https://doi.org/10.3390/heritage6010010
“Several samples coming from the recently discovered (February 2019) Late Bronze Age/Early Iron Age Chovdar necropolis in Azerbaijan were analysed using the X-ray fluorescence (XRF) technique. The analysis allowed a preliminary classification of the samples in eight groups based on their composition, obtained from the XRF spectra using the fundamental parameter method. A more detailed classification was then obtained using the graph clustering method.”
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2. Materials and Methods
The travel restrictions imposed due to the COVID-19 emergency have denied the possibility of performing an in situ study of the metallic findings of the Chovdar necropolis. However, 18 samples from metallic objects were available for spectral analysis, each of them found in a different grave during the 2019 excavation campaign (see Table 1 for a description of the objects).
Table 1. List of the metallic objects analysed by XRF.
The samples were taken from the objects in the form of small metal chips (see Figure 1).
Figure 1. The metal chips taken from one of the samples. The red spot is the laser pointer of the XRF instrument. The plastic support does not contribute to the XRF spectrum.
For the XRF analysis, the chips were loosely compacted on a plastic substrate and fully recovered at the end of the measurement. The spectrum of the plastic substrate was acquired before the analysis, to be sure of not introducing any contribution of the substrate in the spectrum of the samples.
The instrument used for the analysis was the Elio XRF spectrometer from Bruker. The measurement was performed in 60 s, with the X-ray tube voltage set at 40 kV and a current of 80 μA (320 mW). The anode is made of rhodium; the characteristic fluorescence lines of the anode do not interfere with the emission of the samples (Cu-based alloys).
3. Results
The main elements detected in the alloys, in different proportions, are Cu, Zn, Pb, As, Sn, Sb, Fe. Environmental elements such as calcium, strontium and potassium were detected but not considered for classification and analysis (see Figure 2)
Figure 2. The XRF spectra of the 18 samples. The spectra are shifted vertically for a better legibility. The square root of the spectra is plotted to demonstrate the weak lines of the minor elements. The vertical lines mark the XRF emission of the main elements of the samples.
The quantitative analysis of the alloys’ composition was performed using the fundamental parameters method [11] with the open-source software pyMCA [12]. The results obtained have a relatively large uncertainty, since the method would require the use of at least one standard of similar matrix, which in our case was not available. However, by imposing the conditions that the sum of all element concentrations should be equal to 100%, a reliable estimation of the samples’ composition can be obtained, with relative errors in the order of 10% for the elements with concentrations higher than 1 wt%. Larger relative errors (of the order of 50%) are expected for trace elements at lower concentrations. The accuracy of the results is in any case sufficient for a classification of the samples, because the composition of the samples is also qualitatively different. The discriminating elements for classification are zinc (Zn), lead (Pb), arsenic (As), tin (Sn) and antimony (Sb).
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