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
4 COMPOSITION AND MICROSTRUCTURE OF THE ARMATURE AND CORE
AN ATTEMPT WAS MADE to reach the armature through the hole made in the barrel in obtaining a metallographic specimen. At this location, the bronze casting was 15 mm thick. At the point of contact with metal the core was black but at a depth of 35 mm from the outside bronze surface it was gray. From here to 53 mm, the center of the core, it was streaked alternately light and dark gray. After removal of 95 grams of core material, at a depth of about 62 mm, the core was intermittently rust color and black. At 70 mm, it was so hard that it could not be scraped loose without first drilling with a steel drill. The iron armature was located to the right of center of the barrel at about 70 to 75 mm. Small black magnetic pieces were withdrawn along with core material stained red and black. X-ray diffraction demonstrated that the red material basically contained goethite (Fe2O3·H2O) and that the black was magnetite (Fe3O4). The largest black piece was attracted to a magnet but proved to be a mixture of sand and iron corrosion products. Of this hard region a sample initially thought to be a corroded residue of the armature was analyzed by electron microbeam probe by E.V. Sayre, who searched for slag streamer residues characteristic of crude wrought iron. No streamers were observed and the sample proved to be core material. However, iron showed up strongly throughout the finely divided binding material which contained the elemental components of clay minerals. Particles of quartz, calcite including some calcium carbonate shell, and feldspar were observed. The core was moist on removal and contained about 20% water. This was probably the result of seepage occurring when the horse was immersed in water for specific gravity measurements.
Microchemical tests on the core material indicated the presence of chloride. X-ray fluorescence, spectrographic and neutron activation analyses gave silicon major, calcium 17.8%, aluminum 3.8%, which is low for clay, iron 3.0% and a fraction of a percent of magnesium, sodium, copper, titanium, and traces of other elements. Water extractable chloride ion was 1.37 per cent. As a test for ammonium ion was negative and only 0.3 per cent sodium was found, chloride is present in some form other than ammonium or sodium chloride (see Appendix III).
The core material was studied petrographically in thin sections and in powder mounts by F.R. Matson (see Appendix V). He also analyzed it chemically by a glass bead microprobe technique and observed color changes produced by experimental firings. He reported that the core is composed of “approximately equal parts of clear quartz sand and of a calcareous paste that could probably be termed a marl—a sticky naturally occurring deposit derived from an ancient sea bottom.” He found characteristic foraminifers to be present along with relatively small amounts of chalcedony and felspar. However, micas and evidence of organic material were absent. Many of the quartz grains contained well developed rutile needles. Efforts have been made to identify the geographic source of the core material on the basis of the inclusions. Marie Farnsworth in her examination of thin sections of core material found it to be dissimilar to the clays used in the Greek pottery she has studied. A quantitative estimate of the mineral content of the core based upon a rational analysis of the chemical determinations and the petrographic observations is given in Table IV.
A comparison of the color of the core material to the colors produced by laboratory firings indicated that the core had been heated under strongly reducing conditions. In lost wax casting the process of heating the mold to remove the wax produces a reducing atmosphere within the core.
The overall observations on the core material clearly show it to be a natural calcareous sand with a marly binder, probably a marine deposit. Analysis of Mediterranean sands have shown a very similar elemental composition. It is distinctly different in composition and character from modern casting sand, which consists of a nonhardening mixture of pulverized silica and loam. Figures and thin sections compare the diverse microstructure of the core material from the horse to the uniform structure of a typical casting sand. Although little is known about the composition of ancient cores, a crude calcareous sand readily abundant throughout the Mediterranean region would have been a most probable choice for core material.