Chemical Analysis of Archaeological Tars and Ambers

Deirdre Lewis, Vassar College ’11 and Profs. Edie Stout and Sarjit Kaur

Our research focused on the analysis of tar and amber samples, both of which have important archaeological implications. Four tar samples from a 4th century BC Greek ship wreck were analyzed using Gas Chromatography/Mass Spectroscopy (GC/MS) to search for biological markers of their botanical origin. All four samples contained methyl benzoate, a biological marker for the Mediterranean species Aleppo pine (Pinus Halepensis) indicating that the tars were manufactured locally rather than imported from Northern Europe. In addition, the presence of several naturally methylated components indicates that they were made from resinous pine wood in a charcoal pile, and not from tapped resin.

Several select ambers were characterized by Thermogravimetric Analysis, (TGA) Attenuated Total Reflection Infrared Spectroscopy (ATR), and X-Ray Fluorescence (XRF). TGA was used to study the thermal behavior of amber. The ambers were found to be stable to about 260°C, with 5% weight loss occurring between 229-315 °C. The older ambers showed a higher amount of residue remaining at 500°C, which is consistent with the fact that older ambers are more cross-linked. XRF was used to analyze metals in amber. All the amber samples in this study showed the presence of calcium and iron, most also contained lead and sulfur. While no apparent pattern emerged between the various types of amber which could distinguish one from another, the absence of these metals could help in the identification of fake or synthetic ambers. Attenuated Total Reflection Infrared spectra (ATR-IR) were collected to compare with the IRs of ambers previously recorded using the KBr pellet method. No significant differences were seen between the two spectra except for a much larger –OH stretch present in the KBr method. It was concluded that ATR-IR can be used to analyze ambers and is preferred because it is non-destructive and less likely to absorb water. The ratios of the C=O peak (1730 cm-1) to the C-H bend peak (1448 cm-1) were calculated as an indicator of degradation in each sample, and the Baltic amber 46.024 A was found to be more degraded than Gedanite Paris 100.1366.