Perlite is a generic term for naturally-occurring sedimentary rock that contains silicon, aluminum, potassium, sodium and other trace minerals. What sets perlite apart from other volcanic glasses is that when it is heated to its softening range, it expands from four to twenty times its original volume.
This expansion is due to the presence of a small percentage of moisture in the crude perlite rock. When rapidly heated to above 1600°F (871°C), the crude rock pops almost like popcorn as the combined water vaporizes and creates countless tiny bubbles. These bubbles account for the extreme light weight and other remarkable physical properties of expanded perlite. It can be manufactured to weigh as little as two pounds per cubic foot, rendering it suitable for numerous applications — including industrial insulation.