Letter to Amy Byrne reporting on new isotopic analysis of the marble of the head and torso of the Richmond Caligula (April 29, 2013). |
Scott Pike |
April 29, 2013
Amy Byrne
c/o Virginia Museum of Fine Arts
Richmond, VA
Dear Amy,
Good news! I have had the opportunity to analyze the stable isotope data of the marble samples taken from the Richmond Togatus statue of Caligula (Inv. 71.20). One powdered marble sample was taken from both the Caligula's head and torso. The stable isotope analysis was carried out t0 (1) determine if the head and the torso of the statue were of the same marble type and (2), to determine the likely source quarry of the marble. The samples were analyzed by continuous flow mass spectroscopy at the Stable Isotope Laboratory, Department of Geology, University of Alabama using the protocols established by Spötl and Vennemann (2003) and Debajyoti and Skrzypek (2007). The isotope values of 13C/12C and 18O/16O are reported below as per mil (‰) relative to the international PDB standard for carbon and oxygen isotopic ratios.
The stable isotope ratios were compared to the Herz (1985) database using the least-‐ squares statistical program of Pentia (1995). The program provides a statistical probability value for each quarry in the database. The percentage probabilities provided are measurements of the distance between the data and the calculated center of each quarry's two--dimensional δ13C--δ18O isotopic space. The values are only suggestions and are not meant to be statistically accurate probabilities. The data was also compared to the stable isotope database published by Attanasio (2006). The calculated precision for the analyses of these samples is δ13C = 0.06‰ and δ18O = 0.08‰. This is excellent publication--quality precision.
The chart below provides the data as well as the maximum grain size (MGS) I recorded of the head and body during my visit last December.
| WU No. | Sample No. | δ13C | δ18O | MGS(mm) |
| WU13--027 | Caligula torso | 5.14 | --2.86 | ~1 |
| WU13--028 | Caligula head | 4.92 | --2.85 | ~1 |
The very close isotopic values of the two samples, especially for δ18O, strongly suggest that the marble came from the same immediate source area, if not the same quarried block.
The least square's program identified the Parian Lychnites - Marathi quarry region as the only likely source with the Caligula torso having 39% percent probability and the Caligula head having a 30.4% probability. The MGS data also supports a Parian provenance. However, it must be noted that the stable isotope signature of the head and torso also compares well with the stable isotope ratios from the recently discovered larger--scale ancient quarries at Göktepe in Anatolia. The Göktepe isotopic data is not included in the Herz database. Much work is currently being conducted on the Göktepe quarries and the full extent of the marble's export has yet to be determined. Nevertheless, the Caligula's equigranular structure and 1mm MGS suggests Paros as the likely source quarry.
Please let me know if you have any questions regarding these results. As always, I'm enjoying working on these projects with you.
All the best,
Scott Pike
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