Chemical imaging of historical mortars by Raman microscopy

• Chemical images with approx. 500 nm resolution were acquired by Raman microscopy.
• Clinker remnants in 19th-century Roman and Portland cement stone were analysed.
• Gypsum mortars from the Middle Ages and early 20th century were analysed as well.
• Alite, tricalciumaluminate, wollastonite, gypsum, and anhydrite were identified.
• Dedolomitisation of accessory mineral grains in gypsum mortar was visualised.

Raman microspectroscopic imaging was just recently introduced into the analysis of cement stone. Here, we demonstrate this approach on 19th-century Roman and Portland cement mortars and extend it to gypsum-based samples originating from a medieval stucco sculpture (high-burnt gypsum) and a stucco ornament prefabricated at the beginning of the 20th century (plaster of Paris). Furthermore, the distributions of dolomite and calcite were mapped in an accessory mineral grain with approx. 500 nm lateral resolution demonstrating the ability for studying alteration processes such as dedolomitisation. As we would like to make this approach accessible to other researchers, we discuss its present status, advantages, limitations and pitfalls.

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