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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.

Figure 6. Radiocarbon measurements . . .Click to Enlarge Image

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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