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Dating Ancient Mortar

Although radiocarbon dating is usually applied to organic remains, recent work shows that it can also reveal the age of some inorganic building materials

?sa Ringbom, John Hale, Jan Heinemeier, Lynne Lancaster, Alf Lindroos

When in Rome . . .

The city of Rome lies between two extinct volcanic systems. As a result, its builders had acccess to extensive deposits of pozzolana, an unconsolidated volcanic ash that is very rich in silica and alumina. By the first century B.C., Romans were improving their mortar by adding this local material to the mix. When combined with building lime, the silica and alumina in the pozzolana cause a chemical reaction that creates a mortar that is eight to ten times stronger than mortar made with quartz sand.

Like modern Portland cement, pozzolana mortar will harden under water, because it can react with dissolved carbon dioxide. By chance or experiment, Roman builders discovered that a similar mortar with hydraulic properties could be produced without pozzolana, by adding crushed terra-cotta as an aggregate. In this case, the fragments of fired clay from old tiles and pots introduced silica and alumina into the mortar. Less porous than pozzolana, the crushed terra-cotta tended to be less chemically reactive and therefore less strong. It was, however, denser and more resilient to the infiltration of water than pozzolana mortar and was often chosen for waterproofing material in tanks, pools, aqueducts and harbor installations. (Some Roman-era pools and cisterns still hold water today.)

Figure 7. Sampling of mortar . . .Click to Enlarge Image

Our mortar-dating team collected samples of Roman buildings from the provincial capital city of Mérida in western Spain, from Ostia near the mouth of the River Tiber and from Rome itself. Here were to be found buildings that could be precisely dated, thanks to the Roman custom of using datable brick stamps and to their penchant for inscribing structures with the name of the emperor or rich citizen who had paid for them. The buildings we chose for testing included Trajan's Markets, a large-scale imperial complex built about 110 A.D.; summer houses in gardens in Ostia built under Trajan and his successor Hadrian; and in Mérida the spectacular amphitheater and also the mausoleum of Saint Eulalia, built about 430 A.D. to honor a young girl martyred by Roman soldiers stationed in the city.

The walls and vaults of Trajan's Markets are among the most monumental remains of Roman concrete construction, whereas the mausoleum of Saint Eulalia was a tiny crypt. Yet for all these places we obtained mortar dates from AMS analysis that matched the historic dates of the buildings, although on the Roman samples the correct date was indicated by the second rather than the first fraction of carbon dioxide released in the analysis, because the mortar dissolves slowly but contains rapidly dissolving contaminants.

The testing in Mérida presented an opportunity for our team to tackle the same sort of problem that had been raised by the Newport Tower and the Åland churches, namely a building of uncertain date. One of the most impressive Roman monuments at Mérida is the amphitheater, built for gladiatorial combats and spectacles involving wild animals. Like the Colosseum in Rome, Mérida's amphitheater is a vast oval (amphi means "all around" or "on both sides"), with thousands of seats for spectators, elaborate gates and staircases for the crowd, and underground pits for the animals and other performers. Mérida was a new city founded by Caesar Augustus to serve as the capital for the province of Lusitania. Many of its buildings carried inscriptions honoring either Augustus himself or his right-hand man, Marcus Agrippa.

In the case of the amphitheater, archaeologists had discovered an inscription with a Roman date equivalent to the year 8 B.C., thus giving Augustus credit for the building. But perhaps because the Colosseum itself was a much younger building, many scholars maintained that the Mérida amphitheater in fact belonged to the period of the Flavian emperors, almost a century later than the inscription would indicate. The 14C dates from the amphitheater supported a date in the 1st century A.D., well after the original founding of the city. So the inscription denoting the year 8 B.C. appears to be a piece of earlier material deliberately incorporated into the structure, like the inscription naming Marcus Agrippa that the emperor Hadrian had put into the facade of the Pantheon. In each of these cases, the "historical" evidence gives an incorrect date.

Our work within the old Roman province of Lusitania did not end in Mérida. Nearby were many large farms or villas, where the construction and expansion projects over the centuries provide a barometer of Roman economic prosperity. The largest of these villas was discovered in 1947 at Torre de Palma in eastern Portugal, which was excavated by a team from the University of Louisville under the direction of Stephanie Maloney, starting in 1983. The villa at Torre de Palma included a richly decorated house for the owner, slave quarters, barns, granaries, bath houses, stables, work shops, a wine press and an olive press—not one of which could be dated by inscriptions or other documentary evidence. Much excavation was carried out simply in the hope of finding artifacts that might provide clues to the age of the structure, such as a late Roman coin sealed in a floor where it had been dropped during the pouring of the concrete.

The most important building on the site was the early Christian church or basilica, with an adjoining baptistery and cemeteries. German art historians had dated the complex on stylistic grounds to the 6th century A.D., when Visigothic kings had taken over the rule of Lusitania and the rest of Iberia. But during the first season of the Louisville excavations, 10 small bronze coins were found in the mortar under the marble floor near the altar, all of them minted in the middle of the 4th century A.D. during the time of the sons of Constantine, the first emperor to convert to Christianity. Measurements of the basilica showed that it had been laid out on a grid of Roman feet, and the high quality of the masonry there seemed to support the notion that it had been constructed during the years before the fall of the Roman empire.

Here, as with the Åland churches, mortar dating by AMS analysis was able to reveal the complexities hidden under the archaeological surface. The sanctuary around the altar was indeed constructed during the time of Constantius II in the mid-4th century A.D., as was the central part of the baptistery with its unusual "double cross"-shaped pool. But much of the church had been built long after the fall of imperial Rome, after the Visigoths took over control of Iberia in the 6th century A.D. A great building project in about 580 A.D. raised the walls of the nave, with their heavily mortared masonry. From this it follows that in the depths of what are conventionally called the Dark Ages, this remote corner of Portugal supported active quarries, lime kilns, marble cutters and polishers, stone masons, architects and contractors. Such elaborate works could only be carried out in a healthy economy. The mortar dates for the basilica of Torre de Palma thus provide important clues about the survival of Roman technology and social order in the centuries after the fall of the last emperor.

The potential benefits of the new mortar-dating method are great. At a time when archaeologists try to dig less and less in an effort to preserve the world's archaeological heritage for future generations, the method offers the possibility of learning a great deal before excavation is even attempted. In an optimal situation, remains of ancient buildings, whether as isolated ruins or incorporated in later structures, can be dated from samples of no more than a few grams of mortar. An archaeologist carrying out a field survey may be able to determine the age of a building that once stood there simply by collecting fragments of mortar from ancient walls or floors. Buildings with complex histories of expansion and repair can have their stories told. And art works such as frescoes and mosaic pavings can be dated not only on their artistic style but also by determining the moment when the mortar hardened. The results should be significant not only for the history of technology but for human history as a whole.

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