Water absorption in medieval lime plaster: Influence of binder ratio, preparation protocol, and limewashing

Danish medieval wall paintings from the Gothic period are painted on thick limewash layers applied to very lime-rich pure air-lime plasters. This study examines wall painting plasters, replicated for the purpose of conservation trials, particularly focusing on their resemblance to medieval plasters in terms of water absorption properties. Plaster specimens with two different binder-to-aggregate ratios were examined, with and without the application of limewash. Moreover, the impact of the mortar mixing technique was tested, juxtaposing dry hot-mixed and wet hot-mixed mortars. Their resemblance to original wall painting plasters in terms of microstructure, porosity, and water absorption properties was assessed by comparing laboratory measurements to those obtained in situ on medieval plasters. The study shows that the binder content is the most important factor in replicating the general moisture-physical properties of medieval plaster. The preparation protocol has a clear impact on the crystal structure of the lime matrix, but no significant impact on the water absorption properties. Limewashing increases the initial water absorption rate, whereas the subsequent water absorption is affected by the structure of the underlying lime plaster matrix.