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
Even more than digging implements, archaeologists need tools for
finding the age of the objects they study. After all, many sites and
remains—in caves, in deserts, on the sea floor—require
no excavation, but all must be dated. When archaeologists of the
future write the history of their discipline, the second half of the
20th century will stand out for the development of many scientific
methods for ascertaining the age of artifacts. This article is an
account of how our Scandinavian-American team, which includes a
nuclear physicist, a geologist, an art historian and two
archaeologists, developed the means for dating ancient building
materials that contain lime mortar.
In the early days, archaeologists trying to make age determinations
often depended on information supplied by others. Principally, they
relied on historians, who knew the chronologies of literate
societies of the past five millennia, with their written
inscriptions on seals, records, tombs, monuments and coins.
Archaeologists also relied on geologists, who could sometimes make
age determinations based on the association of human remains with
geological features of known age.
Unfortunately, this dependence on historical dates and geological
associations left large areas of the human past untouched. But
beginning in the late 1940s, a new world opened with the development
of radiocarbon dating for organic remains, tree-ring dating for
wood, thermoluminescence for fired clay and potassium-argon dating
for volcanic materials. Of these, radiocarbon dating had the most
universal importance for archaeology. So vital was its discovery
that the pioneer of the field, Willard F. Libby, was awarded the
Nobel Prize for Chemistry in 1960.
The underlying principles of radiocarbon dating are straightforward.
Libby and his coworkers realized that cosmic rays impinging on the
upper atmosphere create a steady supply of the radioactive isotope
of carbon: carbon-14 (14C). Plants absorb traces of the
14C during photosynthesis. Animals in turn absorb
14C by eating plants. Initially, the ratio of
14C to normal carbon in plant and animal tissues reflects
the roughly constant atmospheric concentration. But after an
organism dies, radioactive decay reduces the original amount of
14C by half every 5,730 years. This phenomenon provides a
built-in clock for dating most human foods and many raw materials
for tools, weapons, ornaments and buildings. Libby confirmed the
validity of his dating method using wood fragments of known age,
including heartwood of a stump of a California redwood tree almost
3,000 years old and the deck board from the funeral boat of the
Egyptian pharaoh Sesostris III.
Two subsequent developments greatly enhanced the value of
14C dating. Investigators made radiocarbon measurements
on the yearly growth rings of long-lived bristlecone pines, which
provided an annual record of the varying concentrations of
14C in the earth's atmosphere over the past four
millennia. These results made it possible to account for slight
variations in the atmospheric concentration of 14C and
thus to construct a calibration curve that could translate
"radiocarbon ages" (those determined using only a simple
calculation based on radioactive half-life) into true calendar ages.
Equally important was the introduction of particle accelerators to
separate carbon isotopes and count directly the 14C atoms
in the sample, a technique that came to be known as accelerator mass
spectrometry (AMS). This advance drastically reduced the amount of
material needed: Only one milligram of carbon is required for AMS
analysis, whereas the traditional procedure (the so-called
conventional radiocarbon method), which involves the counting of
particles emitted in the slow radioactive decay of 14C,
requires several grams of carbon to produce a date.
Even with these advances, the study of buildings and other
structures presents special problems. Direct dating of an edifice
usually requires that it be made (at least partially) of wood and
that its original timbers be preserved so that they can subjected to
14C analysis or examined to determine characteristic
patterns in the tree rings the wood contains.
Even when such an analysis provides precise dates, an inherent
uncertainty remains because the wood tested could be older than the
building itself—or it could be younger, if material from later
repairs was misidentified as original. In the case of buildings made
of mud brick, stone, mortar or cement, these methods cannot be
applied at all. In such situations, archaeologists often dig through
vast areas around ancient structures—and in consequence
irretrievably disturb or destroy material—in search of coins,
inscribed objects, fragments of charcoal (which contain carbon) or
other datable items that might lie buried in the builders' trenches
or sealed in the walls or floors.
This reliance on secondary dating, aside from its wastefulness in
time and effort and archaeological resources, is vulnerable to
serious error. Older coins, for example, might find their way into a
new building; later objects too might be introduced long after the
main structure was erected. Even the largest elements of the
structure may cause confusion. For example, the monumental columned
porch of the famous Pantheon in Rome bears a prominent inscription
proclaiming that it was made by Marcus Agrippa during the reign of
the first emperor, Caesar Augustus. But the stamps on the bricks in
the great dome prove that everything visible today was built during
the reign of Hadrian, more than a century later.
Archaeologists must find ways to overcome these difficulties, for it
is of primary importance in many cases to know exactly when a
building was constructed. The complex cultural, technological and
economic systems that lie behind all large-scale buildings can
provide important clues to the nature of the particular culture and
period in question. Whether the archaeologist is dealing with a
decorated pyramid in Mexico, a Moorish palace in Spain or a Roman
market, the study loses much of its value if the time of
construction cannot be pinpointed.
In the 1960s investigators in France attempted to extend
14C dating to certain inorganic substances. In
particular, they knew that all building materials based on
lime—mortar, concrete, plaster, whitewash—absorb
atmospheric carbon dioxide as they harden. In this way
14C is fixed in all these lime- derived substances at the
exact time of construction. And from that moment the 14C
clock begins ticking, just as it does for the remains of any plant
or animal immediately after its death. Thus if 14C
analysis could be applied to mortar, the radiocarbon clock could be
rewound to the point in time when the building came into existence.
The principle was simple enough, but its application proved
surprisingly difficult. Although Robert L. Folk and Salvatore
Valastro, Jr., (both then at University of Texas at Austin)
established many of the prerequisites for this technique in the
1970s, in general the results were so poor that after a few more
years, work on this particular application of 14C
virtually ceased. One investigator who persisted was Mark van
Strydonck of the Royal Institute for Cultural Heritage in Brussels.
He found that although conventional 14C dating could at
times yield accurate results on mortar samples, the process was both
complicated and unreliable. The main difficulty was the presence of
impurities in all lime-derived building materials—impurities
that could seriously affect the outcome of the analysis. Van
Strydonck recommended that 14C traces in mortar, or in
wood or charcoal fragments embedded in the mortar, might be dated by
the AMS method. The difficulty with analyzing charcoal fragments is
that they (just like the timbers used in construction) could come
from old wood and thus could be anywhere from a few years to several
centuries older than the building in which the mortar was found.
Direct analysis of lime mortar would avoid this problem.
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
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
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
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
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.
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
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 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.
The authors wish to express their thanks for generous support to
the Åbo Akademi Foundation, to the local government of
Åland and to Nordkalk Corporation, a company that
produces, among other products, quicklime.
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