Hot-mixed lime mortar: historical and analytical evidence of its use in medieval wall painting plaster

This paper presents evidence supported by scientific analyses and historical documents that medieval plasters used for wall paintings in Denmark were, with high probability, produced by mixing quicklime, aggregate and water in an exothermic process, resulting in a mortar referred to as hot-mixed. This process enables the production of plaster with a very high binder content. Previous thin-section analyses were supplemented by recent analyses of samples collected from three medieval wall paintings in Danish churches. Also included in the analyses were plaster samples produced from hot-mixed mortar and samples produced from lime putty mortar. The analyses of the medieval plasters showed an average binder content in the range of two parts lime to one part aggregate. Moreover, lime inclusions, a characteristic of hot-mix, were found in all samples. SEM analyses showed a similar microstructure in the medieval plasters and the samples produced by hot-mixed mortar, while significant differences were found between the medieval plaster and lime putty samples.