A high temperature furnace adapted to expand mineral ores such as vermiculite, perlite and the like. The furnace preferably includes an upright, vertically oriented frame adapted to support an elongated furnace tube into which mineral ore is delivered for expansion. Raw ore may be routed through a preheating system including a plurality of vertically spaced-apart, inclined baffles removably disposed within a preheating enclosure secured to the furnace. A preheater bypass system is in cluded to facilitate use of the furnace with vermiculite.