Investigation of elemental composition of ancient metal objects found in Khorasan-e Razavi Province of Iran using micro-PIXE technique

Abstract

In this work, elemental composition of 18 metallic objects belonging to the Bronze Age found at the Kalate Shoori site in Neyshabur, northeastern Iran, is investigated using micro-PIXE technique. The aim of the analysis is to identify the type of alloys employed in the fabrication of the investigated objects based on their elemental composition and to measure their contents of trace elements. The results of the micro-PIXE analyses indicate that the investigated objects could be categorized in two main groups: The objects of the 1st group (11 objects) are made of copper–silver alloys in which copper content varies within 30–60 wt% while silver content varies within 25–40 wt%. The objects of the 2nd group (7 objects) are made of nearly pure copper with copper content varying within 85–97 wt%. Though all the investigated metallic objects are found in the Bronze Age layers of the Kalate Shoori site, amazingly, no traces of tin were observed in their elemental composition. The measured impurities in the investigated objects could be either due to the raw materials and minerals used in the extraction of copper and silver or due to the soil surrounding the buried objects. Examination of trace elements contents in the investigated objects reveal that objects with the base of copper–silver alloys (i.e., objects of the 1st group) were all fabricated from the same raw materials, while objects with the base of nearly pure copper (i.e., objects of the 2nd group) were fabricated from different mineral materials with either no arsenic content or with varying arsenic contents of more than 1.5 wt%. Comparison of the results of this work with the published data in the literature indicates that similar samples with the base of copper–silver alloys belonging to Bronze Age have already been found in Altin Tepe, Turkmenistan. It could then be concluded that similar techniques might have been employed in the two regions. Moreover, archeological findings at the Kalate Shoori site reveal that there should be cultural and commercial exchanges between this region and the Central Asia.