Definition of "pummice"

In order to avoid a great deal of trouble, and to perform the work more effectually, by diveſting the new made Cyder of what pummice and other impurities remain; after straining it through a hair ſieve, on its coming from the Wring, or Preſs, it is neceſſary to be provided with a large open vat, keeve, or clive, which will contain a whole pounding, or making of Cyder […]

1753, Hugh Stafford, A treatise on cyder-making, with a catalogue of cyder-apples of character, in Herefordshire and Devonshire. To which is prefixed, A dissertation on cyder and cyder-fruit‎, page 48