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- Raman microspectroscopy (2) (entfernen)
Measuring the Burning Temperatures of Anhydrite Micrograins in a High-Fired Medieval Gypsum Mortar
(2017)
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 microscopic imaging was just recently introduced into the analysis of residual Roman and Portland cement grains in 19th century cement stone, displaying evidence of the experimental adaptation of contemporary technological knowledge and practice to local circumstance. Beyond calcium ferrites, this study deals with ferruginous clinker phases that are atypical compared to present-day commercial conditions of manufacture, such as iron oxides, clinopyroxenes or pyroxenoids. Analog, microtexture, mineralogy and chemical composition of pulverized ferrosilicate slag used in the course of the 19th century as mineral additive in lime mortar reflect local resource utilization, recording the melting history within the furnace and the effectiveness of the reduction process of a single smelting event. In the case of the discussed example, chemical imaging by Raman microscopy allowed deducing a lime-rich, low-silica melt exposed to fairly reducing conditions because of the detection of the pyrometallurgic phases fayalite (Fe2SiO4), kirschsteinite (CaFeSiO4) and calcioolivine (Ca2SiO4) in zoned olivine laths and (with the melilite gehlenite, Ca2Al2SiO7) in the Interstitial matrix, cross-cut by dendritic wuestite (FeO). The presented analytical approach faces the high spatial complexity of such mortar samples by microspectroscopic imaging with micrometer lateral resolution and their chemical complexity by extracting the rich chemical information content from Raman spectra. Intensity maps of marker bands provide spatial phase distributions. Furthermore, maps of peak positions can give access to the dissemination of spectroscopically similar phases of solid solution series (e.g., olivine and calcium Aluminate ferrite) as well as varying amounts of incorporated foreign cations (e.g., in hematite).