ASSESSING THE RATES AND PATTERNS OF TRAVERTINE WEATHERING FROM THE HISTORY OF TIBER RIVER FLOODING AND CONTEMPORARY "REVERSE GRAFFITI" ART IN ROME, ITALY

In April 2016, a large-scale frieze entitled “Triumphs and Laments” by South African artist William Kentridge, in collaboration with American artist Kristin Jones, was created along the western bank of the Tiber River, between the Sisto and Mazzini bridges in Rome, Italy. This work is the artist’s personal representation of the history of Rome in a 500-meter-long procession of more than 80 silhouetted images, some up to 10 meters high, made by power-washing the travertine-faced embankment walls around large figurative stencils. This “reverse graffiti” technique utilizes the color contrast of dark patina on light travertine, and offers a unique opportunity to examine the rates and process of travertine weathering in the context of river flooding history.

The embankment walls were built between 1876 and 1926, after a devastating flood in 1870. The walls are about 12 meters tall and can be breached by flood water levels of >15 meters above sea level (m asl). Since their emplacement Rome has been flooded only 3 times, with the worst flooding in 1937 when waters reached almost 17 m asl. One of the highest recent water levels of 13.5 m asl was recorded in 2008.

Travertine is one the most common building stones in Rome and has been quarried since the ancient Roman times from the nearby town of Tivoli. It formed by calcium carbonate precipitation from hydrothermal fluids during the late Pleistocene (about 115 to 30 ka) and reaches over 85 m in thickness.

Besides rainfall and atmospheric deposition of airborne particles that affect all building stones in Rome, the porous travertine embankments are especially susceptible to dark patination due to groundwater seepage and moisture retention that support prolific growth of organisms such as microbes, moss, lichen, and grass. Occasional flood waters that coat the walls with sediment are an additional moisture and nutrient source for biological growth, but the uniform dark patination of the upper parts of the walls that do not frequently flood suggests this is not the main cause of their discoloration.

As an example of contemporary rock art (i.e., petroglyphs), the “Triumphs and Laments” images represent a unique time marker because they will fade with continuing travertine weathering. Previous work by Kristin Jones suggests that these “vanishing murals” will completely disappear in about 5 years.