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

Schmid, Thomas; Jungnickel, Robert; Dariz, Petra (2019). Raman band widths of anhydrite II reveal the burning history of high‐fired medieval gypsum mortars Journal of Raman Spectroscopy, 50(8), pp. 1154-1168. 10.1002/jrs.5632

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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 · 2 H2O). Although nowadays mainly hemihydrate-based (CaSO4 · ½ 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°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°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.

Item Type:	Journal Article (Original Article)
Division/Institute:	Bern Academy of the Arts Bern Academy of the Arts > Institute Materiality in Art and Culture Bern Academy of the Arts > Institute Materiality in Art and Culture > Technology in Art and Culture
Name:	Schmid, Thomas; Jungnickel, Robert and Dariz, Petra
Subjects:	N Fine Arts > NA Architecture N Fine Arts > NB Sculpture N Fine Arts > NK Decorative arts Applied arts Decoration and ornament Q Science > QD Chemistry Q Science > QE Geology T Technology > TH Building construction T Technology > TS Manufactures T Technology > TT Handicrafts Arts and crafts
ISSN:	1097-4555
Language:	English
Submitter:	Petra Dariz
Date Deposited:	09 Nov 2023 12:57
Last Modified:	14 Nov 2023 15:55
Publisher DOI:	10.1002/jrs.5632
Uncontrolled Keywords:	analytical methods, gypsum dehydration, high‐fired gypsum mortar, Raman band width determination, thermal anhydrite
ARBOR DOI:	10.24451/arbor.20366
URI:	https://arbor.bfh.ch/id/eprint/20366