Perlite is an amorphous volcanic glass that has a relatively high water content, typically formed by the hydration ofobsidian. It occurs naturally and has the unusual property of greatly expanding when heated sufficiently. It is an industrial mineral and a commercial product useful for its light weight after processing. When it reaches temperatures of 850-900 °C, perlite softens (because it is a glass). Water trapped in the structure of the material vapourises and escapes, and this causes the expansion of the material to 7-16 times its original volume. The expanded material is a brilliant white, due to the reflectivity of the trapped bubbles. Unexpanded ("raw") perlite has a bulk density around 1100 kg/m³ (1. 1 g/cm³), while typical expanded perlite has a bulk density of about 30-150 kg/m³. Because of its low density and relatively low price (about US$50 per tonne of unexpanded perlite), many commercial applications for perlite have developed. In the construction and manufacturing fields, it is used in lightweightplasters and mortars, insulation and ceiling tiles. In horticulture, perlite can be used as a soil amendment or alone as a medium for hydroponics or for starting cuttings. When used as an amendment it has high permeability / low water retention and helps prevent soil compaction. Perlite is an excellent filter aid. It is used extensively as an alternative to diatomaceous earth. The popularity of perlite usage in this application is growing considerably worldwide. Perlite filters are fairly commonplace in filtering beer before it is bottled. Small quantities of perlite are also used in foundries, cryogenic insulation, as a lightweight aggregate in mortar (firestop) and in ceramics as a clay additive. It is also used by the explosive industry