Labratorial Studies on the Blue and Green Beads Used in the Prehistoric Ornaments of National Museum of Iran

Abstract
In this work, eight samples in blue, dark blue and green colors that used in three ornaments were analyzed by multi-analytical techniques. One of these artifacts was obtained from the Tepe Hissar, Damghan dated to 3rd millennium BC and two other ones were excavated in the Tepe Nesfi, Gilan dated to first millennium BC and these ornaments belong to the National Museum of Iran. The aims of this study were the identification and characterization of the raw materials, opacifier and colorant agents used in the bead’s samples. Determination and measurement of each of these factors respond to questions raised about the technology and manufacturing recipe, type of the glass and investigation of trade and exchange of beads in prehistoric times. Techniques such as XRD, FTIR, Micro-PIXE and Polarized Light Microscopic (PLM) were applied to characterize the samples. FTIR and micro-PIXE analysis of the blue bead was used in ornaments of Tepe Hesar showed that lapis lazuli used to make the beads. It is a semi-precious stone, and due to the high levels of magnesium and calcium in it, this sample has a considerable mineral content of diopside (CaMgSi2O6). The result of XRD spectra and PLM image of the seven samples of two necklaces of Tepe Nesfi showed that these samples lacked a crystalline structure, indicating the glass structure of the specimens. Elemental composition of the samples of Tepe Nesfi by micro-PIXE show that the all analyzed samples are soda-lime-silica glasses, which is the ash obtained from the plants are considered as the soda source, due to the high potassium and magnesium content. In addition, the results indicated that in these glass beads copper and iron compounds with different weight percentages were used as a colorant agents. Furthermore, antimony (Sb) compounds were used as opacifier in blue beads, but in the dark blue and green glass beads that are translucent glass do not detect any antimony compounds.
Keywords: Bead, Ornament, Lapis Lazul, Micro-PIXE, FTIR, PLM, XRD.
 
Introduction & Method
The use of colored beads in prehistoric ornaments is very common and many studies have been done on elemental analysis, characterization and identification of the structure of beads in the world with different modern techniques. However, archaeological excavation reports in Iran indicated that the large number of beads found in excavations in many archeological sites, especially in the prehistoric cemeteries. On the other hand, the review of researches on historical and prehistorical beads have been excavated in Iran, shows that comprehensive research has not yet been carried using different modern techniques on these kinds of objects.
In the majority of the archeological excavation, the discovered objects are often ornamental items like necklace, bracelet, earring, pendant, hairpin, cloth pin, etc. Most of the ornamental items have been unearthed from prehistoric graveyards because the prehistoric people believed that a man will begin a new life after death and needs provisions for his next life. The use of colored beads in historical ornament is of particular importance, and there are many studies on elemental analysis and characterization of this kind of objects in the world. Due to the large number of beads found in archeological excavations, especially from prehistoric graveyards of Iran, these studies have a high priority and their results are of great help to historians and archaeologists. In this work, eight beads in blue, dark blue and green colors were used in three ornaments analyzed by multi-analytical techniques.  One of these artifacts was obtained from the Tepe Hissar, Damghan dated to 3rd millennium BC and two other ones were excavated in the Tepe Nesfi, Gilan dated to first millennium BC. Tepe Hissar is one of the important prehistoric sites in northeastern Iran, about 2 km south of the city of Damghan in the Semnan province. Tepe Nesfi is an ancient site near Roudbar in Gilan, the northwestern part of Iran. These ornaments belong to the National Museum of Iran. The archeologist in international museums applied different non-destructive techniques to study of ancient materials. In this work, different techniques such as X-ray Diffraction (XRD), Fourier Transform Infrared spectroscopy (FTIR), Proton-induced X-ray Emission in micron scale (Micro-PIXE) and also polarized light microscopic (PLM) were applied to characterize the samples. The aims of this study are to recognize and characterize the raw materials, opacifier and colorant agents were used in the samples of beads.
Identified Traces
FTIR and micro-PIXE techniques show that lapis lazuli used to make of blue colored beads in the necklace were recovered in Tepe Hissar Damghan. The micro-PIXE results show that Si, Mg and Al are the main constituents of blue beads. Other elements such as Ca, Na, K, and S were detected in the analyzed sample in minor contents. Also, two characteristic absorption bands in 997 and 464 cm-1 in the FTIR spectrum confirm the presence of lapis lazuli. FTIR and elemental analysis of the blue sample of Tepe Hesar showed that lapis lazuli used to make the beads. It is a semi-precious stone, and due to the high levels of magnesium and calcium in it, this sample has a considerable mineral content of diopside (CaMgSi2O6). The main ‘true blue’ mineral is lazurite, an aluminosilicate [(Na, Ca)8[(SO4, S, Cl) (AlSiO4)6]) belonging to the sodalite group, associated with some other minerals diopside (CaMgSi2O6), quartz (SiO2), calcite (CaCO3), pyrite (FeS2), to form the lapis lazuli rock. lazurite was the main source of blue for jewelry in the past. Lapis lazuli was brought from Badakhshan in northeastern Afghanistan, indicating that there were a trade and exchange relationships between Badakhshan and Mesopotamia in long distances (about 2,400 km). Furthermore, the XRD spectra and PLM images do not show any crystalline structure in seven analyzed samples from two necklaces in Tepe Nesfi. Also, micro-PIXE analysis shows that the main compositions of these samples are SiO2 (about 65wt%), Na2O (about 10wt%) and CaO (about 7wt%). Therefore, all these samples are Silica- Soda- Lime glass. However, the amounts of magnesium oxide and potassium oxide in all analyzed samples are more than 2.5wt%, and it is clear that the ashes obtained from halophytic plants are considered as supplying source for soda. In addition, the results indicate that in these glass beads copper and iron compounds with different percentages were used as a colorant agent. Moreover, the results show that antimony (Sb) compounds were used as opacifiers in blue beads, but in the dark blue and green glass beads that are translucent glass do not detect any antimony compounds.
 
Conclusion
In this study, eight samples of blue, dark blue and green beads used in three ornaments obtained from excavations of Tepe Hesar Damgahan and Tepe Nesfi, Gilan, which are kept at the National Museum of Iran, are analysed by the instrumental methods such as micro-PIXE, XRD, FTIR and PLM. The results of these studies showed that dark blue beads, from tepe Hesar Damgahan necklaces were made of lapis lazuli. It is a semi-precious stone. The seven samples of two necklace of Tepe Nesfi showed that these samples lacked a crystalline structure, indicating the glass structure of the specimens. Micro-PIXE show that the all analyzed samples are soda-lime silica glasses, which is the ash obtained from the plants are considered as the soda source, due to the high potassium and magnesium content. In addition, the results indicated that in these glass beads were used copper and iron compounds with different percentages as a colorant agent. Furthermore antimony (Sb) compounds were used as opacifier in blue beads, but in the dark blue and green glass beads that are translucent glass do not detect any antimony compounds.