Compositionally unusual slag-like artefacts from polycultural archaeological site Boršice (Chřiby Mts., Czech Republic)
Vol.29,No.1-2(2022)
We present results of laboratory investigation of slag-like artifacts, found on an archaeological site near Boršice, known by occurrence of artifacts from the Early Neolithic (5 700–5 000 BC), Bronze and Early Medieval Ages. The grey-coloured foamy slag-like artifacts reach up to 5 cm in size and often display original lava-like surface. These slags are formed especially by strongly potassic (15–23 wt. % K2O) silicic (54–72 wt. % SiO2) glass with elevated contents of Ca (4–15 wt. % CaO), P (1–5 wt. % P2O5) and Mg (1–5 wt. % MgO) and only traces of Al, Mn and Fe (below 0.14 wt. % of oxides of each of these elements). Glass phase is partly altered due to weathering, which caused the glass hydration and almost complete loss of K. Sparse minute crystals of wollastonite (CaSiO3) and an unidentified Ca-K-Si-P-O phase (with highly variable compositions and poor stoichiometry) occur in places in the glass phase. We suggest that the origin of the studied slag-like artifacts is probably connected with either modern or ancient production of glass. However, it is certainly not modern foam glass, industrially produced worldwide from used waste glass. Based on average chemical composition of glass, we suggest that the studied artifacts could originate by smelting of a mixture of quartz sand, potash, bone ash, calcitic limestone (or calcitic marble) and dolomite in weight proportions equal to 58.9, 24.6, 4.9, 3.3 and 8.3, respectively.
Chřiby Mts.; non-metalurgical slags; wollastonite; chemical composition; glass production
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