ESR investigation of structure and dynamics of paramagnetic centres in lime mortars from Budinjak,Croatia

This study presents the preliminary results of investigation of the types and dynamics of paramagnetic centres in lime mortars from Sveta Petka church in Budinjak, Croatia, using Electron Spin Resonance (ESR) spectroscopy. The excavation in Budinjak discovered a very unique four lobed plan object Sveta Petka, with no additional finds or reliable historical records about the time of its construction. The lime mortars from the church were chosen for analysis in order to characterize the building material and to verify the site chronology by dating them. Lime mortar is valuable but problematic material for luminescence and radiocarbon dating. This type of material has not been dated before using ESR; therefore, careful studies are required to identify the useful paramagnetic centres. The ESR approach suggested in this work concentrates on a calcium carbonate signal. All samples were γ-irradiated in ⁶⁰C bomb with the doses of 1, 10, 20, 50, 80 and 100 kGy. In all spectra signals from Fe³⁺ and Mn²⁺ ions have been observed. Paramagnetic centres which give the ESR signals may be interpreted as CO₂^?, CO₃^?, CO₃^3?, HCO₃^2?, SO₂^?, SO₃^?, PO₂^? and PO₃^2? species. However, all spectra are complex and signals are interfering; therefore, computer resolution enhancement method will be needed in further research. The changes in ESR signals amplitude measured at magnetic field range about 3440–3450 G were analysed versus the dose of irradiation, using Mn²⁺ signals as a reference. Exponential growth of the curve and saturation for doses above 20 kGy were observed; therefore, irradiation with smaller doses is required. These preliminary studies will be helpful in future attempts of dating lime mortars by ESR method.