousand years of use, travertine has more than lived up to Vitruvius’s observations of its durability and suitability for those who want to avoid mistakes when building.

  Robert Folk’s fellow geologists describe him as controversial, eccentric, a publicity hound, and a hyperactive pseudo-elf, but they also agree he is a brilliant observer and a wonderful teacher, and that he draws original scientific connections. He also loves Italy—the food, the scenery, the arts, the music, the architecture. In 1979 he was able to combin
e his interests, energy, and passions in what would become a seminal study of travertine.

  “Well, if you really want to know, I was working in Italy and looking for another excuse to stay,” said Folk.6 He was in Rome in the piazza of St. Peter’s looking at Bernini’s colonnade of Doric columns when he realized he couldn’t place the rock. He knew it was travertine, but the stone didn’t fit into the best-known classification system for carbonate rocks, which was odd because Folk developed that classification in a landmark 1959 paper. “This gave me an excuse to go look at travertine,” he said.

  To study the travertine, Folk teamed up with his former student Henry Chafetz, now a professor at the University of Houston. They traveled to Bagni di Tivoli (the Baths of Tivoli), the most famous travertine deposits in Italy and the source of Bernini’s columns, the Colosseum, and most of Rome’s travertine. The open pits are concentrated in a flat plain about three miles east of Tivoli and just north of the Aniene River. Cavatori during Roman times removed rock from an area next to the river still known as the Barco, a corruption of barga, Latin for barge, in reference to how travertine traveled to Rome.

  Between fifty and sixty quarries operate in Tivoli at present. At the one owned by the Mariotti family, who have been quarrying the site for four generations, cavatori cut rock in a manner similar to what occurs in Indiana, using diamond wire saws, gargantuan front-end loaders and trucks, and gallons and gallons of water.

  Travertine deposition at Tivoli occurred and still occurs in lakes, ponds, and swamps on a flat, volcanic plain pierced by thermal springs. Stone in the primary quarries is younger than two hundred thousand years with a concentration of deposition around eighty thousand years ago. Geothermal activity heats water deep underground and it rises to the surface via faults. Along the way the water passes through and dissolves Mesozoic-age, calcite-rich limestone and transports the calcite in solution to the surface.

  Similar to what happens when you open a soda bottle, the pressure drops when the heated water emerges at the surface, and the carbon dioxide escapes from the fluid into the air. The release in pressure increases the saturation state of the water with respect to the carbonate minerals aragonite and calcite, and after the water degasses, the minerals settle out of the water column. (To simplify the story, I will just refer to calcite.) Depending upon the environment, vent mouth or quiet pond, the calcite accumulates as a fine-grained mud, gloms onto other solid particles in the water, or fills voids.

  Folk said of hot springs, “They can be very fickle. Some days they have an enormous flow when there’s been a lot of rain and you get a very rapid precipitation rate of calcite. On drier days, you get a very slow precipitation rate, perhaps on the order of a tenth of a millimeter a day.” He has recorded calcite deposition of one-sixth inch per day, or a million times faster than calcite accumulates in the ocean. Carbonate accumulation can occur so rapidly in travertine that Folk once described the ghastly scene of algae and mosses “being calcified while still alive.” I don’t recommend letting children see this shocking geologic process.

  Hydrogen sulfide also escapes into the air above the hot springs, and in such great quantities that Folk and Chafetz saw many dead birds littering the pools at Tivoli. Chafetz remembers hearing that three boys died breathing the toxic fumes, a year or so after the geologists finished their study. The fumes smell like rotten eggs and are a classic sign of hot springs, such as at Yellowstone National Park, where travertine forms the falls at Mammoth Hot Springs.

  Geologists refer to this degassing of carbon dioxide and precipitation of carbonates as an inorganic, or abiotic, process. For the last hundred-plus years, geologists accepted that this abiotic process fully explained how most travertine formed. But then in 1984 Folk and Chafetz published their study of Tivoli. Titled Travertines: Depositional Morphology and the Bacterially Constructed Constituents, it proposed the radical idea that bacteria bore primary responsibility for travertine deposition.7

  They were not the first to recognize the connection between bacteria and calcite deposition. As far back as 1914, British marine biologist George Harold Drew proposed that bacteria helped generate calcite in the Bahamas, but not until Folk and Chafetz did anyone present such a thorough study linking significant carbonate accumulations, identifiable structures, and bacteria. “We made this dumb luck discovery. It was exciting,” said Folk. “We didn’t go there with any idea of finding bacteria at all. A lot of scientific studies are just plain dumb luck.” Luck or not, the paper has become the most cited scientific study ever written about travertine.

