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
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.
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.
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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