Elemental Composition of Glass Beads Excavated from Saleh Davoud Tombs in Susa by micro-PIXE: Evidences of Trade of Glass Artifacts During the Parthian Period

Glass beads are unique objects to investigate the trade and exchange of these artifacts between different regions and cultures in the past. The site of Saleh Davoud and its ancient tombs in Susa were excavated in two seasons in 2000 and 2004, under the supervising of Mehdi Rahbar. From these excavations, a significant number of colored glass beads and a number of pieces of glass vessels have been unearthed. In regard to the other objects such as coins, potteries, etc., the period of these glass artifacts dated back to the Parthian-Elymae period. The majority of glass vessels found in Saleh Davoud are simple in shape (undecorated) with green hue in contrast to the glass beads which were considered in much diversity of color. Meanwhile, the different color lines and the gilded layer were used to decorate on the external surface of beads. In this study, totally 13 samples of glass objects from Saleh Davoud, including five samples from glass vessels and eight samples from colored beads have been selected to analyze by micro-ion beam techniques. The aims of the analysis of these objects by micro-PIXE were to focuse on the elemental composition of the main body (bulk glass composition) of the artifacts as well as the decorations which were applied in order to identify the raw materials and get information about the mineral pigments. The thickness and purity of the gold in the gilded layer that was used in a few glass beads were also measured by micro-RBS. The most important questions and hypotheses in this research are: 1- Whether glass beads and glass vessels are locally produced or are imported from other areas; 2- Determination of the pigments and the elements that are responsible for different surface decoration of beads; and 3- Measurement of thickness and the purity of the gold layer using Rutherford Back Scattering (RBS) technique to obtain information about the gilding process. The micro-Ion Beam analysis was performed with microprobe system manufactured by Oxford Instruments using the 3 MV Van de Graaff accelerators at the Nuclear Science & Technology Research Institute in Atomic Energy Organization of Iran. The samples were analyzed in a vacuum chamber using a beam of 2.5MeV protons focused to a diameter less than 10 μm. The beam current was in the range of 30 to 50 pA. Characteristic X-rays were detected using a Si(Li) detector with an active area of 60 mm2 positioned at an angle of 135◦ relative to the incident beam direction and with an energy resolution of 150 eV for Fe-Kα. Elemental analysis and measurement of the constituents of these samples show that all analyzed glasses from the Saleh Davoud are Soda-lime-silica glass type. However, the weight percentages of magnesium oxide (MgO) and potassium oxide (K2O) in the analyzed samples suggest that the glass beads are manufactured by the mineral soda (natron) flux, and glass vessels were used the plant ashes as a flux. The amount of MgO and K2O are the main reason for this identification. These two oxides in the glass beads are less than 1.5wt. %, while in the glass vessels are more than 2.5wt. %. Therefore, the glass beads were most likely imported from other areas such as Egypt or the eastern Mediterranean Sea through the trade and exchange of glass products to the Saleh Davoud. Moreover, based on the elemental maps from the micro-PIXE analysis, the elements which are responsible for, red, green, yellow and white colors might be identified as Fe, Cu and Pb.