Raman band widths of anhydrite II reveal the burning history of high-fired medieval gypsum mortars

When used as a mineral binder, gypsum is thermally dehydrated and mixed with water, resulting in a paste hardening in the backreaction to calcium sulphate dihydrate (CaSO4 center dot 2 H2O). Although nowadays mainly hemihydrate-based (CaSO4 center dot 1/2 H2O) binders are employed, higher firing temperatures in medieval kilns yielded anhydrite II (CaSO4). Except for the discrimination of the metastable phases anhydrite III and I due to different crystal structures, variations within the production temperature range of anhydrite II (approximately 300 to 1180 degrees C) were not analytically accessible until recently. This study describes the development of an analytical technique, which is based on steady changes of band widths in room-temperature Raman spectra of anhydrite II as a function of burning temperature. Raman microspectroscopic mapping experiments enable to pinpoint individual unreacted grains of thermal anhydrite in mortars and to discriminate them from natural anhydrites originating from the raw gypsum. The determination of band full widths at half maximum of down to 3 cm(-1) and differences between them of a few tenths of wavenumbers is not a trivial task. Thus, a focus of this work is on peak fitting and strategies for correction of instrument-dependent band broadening, which is often neglected also beyond the field of mortar analysis. Including other potential influences on band widths, burning temperatures of 400 to 900 degrees C can be retraced in high-fired medieval gypsum mortars with an uncertainty of approximately +/- 50 K, as demonstrated with sample material of a stucco sculpture dated around 1400.