Crystallinity mapping
The research question of the previously unknown calcination temperatures of high-fired medieval gypsum mortars was found to be accessible by quantitative measurements of anhydrite II crystallinities by Raman band width determination and thus, triggered detailed studies on the evaluation of Raman band shapes. A detailed study published in the Journal of Raman Spectroscopy (JRS 50 (2019) 1154-1168) describes strategies for selecting appropriate fit functions and correcting band width data with respect to instrument-induced broadening. Finally, application of the analytical method for firing-temperature determination is demonstrated based on a sample from a high-medieval Pietà sculpture at the Benedictine abbey Marienberg (Burgeis, South Tyrol, Italy). The story made it onto the journal cover.
Excerpt from the cover image of the Journal of Raman Spectroscopy 50(8) (2019) showing different aspects of Raman band width determination of anhydrite II in a high-fired medieval gypsum mortar.
The method was applied to stucco ornaments and sculptures from South Tyrol (Italy) based on high-fired medieval gypsum mortar : St. Peter above Gratsch, Marienberg above Burgeis, and St. Benedict in Mals. The burning temperatures were confirmed by studying products of pyrometamorphic conversions of accessory minerals from the gypsum stones used as raw material of the calcination process. The study yielded calcination temperatures in the range of 600°C to 900°C.
As a side-product of the research on historical high-fired gypsum, the same authors published a comprehensive study on the phases of the CaSO4–H2O system and postulated a two-fold sub-division of anhydrite II into disordered AII and crystalline AII based on differences in crystallinity and reactivtiy. The first spectroscopic differenciation of the α and β forms of CaSO4∙½H2O is based on Raman crystallinity measurenements too. If you would like to learn more about the system gypsum (CaSO4∙2H2O)–bassanite (CaSO4∙½H2O)–anhydrite (CaSO4), this open access article is our recommended read: Minerals 10 (2020) 115.
Selected publications:
T. Schmid, R. Jungnickel, P. Dariz, J. Raman Spectrosc. 50 (2019) 1154-1168.
P. Dariz, T. Schmid, Mat. Char. 151 (2019) 292-301.
T. Schmid, R. Jungnickel, P. Dariz, Minerals 10 (2020) 115.
Cover image:
J. Raman Spectrosc. 50(8) (2019).