  Bacteria influence travertine formation by altering the microenvironment around themselves. First, they excrete carbon dioxide during photosynthesis, which provides carbon and oxygen, the main building blocks of calcite. Second, bacteria can process inorganic nitrogen and generate ammonia gas, which raises the water pH, makes the water supersaturated in calcite, and generates calcite deposition. The microbes must achieve a balancing act because if they induce mineral precipitation too quickly, they get entombed, “to the dismay of the bacteria,” as a sentimental pair of later geologists wrote.

  Bacterial-stimulated crystallization most often occurs in nonflowing water, where it produces branching, shrublike growths. The wee forests often form in ponds and can extend laterally for tens of yards with shrubs growing an inch high, although they can also skyrocket to three inches in the right conditions. Some forests even tilt in the direction of water currents. To extend the forest analogy, inorganic calcite precipitates on and around the shrubs, eventually accumulating deeply enough to bury the shrubs and to provide a substrate for the next layer of bacterially generated shrubs to develop. Chafetz and Folk hypothesized that each shrub layer, or bed, represents one year of deposition with maximum shrub growth in summer and maximum “snowfall” of calcite in winter.

  By cutting the stone perpendicular to the bedding, quarrymen exploited the dense, well-cemented shrub-and-snow texture. In ancient times, builders took advantage of travertine’s low compressibility by placing blocks with the beds horizontal. For modern travertine used as cladding and not for structural purposes, architects disregard geology and structural integrity and attach the thin sheets to walls any way that looks good, with the result that bedding planes may run vertically and not horizontally.

  Architects don’t appear to share geologists’ affinity for travertine’s characteristic holey texture, though the voids are a good place to see where calcite crystals have grown. When I lead geology-oriented walking tours in downtown Seattle, I encourage people to use a hand lens to explore the pockets and look for six-sided calcite crystals. (I have also been known to sprinkle balsamic vinegar on the crystals. Geologists more typically use dilute hydrochloric acid to create a reaction with the calcite, which bubbles off carbon dioxide, but I like balsamic vinegar; it’s always available and has multiple uses.)

  Builders don’t like voids because they provide a good spot for pollution-related particulates to collect. The dirt makes many buildings with exterior travertine look blotchy, as if they had the bubonic plague. To counter dust and soot accumulation, builders often fill in the holes in travertine with grout, which also prevents damage from water pooling, freezing, and expansion.8

  Reacting to Chafetz and Folk’s radical shift in explaining how travertine formed, geologists from around the world tried to verify the Texas geologists’ conclusion. Within a few years, bacteria had been reported from travertine in Germany, Idaho, Yellowstone National Park, and Morocco, as well as at other travertine deposits in Italy. All occurred in warm, chemically harsh waters. However, Folk and Chafetz had one significant problem with defending the conclusions in their 1984 paper; they didn’t see any solid bacterial bodies in Italy, only the voids left behind by decayed bacteria. What was missing was good fossil evidence.

  Folk returned to Italy in June 1988 to look for more bacteria in travertine. This time he traveled north of Rome to Viterbo and the Bagnaccio and Bulicame hot springs, both of which have been
well known for their therapeutic waters since Etruscan times.9 “Bob wanted to pursue what he called the ‘lesser rocks,’ those with complex histories that formed under enigmatic circumstances. They also happened to be the prettiest,” says his former student Paula Noble, now a paleontologist at the University of Nevada,Reno.10

  At Viterbo, Noble and Folk collected mud and samples of recently formed travertine. They also dropped coins and pipes in the water to test how quickly travertine formed. On the pipes, travertine precipitated faster in areas hit by sunlight, which were richer in bacteria. No travertine accumulated on the copper coins because bacteria don’t like copper. “He was so excited to see these examples of how bacteria affected travertine,” says Noble.

  When he and Noble returned to Austin, Folk began to examine his samples with an aged electron microscope. At 5,000 magnification, the most powerful the old scope could manage, he saw beautiful calcite and aragonite crystals. Folk described them as “resembling an ear of corn with a tattered shuck enclosing it” and “covered with a forest of spikes like a fakir’s bed.” The scope also revealed calcified bacteria. He now had the evidence that had eluded him for many years. Better material was yet to come.

  In early 1989, the University of Texas acquired a new scanning electron microscope. The SEM’s 100,000 magnification revealed tiny spheres and bumps studding the calcite. Folk initially ignored them, as had other SEM researchers, thinking they were sampling artifacts or lab contamination. Not all of the samples, however, exhibited the microballs. In some samples the spheres clumped together. He continued to look at the odd balls but also retreated to the library, where he found descriptions of similar-looking objects that microbiologists called ultramicrobacteria, or dwarf bacteria. The microbiologists hypothesized that the dwarf bacteria had shrunk in response to toxic conditions, incompatible temperatures, altered pH, or nutrient stress. Upon a return to normal conditions they would revert to their normal size.