TECHNICAL EXAMINATION OF THE CLASSICAL BRONZE HORSE FROM THE METROPOLITAN MUSEUM OF ART
Kate C. Lefferts, Lawrence J. Majewski, Edward V. Sayre, Pieter Meyers, R.M. Organ , C.S. Smith , Edward V. Sayre , Robert H. Brill , I. Lynus Barnes , Thomas J. Murphy , & Frederick R. Matson
5 THERMOLUMINESCENCE DATING OF THE CORE MATERIAL
THE POSSIBILITY OF DATING the core material of the horse by thermoluminescence, ∗ an established technique for dating of ceramic materials, was investigated. However, it was initially felt that thermoluminescence would not be applicable in the case of the core of the bronze horse, because gamma ray and x-ray radiography applied to the horse had supplied an artificial radiation dose in addition to and not distinguishable from the natural accumulated dose received by the ceramic core, thereby interfering with an accurate dating or reliable authentication. Fortunately, a sizable sample of the core material was extracted in the early stage of the technical examination before most of the radiography was done. Furthermore, recent improvements in the thermo-luminescence techniques developed at the Research Laboratory for Archaeology and the History of Art, Oxford, and at the Laboratory for Space Physics of Washington University, St. Louis, Missouri, have made possible such dating of the core material despite the radiographic dose it has received.
∗Thermoluminescence dating is based on the following principle. All ceramic materials contain small amounts of the natural radioactive elements uranium, thorium, and potassium. A fraction of the energy released in the decay processes of these radioactive elements is absorbed and stored by the minerals forming these materials. When these minerals are heated to about 500°C this stored energy will be released, some in the form of emission of light. This emission is called thermoluminescence. Dating can be applied to ceramic objects which have been fired at the time of their production; all existing stored energy will be released at that time and the dating clock is set to zero. From this moment energy will be stored at an essentially constant rate, proportional to the “dose rate,” i.e., the intensity of the radiation originating predominantly from the natural radioactive elements. The total amount of absorbed radiation is called the accumulated dose, expressed in rads. The age of an object can be determined from a simple relationship: age = accumulated dose/dose rate. The accumulated dose can be determined by measuring the amount of emitted light as a small sample is heated to approximately 500°C and comparing it with the light produced by a known lab dose. The dose rate can be calculated from the experimentally determined concentrations of the radioactive elements.
The recent thermoluminescence measurements were initiated by Pieter Meyers and were carried out as a collaboration between him and David W. Zimmerman and Marjorie P. Yuhas at the Laboratory for Space Physics.15 A careful estimate was made of the received dose from radiography prior to extraction of the core sample, based on the available radiographic records. This dose was calculated to be 15 rads, but could possibly have been somewhat higher because not all exposures, such as trial exposures, were necessarily recorded. All particles within the core would have received a roughly similar dose from this radiography. The thermoluminescence measurements were performed on three quite different types of particles separated from the core material: zircon grains which were found to contain extremely high concentrations of internal radioactivity, relatively large quartz particles containing very little internal radioactivity, and mineral fractions of very small grain size containing intermediate concentrations of radioactivity. The measured accumulated doses for each of these particles were approximately: 100,000 rads for the zircon grains, 625 rads for the quartz particles and 950 rads for the fine grain particles. These values, because of their inequality and magnitudes, clearly show that the core material has been subjected to a considerable amount of natural internal radiation. For example, the zircon grains were found to have received a radiation dose several thousand times greater than the estimated radiographic dose and more than a hundred times greater than most other particles within the core. This exceptionally large dose could only have come from the exceptionally high internal radioactive content of these particular particles. A significant difference in accumulated dose of approximately 325 rads was observed between the accumulated doses of the fine grain particles and the quartz grains. The dose in the fine grain particles is due to radiation from alpha particles in addition to beta and gamma radiation from radiography while the dose in quartz grains does not include the contribution due to alpha radiation. (Quartz grains contain a negligibly small amount of alpha particle emitting radioactivity, and the outer layer of the grains which would have absorbed all of the alpha radiation from the surrounding materials was removed prior to thermoluminescence measurement by etching.) Therefore the difference in accumulated dose is due to the alpha radiation only. The alpha radiation can only originate from the decay processes of the natural radioactive elements within the core material.
From the measured doses corrected for the dose received by radiography, estimates of the age of the core material were made for the various separated particles. A precise dating could not be expected because of limitations in the experimental procedures and also due to uncertainties of some factors such as the exact radiation dose due to radiography. Initial age determinations made upon the basis of three individual zircon particle measurements varied within the range of 1900 and 3700 years. Estimates of minimum and of maximum based upon measurements of fine grained particles were 1300 and 2800 years. Similar dating of the quartz particles was necessarily less accurate because of the relatively low natural dose they had received. However, the relatively rough estimates of minimum and maximum age based upon quartz particle measurements were 2000 and 5500 years, which under the circumstances was also felt to be reasonably consistent with other results.
Improvements made recently in the zircon dating technique have allowed a more accurate estimate of the date of manufacture of the horse. Zimmerman16 has reported an age of 2250 ± 210 years BP. This number was obtained as an average of measurements on a new set of three individual zircon grains. The uncertainty of ± 210 years represents, according to Zimmerman, a reliable estimate of one standard deviation. He states: “the results show only that the horse was probably made between c. 50 B.C. and 500 B.C. with, for practical purposes, equal probability for any time in this period.”
From the thermoluminescence results it can be concluded that the bronze horse was cast in classical antiquity. The three sets of thermoluminescence measurements are all inconsistent with the hypothesis of a recent origin for the bronze horse.