Laboratory-prepared lime-gypsum mixtures based on the know-how of traditional technology

Stratigraphically, the oldest layer of paint which was applied to the facade walls of the Novo Celje Baroque Mansion contains substantial amounts of gypsum, as well as calcium carbonate and iron pigment. Microstructural, isotopic, and thermal analyses of facade samples were performed in order to exclude the possibility of an anthropogenic source of gypsum crystallisation, and to prove that natural gypsum was used to prepare the original lime-gypsum suspension. This is because, in the exothermic process of quicklime hydration, natural gypsum is converted into ß-hemihydrate and then, after cooling of the suspension, this compound is converted back into gypsum. The technology which was used for the laboratory-level preparation of the investigated lime-gypsum suspensions is simple, efficient, and sustainable, and could thus be used instead of the more energy-consuming dehydration processes which are presently used in the industrial production of gypsum.