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

Abstract

ABSTRACT Five clay poultices used in built heritage, stone, and wall painting conservation were examined for their potential to extract impurities from medieval lime-rich wall painting plaster. Since the most efficient extraction is achieved by capillary advection, this evaluation compared the pore size distribution of the clay poultices with that of medieval plaster. The pore size distribution was measured using the pressure plate method, a method with a long history of use in soil and building science. The present study demonstrates for the first time a successful application of this method in the evaluation of clay poultices for conservation. In addition, the retentive capacities of the five poultices were examined by measuring lateral migration and penetration depths as a function of time, using customised plaster samples and a blue dye to facilitate migration measurements. Of the five poultices, bentonite showed the least promising pore size range and was estimated to have little or no effect as a drying poultice for medieval wall painting plaster. The remaining four poultices showed promising results, although with various shortcomings that might influence the results of the intervention, as the best fit is dependent on the aim of the poulticing. For a cleaning poultice, where dirt is to be extracted solely from the surface of the plaster, attapulgite and PANGEL® S1500 seemed to be best suited due to their high retention. The pore size distribution and migration pattern of PANGEL® S9 and sepiolite indicated that they could be suitable as poultices for desalination.