Perlite is a petrographic term (not a trade name) for a siliceous volcanic rock which naturally occurs in certain regions throughout the world. The distinguishing feature, which sets it apart from other volcanic minerals is it's ability to expand four to twenty times it's original volume when heated to certain temperatures.When heated above 1600°F, crushed perlite rock expands in a manner similar to pop corn. This expansion is due to the presence of combined water within the crude perlite rock. The combined water vaporizes during the heating process and creates countless tiny bubbles in the heat softned glassy particles. It is these diminutive glass sealed bubbles which account for it's light weight and other exceptional physical properties of expanded perlite. Expanded perlite can be manufactured to weigh as little as 2.5 lbs. per cu. ft. RYOLEX® is a trade name applied to expanded perlite processed exclusively by Silbrico Corporation.