Measuring the Burning Temperatures of Anhydrite Micrograins in a High-Fired Medieval Gypsum Mortar

Abstract

Typical feature of high‐fired medieval gypsum mortars is a compact microstructure of squat gypsum crystals containing firing products as remains of the calcination process. So far, the burning history of the binder is estimated based on morphological characteristics of the latter. A novel Raman microspectroscopy approach provides access to the calcination temperatures of individual anhydrite grains based on quantifiable spectroscopic changes appearing due to gradual variations of crystallinity, as independently confirmed by X‐ray diffraction analysis of anhydrites synthesised at temperatures between 500 °C and 900 °C. The approach was successfully applied to the high‐fired gypsum mortar of a South Tyrolean stucco sculpture of a pieta dated around 1420. Microparticles of burned anhydrite II with firing temperatures scattered around 650 °C and clusters of thermally damaged natural anhydrite II crystals from the raw material were identified and imaged.

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

“…Within this study, except for Figure and Table S1 (Supporting Information), uncorrected Horiba 532‐nm (see Table ) values are presented in order to keep them comparable with the results of previous studies . The measurements can be reproduced with other instruments and measurement parameters by determining the according w IPF (ideally with the emission lines proposed here, if applicable) and recalculating the instrument‐independent values in Table S1 into the band widths expected for that experimental configuration by employing the following slight variation of Equation : …”

“…The most prominent band of anhydrite II has several advantages as a marker for burning temperature determination compared with the other signals in the spectrum. Not only being the strongest band, which so far was detectable even in spectra with significant contributions from luminescence emission (see Supporting Information of Dariz et al), together with the 1,129‐cm −1 band, it shows the most pronounced shape change at varying temperatures. The less intense signal at 1,129 cm −1 has the disadvantage of being strongly orientation‐dependent, and therefore almost cancelling out at certain orientations of anhydrite crystals with respect to the laser polarisation direction (see Section ).…”

“…Finally, the analytical method is demonstrated by analysing the high‐fired gypsum mortar constituting a pieta sculpture at the Benedictine abbey Marienberg above Burgeis in South Tyrol (Italy), dated around 1400 . From the analytical point of view, the sample material is challenging as it contains a significant amount of primary anhydrite, occurring as natural accessory mineral in the raw gypsum, spread in the binder matrix together with nonreacted secondary, thermal anhydrite …”

“…Although Morris demonstrated the analytical discrimination of two types of bassanite, that is, α‐CaSO 4 · ½ H 2 O and β‐CaSO 4 · ½ H 2 O, by XRD due to their different degrees of crystallinity already in 1963, it took until 2017 that temperature‐dependent quantifiable gradual changes were found within the stability range of anhydrite II. A proof‐of‐concept study revealed a steady increase in crystallite size accompanied by a decrease in stress and strain (as determined by XRD), of the specific surface area (according to the Brunauer–Emmett–Teller method) and of the width of the most prominent Raman band of anhydrite II at 1,017 cm −1 in the burning temperature range of 500 to 900°C …”

“…Morphological changes of thermal anhydrite grains observable by polarised light and scanning electron microscopy (SEM) are discussed in the literature, but quantitative results can only be gathered by a spectroscopic analysis. Raman microspectroscopy turned out to be extraordinarily suitable, because it combines high structural sensitivity with submicrometric lateral resolution enabling to pinpoint individual anhydrite crystallites in mapping measurements of thin‐sectional samples of medieval mortars …”