Measuring Water Absorption in Replicas of Medieval Plaster - Assessing Their Reliability as Models for Conservation Trials

Using laboratory produced mortar samples in wall painting conservation research requires sufficient chemical and physical resemblance to the original object to make the tests on the samples relevant. This study applies the contact sponge method comparing the water absorption rate of limewashed medieval wall painting plaster with mortar samples of varying binder-to-aggregate ratios produced by the hot-mix technique and with lime putty mortar. Different types of limewashes are also examined. Based on this comparison, the study evaluates the samples’ suitability as models for wall painting conservation research. Moreover, the study explores the validity of the contact sponge method for measuring highly porous mortars.

The experiment demonstrates that the contact sponge method can be used for measuring the water absorption rate of highly porous mortar samples, and that the fastest water absorption rates are found in the mortar samples with the highest lime content. Moreover, the closer the sample’s binder-to-aggregate ratio is to the historical plaster, the more similar the values are. The poorest correlations are found for lime putty mortar samples with a 1:3 ratio heretofore used in conservation trials. Furthermore, the absorption rate is significantly influenced by the type of limewash on the samples.