ART+Science 140 Concrete Examples BEFORE THE SEVENTEENTH CENTURY The dating of a Sapi ivory salt-cellar belonging to Entwistle Gallery in London was successful in establishing the period of its manufacture. The calibrated dates of the sample taken from under its cover include two intervals: AD 1438–1518 (82.5%) and AD 1593–1619 (12.9%). Based on a combination of stylistic approach, consideration of the materials involved, and the carbon-14 date, it can be said that this object most probably dates to the sixteenth century. The Dogon-Tellem cultures produced objects for an extended period of time, and that is why carbon-14 dating is very appropriate for defining these sculptures’ chronology. The analysis done on Galerie Alain Bovis’ male figure with upraised arms made it possible to confirm that the statue dates from the fifteenth century. Its radiocarbon age is 440 ± 20 years BP. Calibration of this result furnished only one very precise interval: AD 1427–1469 (95.4%). AFTER THE SEVENTEENTH CENTURY Carbon-14 dates are not always as precise for this period because the calibration curve does not have a uniform incline. There is a “plateau” zone between the eighteenth and twentieth centuries that does not allow for differentiation in this period. The example of a Kwele mask tested for Galerie Claes in Brussels is typical. The object’s radiocarbon age is 190 ± 20 years BP, which would place it between AD 1740 and 1780. Calibration yields three different intervals: 1661–1664 (20.4%), 1735–1806 (54%), and 1930–1954 (21%). The test confirms that the mask most probably dates to the beginning of the nineteenth century. to a number of processes: soxhlet extraction, acid and base, and demineralized water treatments. The CO2 extract obtained is purified and converted into graphite. These procedures allow for the elimination of potential pollutants (such as glue, resin, wax, polish, and other materials used in restoration and conservation). The cleaning of objects is essential to obtaining credible and accurate results, especially where artworks are concerned. As an illustration of this, imagine an object that was recently covered with a petroleum-based microcrystalline wax, which we will endeavor to test without prior treatment. The date we might obtain could be very old, because petroleum contains very old fossilized carbon. The mixture of this very old material with recent wood might give us an average age that would be much older than the object’s true date of manufacture. Calibrating Results The amount of carbon-14 that an object retains is proportional to its age. What is called the “radiocarbon date” is expressed in years “before present,” or BP, (730 ± 20 years, for example). The “carbon-14 present” was fixed by Libby in 1950 and radiocarbon age is calculated on the assumption that the concentration of carbon-14 has remained constant over time. However, today we know that this is not at all the case and that carbon-14 concentrations vary as a function of solar activity, climate change, and industrial activity. Preliminary results need to be corrected, or “calibrated.” Calibration curves have been established which allow for BP ages to be transformed into calibrated dates associated with a percentage of probability. For instance, 730 ± 20 years BP corresponds to the time interval AD 1260–1290, with a probability of 95.4%.
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