Phase composition and burning history of high-fired medieval gypsum mortars studied by Raman microspectroscopy

Dariz, Petra; Schmid, Thomas (2019). Phase composition and burning history of high-fired medieval gypsum mortars studied by Raman microspectroscopy Materials Characterization, 151, pp. 292-301. Elsevier 10.1016/j.matchar.2019.03.013

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The use of high-fired gypsum as binder for masonry and joint mortars or stuccowork in Central Europe in the Early and High Middle Ages was a regional specific as it depended on local gypsum deposits. The calcination technology possible at the time resulted in an assemblage of calcium sulphate phases dehydrated to different degrees and partly thermally damaged accessory minerals of the raw gypsum. Not hydrated clusters of firing products preserved in the binder matrix are a typical feature of such mortars. A novel Raman microspectroscopic approach, providing access to the burning history of individual anhydrite grains, was applied to samples from medieval South Tyrolean stucco decorations and sculptures. Beyond that, Raman microspectroscopy was employed for tracing and visualising pyrometamorphic reactions in natural impurities of the kiln run. In the discussed examples mineral thermometry indicates process temperatures above 800°C: the breakdown of magnesium-rich chlorite led to the formation of forsterite Mg2SiO4, while the thermal decomposition of dolomite CaMg(CO3)2 to periclase MgO and lime CaO yielded – after hydration and carbonation – magnesite MgCO3, CaCO3 polymorphs and magnesian calcite. Hydration of periclase in the mixed gypsum paste containing sulphate ions also resulted in magnesium sulphate hydrates, here identified in the form of hexahydrite MgSO4·6H2O. Lower burning temperatures left the accessory minerals in their pristine form, but can be traced by measuring the spectra of individual anhydrite crystals in grains of firing products and evaluating Raman band widths. Throughout the present study, calcination temperatures ranging from approx. 600°C to 900°C were determined.

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:	Dariz, Petra and Schmid, Thomas
Subjects:	N Fine Arts > NA Architecture N Fine Arts > NB Sculpture N Fine Arts > NK Decorative arts Applied arts Decoration and ornament Q Science > QE Geology T Technology > TH Building construction T Technology > TS Manufactures T Technology > TT Handicrafts Arts and crafts
ISSN:	10445803
Publisher:	Elsevier
Funders:	[UNSPECIFIED] Stiftung zur Förderung der Denkmalpflege
Projects:	[UNSPECIFIED] Die Phasenzusammensetzung mittelalterlicher Hochbrandgipse
Language:	English
Submitter:	Petra Dariz
Date Deposited:	08 Nov 2023 10:46
Last Modified:	08 Nov 2023 10:46
Publisher DOI:	10.1016/j.matchar.2019.03.013
Uncontrolled Keywords:	High-fired gypsum mortar pyrometamorphism anhydrite dolomite forsterite Raman microspectroscopy
ARBOR DOI:	10.24451/arbor.20344
URI:	https://arbor.bfh.ch/id/eprint/20344