Advanced materials engineering in historical gypsum plaster formulations

We show how historical gypsum plaster preparation methods affect the microstructure and the nanoscale hydrophilic properties of the final stucco materials. We reproduced a traditional Persian recipe (Gach-e Koshteh, ~14th c. AD), which involves a continuous mechanical treatment during plaster hydration. The Koshteh samples were compared with a lab-replicated historical recipe from Renaissance Italy (Gesso Sottile, ~15th c. AD), and contemporary plaster. The Iranian recipe induces the formation of gypsum platelets displaying preferential orientation in the plaster bulk. In contrast, the Italian and regular plasters comprise a typical needle-like morphology of gypsum crystals. The platelets in Koshteh expose the hydrophilic {010} face of gypsum in a much more pronounced manner than needles. Consequently, the Iranian plaster exhibits enhanced wettability, enabling their direct use for water-based decoration purposes without mixing it with any binder material, as is the case for Sottile. Contrary, the preparation of Sottile promotes the exposure of the more hydrophobic gypsum faces conferring a more water repelling character to this type of plaster. These observations highlight the development of advanced functional plaster materials already during the pre-modern era.