The Berkeley Plato: From Neglected Relic to Ancient Treasure, An Archaeological Detective Story

Appendix B. Technical and Scientific Analysis

a p p e n d i x b TECHNICAL AND SCIENTIFIC ANALYSIS OF THE BERKELEY PLATO John Twilley The objective of the program of examination and testing reported here was to attempt an interpretation of the past history of the marble through the independent perspective of physical and compositional analysis and thereby to suggest whether the herm is, in fact, an ancient artifact and whether the head and shaft can be said unequivocally to belong together. Tests that had previously been carried out served to demonstrate that the head and herm are carved of marble with the same Parian lychnitis origin , a source known to be active in the ancient world and to have become exhausted later (above, pp. 11–12). The objectives of the present testing were to go beyond that simple fact to explore the possibility that the herm may have been carved through reuse of, for example, an architectural element and, if not, to explain the circumstances of its survival.1 Sixteen discrete areas of the exterior surface were examined in detail (plate 2), with samples taken and analyzed. Surfaces were also examined microscopically with a view toward comparison of degrees of weathering.2 69 1. The full report, including detailed analyses, is on file and available at the Phoebe Apperson Hearst Museum of Anthropology at the University of California , Berkeley. 2. The main analytical steps undertaken involved the following. Scanning electron microscopy (SEM) of particulate samples was carried out in which the First, however, examination of the sculpture using ultraviolet light demonstrated a wide variety of fluorescence behaviors on the surfaces of both head and shaft (plate 3) The strong and variable fluorescence of the shaft is characteristic of marbles that have been exposed to the uptake of organic matter from environmental sources. Additional contributions to the fluorescence behavior come from residues of materials that later testing has shown to be mortar remnants of several types. The treatment residues on the head also exhibit strong and variable fluorescence. However , the variations in fluorescence behavior there, like the variations in their visual appearance, do not correlate with any obvious compositional 70 / Appendix B surfaces, undersides, or fracture sections were examined and elemental analysis carried out through the use of an associated X-ray spectrometer. Optical microscopy and SEM examination with elemental analyses of crosssections were undertaken on embedded fragments of surface residues that had been oriented so as to give a stratigraphic view. As a result of using nonstandard sample-preparation protocols, including strict avoidance of water, the cross-sections preserved evidence of soluble salts derived from magnesite cement in various locations. Cross-sections of the magnesite repair materials from the head were also examined and analyzed in order to compare with encrustations and mortar residues found on the surfaces. In some cases, chiefly those involving potentially ancient mortar residues, the cross-sections were reduced to thin sections for study by polarized-light microscopy . Individual phases in these sections were probed using Raman spectroscopy, which yielded confirmatory identifications for some of the detrital minerals but proved not to be very useful for secondary mineral formations because of a high fluorescence background. This is a typical problem with fine-grained or secondary minerals in Raman analysis. Subunits of some of the samples were crushed and mounted for polarizedlight microscopy as dispersed particulates in order to gain an impression of the quantities of the different species present. Fourier transform infrared spectroscopy (FTIR) was used to confirm the compositions of some of the samples by distinguishing between possible polymorphs such as calcite and aragonite and between hydration states such as gypsum and hemihydrate. Surface-replica peels were taken from a few locations for possible examination and analysis of their residues in the SEM, but these were of low priority. or microstructural variations. Laboratory testing has shown that deposits on the head that were sampled on the basis of their differing fluorescence behavior and appearance share the same compositional features. A thick orange patina on parts of the herm, including the interior of the right shoulder socket (plate 4 and fig. 91), has been shown to consist of calcium phosphate (hydroxylapatite), a widely underreported form of secondary mineralization encountered on calcareous antiquities from a variety of contexts around the world. In addition, there was strong microstructural evidence for the formation of the hydroxylapatite as a replacement mineral after calcite of the marble. Some thin residues of what appears...