The Potential of Clay Poultices as Sorbents for Medieval Plaster: A Comparative Study Using the Pressure Plate Method and Dye

Five clay poultices used in built heritage, stone, and wall painting conservation were examinedfor their potential to extract impurities from medieval lime-rich wall painting plaster. Since themost efficient extraction is achieved by capillary advection, this evaluation compared the poresize distribution of the clay poultices with that of medieval plaster. The pore size distributionwas measured using the pressure plate method, a method with a long history of use in soil andbuilding science. The present study demonstrates for the first time a successful application ofthis method in the evaluation of clay poultices for conservation. In addition, the retentivecapacities of the five poultices were examined by measuring lateral migration andpenetration depths as a function of time, using customised plaster samples and a blue dyeto facilitate migration measurements. Of the five poultices, bentonite showed the leastpromising pore size range and was estimated to have little or no effect as a drying poulticefor medieval wall painting plaster. The remaining four poultices showed promising results,although with various shortcomings that might influence the results of the intervention, asthe best fit is dependent on the aim of the poulticing. For a cleaning poultice, where dirt isto be extracted solely from the surface of the plaster, attapulgite and PANGEL® S1500seemed to be best suited due to their high retention. The pore size distribution andmigration pattern of PANGEL® S9 and sepiolite indicated that they could be suitable aspoultices for desalination.