Perlite is an amorphous volcanic glass which has a relatively high water content and occurs naturally. It has one of the most unusual property, i.e. it greatly expands when sufficiently heated.
Perlite becomes soft, as it is a glass, at a temperature of 850-900°C. The trapped water in the structure of perlite escapes and makes the material to expand 7 - 15 times of its original volume. The expanded material is brilliant white in color because of the reflectivity of the trapped bubbles.
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