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
Åland Revisited
When Ringbom, still working on the Åland churches, learned of
the promising results from the Newport Tower, she resolved to
abandon the earlier approach and start over again using only AMS
14C dating. After doing so, the age determinations proved
plausible and consistent. Mortar dating indicated that the naves of
all eight churches had been completed during a very short interval,
from 1280 to 1300, matching the age that ecclesiastical activity
began at Kökar. Studies of the tree rings in timbers found in
the bell tower of one of these churches (at Jomala) dated the
structure to 1281. Five samples of mortar from that tower yielded
14C dates of 1279 to 1290—the most remarkable
bull's-eye yet achieved with the newly developed method.
Indeed, AMS-based mortar dating appeared to yield a full history for
these previously enigmatic structures. The bell tower at Jomala was
later copied in the other parishes. Hammarland church got its west
tower in 1310 and Lemland in 1316. Then after a long gap, towers
were added to the other churches between 1381 and 1467. Porches were
added later still. Thus earlier conflicts about the ages of the
churches could be explained in part by incremental building, a
practice fully revealed by AMS dating of the mortar.
Initially it seemed surprising that all these churches should have
been established in one great burst of concentrated energy,
considering the costs, effort and expertise involved. But Ringbom
found a possible explanation. In about 1280, these islands began to
enjoy an economic boom as the Ålanders supplied timber and
lime mortar for the building of two new cities: Stockholm to the
west in Sweden and Åbo (Turku) to the east in Finland. The
financial fruits of this windfall seem to have found their way into
the eight monumental churches, symbols of the Ålanders'
communal pride and pious gratitude.
This work on the Åland churches brought important refinements
to the mortar-dating method. For example, finer meshes than had
previously been used aided the mechanical separation of pure fired
lime from contaminants, as did adding the steps of dry and wet
sieving. And a technique called
cathodoluminescence—essentially bombarding a sample with
electrons and viewing the light given off—allowed impurities
that could affect the date to be made readily visible. Also, it
proved worthwhile to produce a sequence of subsamples of the carbon
dioxide released from the mortar after the application of an acid so
as to test the consistency of dates derived from various fractions.
It turned out that for most of these samples the very first gas
fraction came from rapidly dissolving carbonate in the hardened
lime, thus yielding the correct date of the building. The second gas
fraction was contaminated with carbon dioxide from slowly dissolving
fossil limestone, thus giving an erratic result that tended to be
too old.
With promising results from Kökar, the Newport Tower and the
Åland churches, the mortar-dating method was securely
established. But from an archaeological point of view, the work was
just beginning. Ahead lay the application of this method to mortar
samples from different periods and environmental settings (including
under- water structures) and the development of precise procedures
for collecting the samples. It was already clear that success might
require site visits by a number of specialists to verify the
original position and condition of each sample: where it lay in the
structure, whether it remained chemically pristine, what the local
sources of raw materials and potential contamination were and so on.
Beginning in 1999, we formed an interdisciplinary team to test this
method on mortars from more ancient sites. Our group includes a
physicist (Heinemeier), an art historian (Ringbom), a geologist
(Lindroos) and two archaeologists (Hale and Lancaster). Our focus
has been on the Mediterranean and the territory of the Roman Empire.
By the time we assembled our group, the method had proved reliable
on sites from the medieval and early modern periods; yet it remained
to be shown that it could work equally well on material from the
classical age. Moreover, the Romans were famous for having used an
alternative to normal sand as aggregate, and there was interest in
seeing how this Roman mortar would behave during analysis.
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