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 (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.
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