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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

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.

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 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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