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

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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.
Original languageEnglish
JournalStudies in Conservation
Number of pages16
Publication statusPublished - 9 Mar 2024


  • pressure plate method
  • pore size distribution
  • medieval plaster
  • clay poultice
  • migration
  • rentention
  • capillary advection
  • cleaning
  • desalination

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