Bagni di Tivoli Travertine: The white stone that you see all over Rome, from the fountains to the old noble palazzos and from St. Peter's church to the Colosseum, is almost always travertine rather than marble. Although travertine looks porous, with its gaps, bubbles, grain, and inclusions, the stone actually is much less permeable than marbles, limestones, and sandstones. It is not as strong and impermeable as granites or basalts, but it is much cheaper because it is more easily quarried and worked and much more available. It has been the structural stone of choice in Rome for 2000 years because it is stronger than marble and because one of the largest deposits of good travertine in the world is just twenty miles down the road at the base of the Tiburtine mountains.

Travertine is a product of the earth's water and carbon cycles. As carbon dioxide-rich rainwater percolates through soil and stone, it slowly dissolves tremendous quantities of limestone along underground fissures. This water, now saturated with dissolved limestone, reemerges at the surface at warm or hot springs and releases carbon dioxide gas into the atmosphere -- much like carbonated mineral water. Once the carbon dioxide is released, the limestone can no longer remain in solution. It re-crystallizes, typically as the water cascades over organic films made of bacteria, algae, and mosses. A dense, banded carbonate stone is built up over time as new material covers older layers. This process has continued at the Bagni di Tivoli springs for at least 80,000 years. (This is the same process that builds up the terraces around the springs at Yellowstone National Park in the United States. The "yellow stone" there is fresh sulfurous travertine of almost the exact same chemical composition as Bagni di Tivoli stone.)

Calcite and gypsum, the minerals that make up about 99 percent of travertine stone, are colorless. The beautiful honey color of Tivoli travertine is from the other 1 percent of the stone: traces of yellow sulfur, brown iron compounds, and organic pigments. The intricate "Swiss cheese" texture of travertine is partly the result of gas bubbles, which are often trapped between layers of stone, creating spherical voids. Minerals crystallizing on the ever-present bacteria in travertine deposits -- like granular snow blanketing a miniature landscape -- preserve organic growth forms, called "shrubs," and produce much of the rugged relief evident if the stone is split for use along its "grain". In some cases, travertine layers are similar to tree rings, with lighter and darker laminations representing seasons of growth.

Travertine is found in greatest abundance where hot and cold springs have been active for tens of thousands of years. The most famous travertine location, is Bagni di Tivoli, 20 kilometers east of Rome, where travertine deposits over 90 meters thick have been quarried for over two thousand years. The name of the stone is, in fact, derived from the ancient Roman name of Tivoli, which was Tibur. "Tibur stone" in Latin was "lapis Tiburtinus", which with only a little corruption became "Travertine".

Because travertine is plentiful, weighs less than marble or granite, and is relatively easy to quarry, it was the stone most commonly used by the ancient Romans. Famous structures constructed with Tivoli travertine include the Colosseum, the Trevi Fountain, the façade and colonnade of Saint Peter's Basilica, and many Roman aqueducts. In the last century, Lincoln Center in New York and the ABC Entertainment Center in Los Angeles were faced with travertine from the same Tivoli quarries.

Travertine is split with the grain of the stone, making visible many more fossils than are seen in the more common banded travertine, which is cross-sectioned and polished. At least two species of fossilized leaves are fairly common in stone from Bagni di Tivoli -- evidence of a lakeside environment at the time of the stone's formation. Fossilized animal material is less common, but an occasional feather or bone is discovered. The rapid deposition of the travertine layers acts as a natural preservative for these traces of prehistoric life.

In order to remove the travertine from the vertical quarry face, workers drill holes into the stone, outlining a block 6 meters high, 12 meters wide, and 2 meters deep. A diamond-studded cable is then threaded through the holes, lubricated with water to prevent heat buildup, and pulled against the stone with a set of pulleys. A large cut may take a day and a half, but eventually diamond wins out over the softer travertine. When the cuts are completed, the slab is pushed away from the quarry wall and falls onto mounds of earth, which help cushion the fall. The slab is then broken up into more manageable cubes, which are taken to factories for honing and cutting or splitting. An automated guillotine has been invented to split the stone along its natural bedding plane, or diamond saws can be used to cut standard or specially ordered cross-cut stones and slabs.

Travertine floor surfaces are anticipated to last at least 50 years before replacement or refinishing is needed in high-traffic areas. High-quality travertine is very durable, since it is formed at the earth's surface in relative equilibrium with the environment. Most other building stones are formed under different conditions, deep underground and may be less stable in surface temperature and pressure conditions.

In modern installations, travertine blocks may be treated with silicon-based water repellents that are expected to ease cleaning. Paving stone and lower parts of walls in public areas may also be treated with oil and paint resistant coatings to reduce the effects of soiling and graffiti. It's important, however, not to completely seal surfaces since that could cause moisture to accumulate inside the stone or to migrate to inner walls where it could damage plaster or stucco surfaces or frescoes.

Over time, the honey color of fresh Bagni di Tivoli travertine will change as the stone weathers and natural patinas forms. We, of course, are more familiar with the "un-natural" grimy patina on those buildings that did not get cleaned for Rome's Jubilee Year.

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Adapted (by permission) from http://getty.edu/conservation/publications_resources/newsletters/11_2/news2_3.html.
For further information on travertine stone ("sinter" or "travertino" in Italian, lapis tiburtinus in Latin), see
http://en.wikipedia.org/wiki/Travertine, or
http://www.stonesourceusa.com/travertine_slabs.php (with images), or,
for more detail on Italian travertine deposits,
http://www.andreabilli.com/2008_FaccennaetaliiGPC.pdf.