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
3 COMPOSITION OF THE METAL
A SAMPLE OF BRONZE was drilled from the barrel of the horse at 50 mm behind the forelegs and was analyzed by E.V. Sayre by emission spectroscopy (see Appendix III). A second sample from the drilling beneath the plug in the left rear leg was also analyzed, as well as samples from the break of the right foreleg and the bottom of the left rear leg, where the radiographs indicate a region of exceptionally high density. These analyses are given in Table I.
It is apparent from these data that the first three samples are similar in composition. The differences occurring among them are not greater than one might expect from the non-uniformity frequently present in ancient metal alloys.
As indicated in the discussion of the structure of the horse, the right foreleg was cast-on. The analytical data indicate that the metal used in this leg is not significantly different from the alloy used generally throughout the horse and hence supports the conclusion that the added leg was probably made at the time of the fabrication of the statue. The slightly lower tin and lead content in the foreleg might have resulted from removal of these metals from the alloy through oxidation upon remelting. In contrast, the high lead alloy of the repair to the left rear leg is strikingly different from that of the rest of the horse. The difference in the composition of the two leg castings would suggest they were made at quite different times.
The correlations of elemental compositions to the chronology of ancient metal casting practices are published by E.R. Caley in his summary of the composition of ancient statuary bronzes.11 Based on this work, it would appear that the metal of the main body of the horse and the right foreleg was similar to that used in classical Greek bronzes but with a slightly higher lead content, while the high lead alloy of the left rear leg repair approximated that of Roman or later date. However, more recent analyses of classical bronzes modify these conclusions. The lead content of the original structure of the horse, which averages approximately 0.4%, at first seemed to be unusual because the analyses of classical Greek bronzes reported by Caley indicated a nearly total absence of lead. Also, the range of concentration initially seemed unexpected in that it appeared to be too low to represent a deliberate addition of lead, but too high to be an accidental inclusion. However, the work of P.R. Fields has shown that a small portion of natural copper ores do contain this amount of lead.12 Regardless of how this level of lead may have been introduced, it now appears upon the basis of more recent analyses to be a characteristic lead level for many classical bronzes. In no less than 17 out of 19 early Greek, 13 out of 17 Etruscan and 6 out of 25 Roman bronzes, lead concentrations ranging between 0.013 and 3.6% have been reported.13 A summary of these analyses appear in Table II, prepared by E.V. Sayre. This summary indicates that although it is more probable that bronze of the low lead content of the horse would be encountered in a pre-Roman object, it is nevertheless possible that it could occur in an object of a later period. It also indicates that bronze alloys with a lead content as high as that of the repair on the left rear leg are only rarely encountered in classical objects of dates earlier than the Roman period.
Table II Average Compositions of Classical Statuary Bronzes of Various Regions and Periods Compared to M.M.A. Classical Bronze
Of particular interest to this case is the fact that only recently have we become aware that classical bronzes are likely to contain this intermediate concentration of approximately 1% of lead. All of the early analyses of classic Greek bronzes, in particular those published before the date when The Metropolitan Museum of Art acquired the bronze horse, reported no lead to be present. (Of course these early analyses were carried out by chemical methods in which low concentrations of lead might have gone undetected.) Therefore, in 1923 or before, a knowledgeable forger attempting to duplicate a classic Greek bronze alloy upon the basis of published analyses, would have tried to make his alloy free of lead. The lead content, therefore, provides an indication that the metal used in the horse was indeed of ancient origin. It must be remembered, however, that a forger could have remelted ancient metal scrap to cast the horse. There is also a possibility that lead at this level could occur in a modern forgery accidentally. Modern casting bronze sometimes contains about 1% lead, but it usually also contains at least a like amount of zinc. The fact that zinc was not detected in the metal of the horse even by the relatively sensitive spectrographic technique used for the analysis provides a degree of confirmation that the metal is of ancient origin. Although appreciable concentrations of zinc have been found in some cases in first millennium B.C. bronzes from Anatolia, the use of zinc in copper alloys did not become prevalent in the Mediterranean area until the Roman period.14
Lead isotope ratios from samples of the horse and from the left leg repair were studied jointly by R.H. Brill of the Corning Museum of Glass and I.L. Barnes, J. Murphy and W.R. Shields of the National Bureau of Standards. From such isotope ratios it is often possible to infer the geographic origin of the lead present in the alloy. The method does not provide a date for fabrication of the alloy as lead mined today from a given site will have the same isotope ratios as that mined in the same place in antiquity. Therefore, the method cannot produce proof of authenticity, but if a highly improbable geographic source had been indicated by the measurements, it would have cast serious doubt upon the authenticity of the horse. The measurements, which are described in detail in Appendix IV, indicate that the Levant and Italy are both probably sources of the lead occurring in the high-lead repair and possibly in the body of the horse. Neither lead had the isotope ratios of that of the ancient mines at Laurion in Greece thought to have been the major source of ancient Greek lead. Had lead from the European Greek mainland been added to the alloy, its isotope ratios would very probably have been those of Laurion lead. The lead in the repair must have been added; however, there is no firm evidence that there was any deliberate addition of refined lead to the alloy of the body of the horse. One cannot be sure that lead occurring as an impurity in copper ores would necessarily relate isotopically to that from a lead ore of the same region; but even if such an isotopical correlation should be proven, the indication of an Anatolian or an Italian source for the bronze does not exclude a classical origin nor one directly associated with Greek culture, since there were many major Greek cities in Anatolia and Italy.
Both the elemental composition and lead isotope ratios of the bronze in which the horse was cast are clearly compatible with a classical Mediterranean origin. The fact that the alloy in its low lead content corresponds to an ancient composition which has only been recognized within the last few years is in particular indicative of the metal having been of ancient origin. There is no conclusive evidence as to when the high lead bronze repair on the left rear leg might have been applied although on the basis of our present evidence it is likely to have been Roman or later, perhaps even modern.∗
∗Dr. Bernard Keisch ran a lead 210 dating of the lead from the cast-on repair. He reported that the radioactivity level of polonium 210 was less than that of radium 226 in this lead and inferred from this observation that the repair was relatively modern. However, most of the difference in radioactivity levels was the result of a negative activity measurement for the radium 226 (−0.9 ± 0.7) disintegrations per minute per gram of lead. Since a measurement of less than zero radioactivity is physically impossible and can only result from experimental error, it is questionable whether the measured difference is significant relative to the error in the measurements involved.