Application of fresh mortar tests to poultices used for the desalination of historical masonry

An European enquiry revealed that workability and adhesion of poultices is the main criteria for conservators to select poultice recipe. The poultice needs first to adhere to the substrate and be easy to use and prepare before presenting a real effect on salt extraction. However, a procedure to test the consistency and workability of fresh poultices does not exist neither in practice nor is it documented. Users usually rely on their empirical know—how to determine the most adapted consistency of poultices for a given type of substrate. The purpose of the following study was to scientifically determine the consistency and workability of fresh poultices. A test procedure was adapted from the fields of mortar, cement and concrete. In the same way as for cement and mortar standards, poultices can be classified in different categories according to their composition. It has been demonstrated that the determination of consistency helps to optimise poultice water content, while the workability has a direct influence on the adhesion of the poultice to the substrate. The knowledge of these properties is essential to determine the best conditions for a desalination treatment. Finally, optimal ranges of consistency and workability are established for the main categories of poultices (cellulose and mineral) and may help end-users to improve their own poultice recipes. Shrinkage limits of poultices are determined to insure their good adherence of the substrate during drying.