Abstract: Obsidian having less than one percent of combined water is expanded to a volume many times the original by a two stage heating process carried out in at least one refractory-lined, rotary kiln. It is heated gradually by a first flame to a temperature of 1800-1900 F., then subjected to a second flame at a temperature of 2400-2500 F. for a very few minutes. The heating is carried out in the presence of dust which has a softening point considerably above those temperatures. The dust prevents the softened obsidian from sticking to itself or to the lining of the kiln. In some embodiments, the entire process is carried out in a single kiln with both flames entering the discharge end of the kiln, the first flame being directed toward the feed end and the second flame directed transversely or toward the discharge end. In another embodiment, the obsidian is gradually heated in a first kiln, then immediately fed to a second rotary kiln where the second flame enters the feed end and is directed toward the discharge end.