Method for Qualitative and Quantitative Analysis of Ancient Lead Enamel Using Laser Inducted Breakdown Spectroscopy

Abstract

The method of laser induced breakdown spectroscopy (LIBS) was used in analysis of the archaeological enamel samples from the ancient settlement of Jochi Khan (XIV century). During the qualitative analysis, it was found that the elements of the enamel matrix are Si, Pb, K, Na, Mg, Ca, Al. The glaze color is due to the presence of copper and iron. Clustering by the k-means method revealed two groups of samples similar in composition of enamel, but differing in place of origin within the settlement. For the studied samples the semi-quantitative composition of glazes was established from the spectra of LIBS using a method based on the hypothesis of local thermodynamic equilibrium (LTE) and a chemometric approach (Projection to Latent Structures, PLS). Probabilistic deterministic design of experiment was used to search for pairs of element lines that are not subject to changes when the conditions for spectra registration vary, and to determine the coefficients in the equations based on LTR. Calculations using the PLC method were carried out in the “R” programming environment. The following content of the matrix elements was obtained in terms of the most stable oxides, %: SiO2 — 49–61, PbO — 23–31, MgO — 1.7–2.3, CaO — 4.6–6.9, Na2O — 4.1–5.3, K2O — 5.1–6.4, Al2O3 — 0.8–1.7, CuO — 0.32–0.4, Fe2O3 — 0.09–0.16. The results of both methods are generally consistent with each other. The inaccuracy between 4 parallel determinations were 15-24 % for LTE, and 9–14 % for PLS. Taking into account the data of semi-quantitative analysis, it was concluded that the division into two groups is based on a different ratio of copper and iron in enamel.