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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
Radiocarbon Basics
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 (^14 C). Plants absorb traces of the ^14 C during
photosynthesis. Animals in turn absorb ^14 C by eating plants.
Initially, the ratio of ^14 C 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
^14 C 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 ^14 C 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 ^14 C 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 ^14 C 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 ^14 C 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 ^14 C,
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 ^14 C analysis or
examined to determine characteristic patterns in the tree rings the wood
contains.
Figure 2. Mortar is made using limestone . . .Click to Enlarge Image
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.
Figure 3. Mortar dating . . .Click to Enlarge Image
In the 1960s investigators in France attempted to extend ^14 C 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 ^14 C is fixed in
all these lime- derived substances at the exact time of construction.
And from that moment the ^14 C clock begins ticking, just as it does for
the remains of any plant or animal immediately after its death. Thus if
^14 C 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 ^14 C virtually ceased. One investigator who persisted
was Mark van Strydonck of the Royal Institute for Cultural Heritage in
Brussels. He found that although conventional ^14 C 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 ^14 C 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.
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This Article from Issue
March-April 2003
Volume 91, Number 2
Page: 130
DOI: 10.1511/2003.2.130
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Comments
I am interested in seeing other applications of this.
- Corrie Sloot
posted by Corrie Sloot
March 21, 2009 @ 9:11 PM
I am interested in seeing other applications of this.
- Corrie Sloot
posted by Corrie Sloot
March 21, 2009 @ 9:11 PM
I am excavating a Roman period lime kiln in Italy and wonder if this
dating technique could be applied to the re-solidified lime we have
recovered within the kiln.
posted by Myles McCallum
June 11, 2009 @ 2:10 PM
View all 3 comments
*Of Possible Interest*
*Marginalia*: The Woof at the Door
*Feature Article*: Human History Written in Stone and Blood
*Engineering*: Machu Picchu
*Related Internet Resources*
David Moore's Roman Concrete.com
Definition of Portland Cement
The Churches of Aland
Oxford Radiocarbon Accelerator Unit's Radiocarbon Calibration Page
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