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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

Lime Is Key

Lime is created by heating limestone or marble in a kiln to a temperature of 900 degrees Celsius, well above the temperature reached in open wood fires. Charcoal or forced air are thus prerequisites for the making of lime. When the heat reaches 900 degrees, carbon dioxide is completely released, leaving quicklime (calcium oxide) behind, a substance much whiter and more powdery than the original stone.

The quicklime is slaked with water to produce building lime (calcium hydroxide, the source of whitewash and plaster), which absorbs carbon dioxide from the atmosphere as it sets. Unfortunately, most lime samples contain impurities in the form of incompletely burned limestone fragments or particles. Because this limestone derives from fossil carbonate deposits, even small levels of contamination will make the sample appear far too old when subjected to 14C dating.

An additional source of contamination may be introduced when the builder decides to make mortar rather than plain lime. This is done by adding to the quicklime an aggregate—typically sand, gravel or crushed ceramic material—along with the water. Any of these substances can affect the 14C analysis of the resulting mortar, with the limestone often found in beach sand being perhaps the most troublesome.

Whether pure lime or mortar is used, the chemistry remains the same. The building lime (calcium hydroxide) reacts with carbon dioxide in the atmosphere to form calcium carbonate. But even in the hardening process there are potential problems. Mortar lying on the insides of walls or behind stone facings may take years or even decades to solidify, thus yielding a date that is too recent for the building as a whole. Also, mortar exposed to rain may recrystallize, thus resetting the radiocarbon clock long after the original hardening, making the sample again appear too recent.

Such complications probably dissuaded many people from attempting to determine 14C ages for mortar. But it sometimes happens in the course of scientific research that an illusory initial success leads to a genuine advance. Those involved must then attribute part of their progress to a strange combination of error and luck. Such was the case with more recent efforts to develop a reliable method for dating building lime and mortar.

In the late 1980s two scientists from the Åland Islands (a Swedish-speaking autonomous province of Finland) and from Finland proper were seeking to date a medieval Franciscan monastery on the remote island of Kökar, on the edge of the Åland archipelago. This island had been important during the Bronze Age, when seal hunters from Germany and Poland established a hunting and oil-processing station there. Traditional dating placed the construction of the monastery and its church in about the year 1450.

Archaeologist Kenneth Gustavsson of the Åland Museum in Mariehamn and physicist Högne Jungner of the Helsinki University Radiocarbon Laboratory took large samples of mortar from the masonry of medieval ruins surrounding the church at Kökar and submitted them for conventional 14C dating. Gustavsson and Jungner were astonished when the laboratory reported a date of about 1280—more than a century and a half older than expected. And they were further surprised when Gustavsson's subsequent excavations around the church yielded jewelry and other artifacts of types that supported such an early date. Later, thermoluminescence dating of roof tiles from the church's outbuildings also indicated that they had been built in the 13th century. Thermoluminescence dating (a procedure that uses the small amount of light released during heating to measure the dose of natural radioactivity a ceramic sample has received since it was fired) has its own built-in uncertainties, but the agreement with the radiocarbon determination was compelling. The extraordinary value of mortar-dating for archaeology seemed to have been proved.

Only long afterward did these investigators realize how lucky they had been. Although Kökar and the rest of the Åland islands are made mostly of granite, some of this bedrock has been overlaid since the Ice Age with blocks of limestone deposited by glaciers. Erosion of this glacial cover contributed limestone particles to most Åland beaches, so the builders of the medieval stone churches on these islands typically introduced fossil limestone into the mortar they used when they added beach sand as aggregate to their quicklime. The little island of Kökar, however, is different: It has beach sand and gravel composed almost exclusively of quartz and feldspar. The medieval masons who laid the early foundation there used the local beach sand, with the result that the aggregate in their mortar did not throw off Gustavsson and Jungner's 14C analysis.

Figure 4. Stone churches of the +land archipelago . . .Click to Enlarge Image

This promising start led to a new project intended to date the eight great medieval "Mother Churches" scattered through the Åland islands. For that Jungner and Gustavvson joined forces with two of us (Ringbom and Lindroos). Lindroos, being a geologist, was well prepared to study the physical, mechanical and chemical properties of the various carbonate minerals in the mortars, including the contaminants. Ringbom, in addition to being an art historian, was drawn to the project because she had a family interest in mortar: Her father had been a cement engineer.

The Åland churches are important repositories of medieval sculpture, painting and manuscripts, but no records survive that document the erection of the buildings themselves. Modern scholarly estimates of their age have ranged over a four-century span, from about 1100 to the end of the 15th century. Thus they were prime candidates for mortar dating. In addition, the Åland churches offered the possibility—very important for the development of this method—of comparing 14C dates for mortar samples with extremely precise dates derived from the tree rings in the roof beams and tower joists, although it was evident that some of the timbers were replacements inserted after damage to earlier beams caused by fire or rot, or as part of a remodeling campaign. Some of these timbers are as young as the late 16th century and represent rebuilding during the Lutheran era following the Protestant Reformation.

Mortar is abundant in all the Åland churches. But the dates provided by conventional 14C dating of this mortar seemed suspiciously—sometimes impossibly—early. Why this was so is now clear: The beach sand on these islands (except for Kökar) was a constant source of fossil limestone in the mix, and the conventional method required such large samples that some contamination always seemed to get through.

While struggling with unsatisfactory results from the Åland churches, Jungner received an invitation to travel to the United States and analyze the mortar in the famous and mysterious Newport Tower in Rhode Island. This unusual structure—a large open cylinder of rough masonry with an arcade of columns at ground level—was involved in a chronological and archaeological controversy.

Since the early 19th century, enthusiasts of the Viking sagas had claimed that the tower was built by Vikings who had come south from the settlement at Vinland that Leif Ericsson established in about the year 1000. Henry Wadsworth Longfellow even wrote a poem about this Viking legend called "The Skeleton in Armor." But when archaeologists from Harvard excavated around the foundations of the Newport Tower in the early 1950s, they discovered not Viking artifacts but Anglo-American colonial pottery dating to the late 1600s. These archaeologists concluded that the tower was nothing more romantic than the remains of the "stone-built windmill" that the great-grandfather of general Benedict Arnold had mentioned in his will as standing at Newport.

Figure 5. Particle accelerators . . .Click to Enlarge Image

After arriving at Newport in 1993 and being feted by the pro-Viking party, Jungner drilled into the mortar between the stones in the columns of the tower, going deep so as to get past any recent mortar that might have been applied during tuck pointing. But the samples taken from the Newport Tower proved to be too small for conventional 14C dating. So Jungner sent them to the AMS laboratory in Aarhus, Denmark, where samples as small as one gram of prepared mortar powder could be dated, thanks to the fact that the AMS method requires less than one milligram of carbon. At Aarhus, one of us (Heinemeier), being director of that laboratory, first became involved in mortar dating. Although a physicist, Heinemeier was already engaged in archaeological pursuits, namely studies of the bones of Greenland Vikings.

The samples from Newport Tower were crushed, sieved and then combined with acid, yielding carbon dioxide, which gave a date of about 1680. This finding provided additional scientific support for the late 17th-century date derived from the archaeological evidence. No Vikings at this site—the tower was a Colonial windmill after all.

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