A pale grey, porous, volcanic stone. Pumice is composed of potassium aluminium silicate with small amounts of iron and alkalis. Its spongy texture is due to numerous cavities formed by gas bubbles that were trapped when the stone solidified. Pumice is used as an abrasive. It has also been used as a coarsening agent for texturising painted surfaces. It is mentioned as grinding stone for polishing stucco marble by repeated grinding and polishing, performed with the use of a lot of water and various grinding stones, including pumice and sandstone for the first and second grinding; serpentine, limestone, hornblende, jasper, bloodstone for dark surfaces and meerschaum for white surfaces. Today also synthetic pumice and sanding paper are used for some of the grinding and polishing steps. Pumice can also be added to lime to obtain a hydraulic set. Synonyms: volcanic glass. Wittenburg (1999); Trench (2000); Museum of Fine Arts, Boston (2000). [J. Cassar, R. de Angelis]