*MY AMERICAN SCIENTIST* LOG IN! REGISTER! * Keep me signed in* ? Would you like us to keep you signed in on this computer ? Yes No *SEARCH* RSS Logo * Current Issue * Past Issues * On the Bookshelf * Science in the News * About * Subscribe * Advertise * Sigma Xi HOME > PAST ISSUE > March-April 2003 > Article Detail printer friendly fontSizesmallA <#>midA <#>largeA <#> 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. ‹ PREVIOUS 1 2 <#> 3 4 5 NEXT › » » Post Comment *IN THIS SECTION* /American Scientist/ Classics Authors Purchase a Back Issue This Article from Issue March-April 2003 Volume 91, Number 2 Page: 130 DOI: 10.1511/2003.2.130 Purchase Article Printer Friendly Save To Library EMAIL TO A FRIEND : Subscribe Sending... Your email has been sent 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 Subscribe to American Scientist poweredBy CURRENT ISSUE PAST ISSUES ON THE BOOKSHELF SCIENCE IN THE NEWS ABOUT SUBSCRIBE ADVERTISE PRIVACY POLICY © Sigma Xi, The Scientific Research Society