TY - CONF A1 - Schmid, Thomas A1 - Dariz, P. T1 - Raman band widths of anhydrite II reveal the burning history of high‐fired medieval gypsum mortars N2 - An interdisciplinary collaboration between art technology and analytical sciences yielded an approach based on Raman microspectroscopy for the determination of the burning temperatures applied during the production process of high-fired medieval gypsum mortars. Analytical challenges and applications of the approach to examples from the cultural heritage of South Tyrol are presented. T2 - 10th International Congress on the Application of Raman Spectroscopy in Art and Archaeology CY - Potsdam, Germany DA - 03.09.2019 KW - Raman microspectroscopy KW - High-fired medieval gypsum mortars KW - Thermal anhydrite PY - 2019 AN - OPUS4-49624 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Schmid, Thomas A1 - Dariz, P. T1 - Raman Microspectroscopic Imaging of Binder Remnants in Historical Mortars Reveals Processing Conditions N2 - Binder remnants in historical mortars represent a record of the connection between the raw materials that enter the kiln, the process parameters, and the end product of the calcination. Raman microspectroscopy combines high structural sensitivity with micrometre to sub-micrometre spatial resolution and compatibility with conventional thin-sectional samples in an almost unique fashion, making it an interesting complementary extension of the existing methodological arsenal for mortar analysis. Raman spectra are vibrational fingerprints of crystalline and amorphous compounds, and contain marker bands that are specific for minerals and their polymorphic forms. Relative intensities of bands that are related to the same crystalline species change according to crystal orientations, and band shifts can be caused by the incorporation of foreign ions into crystal lattices, as well as stoichiometric changes within solid solution series. Finally, variations in crystallinity affect band widths. These effects are demonstrated based on the analysis of three historical mortar samples: micrometric distribution maps of phases and polymorphs, crystal orientations, and compositional variations of solid solution series of unreacted clinker grains in the Portland cement mortars of two 19th century castings, and the crystallinities of thermal anhydrite clusters in a high-fired medieval gypsum mortar as a measure for the applied burning temperature were successfully acquired. KW - Cement clinker remnants KW - High-fired gypsum KW - Thermal anhydrite KW - Spectroscopic imaging KW - Raman microscopy PY - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:b43-515038 VL - 2 IS - 2 SP - 1662 EP - 1683 PB - MDPI CY - Basel AN - OPUS4-51503 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - CONF A1 - Schmid, Thomas A1 - Dariz, P. ED - Ziemann, M. T1 - Raman band widths of anhydrite II reveal the burning history of high-fired medieval gypsum mortars N2 - 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. Because of the absence of medieval textbooks, the observation of high-temperature, low-pressure mineral transformations and the correlation of phases coexisting in not hydrated binder relicts in the gypsum matrix to the mineralogy of the raw material and the burning conditions constitute the only source to the historical technological know-how. The CaSO4–H2O system consists of five crystalline phases, which can be discriminated by structural analysis methods, such as Raman spectroscopy, due to obvious differences in their spectroscopic data: gypsum (CaSO4 ⋅ 2 H2O), bassanite (hemihydrate, CaSO4 ⋅ ½ H2O), anhydrite III (CaSO4), anhydrite II (CaSO4), and anhydrite I (CaSO4). Only recently, it was possible to demonstrate that small spectroscopic variations exist also within the relatively large stability range of anhydrite II from approx. 180°C to 1180°C: all Raman bands narrow with increasing burning temperature applied in the synthesis from gypsum powder. The determination of band widths of down to 3 cm-1 and differences between them of a few tenths of a wavenumber is not a trivial task. Thus, this contribution discusses peak fitting and strategies for correction of instrument-dependent band broadening. Raman maps of polished thin sections of gypsum mortars provide access to the burning histories of individual remnant thermal anhydrite grains and enable the discrimination of natural anhydrite originating from the gypsum deposit. This novel analytical method was applied to samples from medieval South Tyrolean stucco decorations and sculptures. Beyond that, Raman microspectroscopy was employed for following pyrometamorphic reactions in natural impurities of the raw material. In the presented 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 yielded – after hydration and carbonation – magnesite MgCO3, CaCO3 polymorphs and magnesian calcite. Lower burning temperatures, which leave the accessory minerals in their pristine form, can be traced by measuring the spectra of anhydrite crystalites in grains of firing products and evaluating Raman band widths. Throughout the applications of this analytical method so far, calcination temperatures ranging from approx. 600°C to 900°C were determined. T2 - 10th International Congress on the Application of Raman Spectroscopy in Art and Archaeology CY - Potsdam, Germany DA - 03.09.2019 KW - Raman microspectroscopy KW - High-fired medieval gypsum mortars KW - Raman band width KW - Gypsum dehydration KW - Thermal anhydrite PY - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:b43-496204 SP - 36 EP - 37 PB - University of Potsdam CY - Potsdam AN - OPUS4-49620 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Schmid, Thomas A1 - Jungnickel, R. A1 - Dariz, P. T1 - Raman band widths of anhydrite II reveal the burning history of high‐fired medieval gypsum mortars N2 - 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 hemihydratebased (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. KW - Analytical methods KW - Gypsum dehydration KW - High-fired gypsum mortar KW - Raman band width determination KW - Thermal anhydrite PY - 2019 U6 - https://doi.org/10.1002/jrs.5632 SN - 1097-4555 VL - 50 IS - 8 SP - 1154 EP - 1168 PB - Wiley AN - OPUS4-48757 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -