FEATURE ARTICLE
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.)
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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