Pumice

PUMICE (Pumex, Lapis Pumicis), a mineral of volcanic origin belonging to the rhyolite or liparite rock family. Rhyolites have a composition similar to granite: SiO₂ — 72%, Al₂O₃ — 16%, FeO.Fe₂O₃ — 1.5%, CaO — 1.5%, MgO — 0.5%, K₂O — 2.5%. A variety of rhyolite, consisting chemically of acidic volcanic glass and passing into a homogeneous light-colored and vesicular glassy rock, is called pumice. P. is formed during the rapid cooling of lava and the presence of a large amount of water vapor in it; in this case, the rapid release of vapor promotes the formation of a spongy, foamy mass consisting of intersecting thin glassy films. P. consists of light pieces of white, gray, and yellow color, easily turning into powder. P. enters into the composition (5–10%) of pumice soap for washing heavily soiled hands and for rough skin; in dentistry, P. is applied for cleaning teeth; the application of P. in tooth powder is harmful, since P. damages tooth enamel. In cosmetics, P. is used for the destruction of hair on the face by means of daily careful wiping of the skin with a piece of P. P. is also applied for polishing wood, bone, horn, metal, etc., and in chemistry for the absorption of gases. In recent times, P. is widely applied in construction, giving a building material with cement.