1. A vertically upright furnace for processing heat expandable ore such as perlite, vermiculite or the like, the furnace comprising: a rigid, upright frame adapted to be disposed upon a supporting surface; an elongated furnace tube adapted to be supported by said frame, the furnace tube including an upper product output orifice and a lower, reduced diameter heat input orifice; means for preheating a raw ore and inputting pre heated ore into said furnace tube for expansion, the preheating means including: an elongated preheating enclosure secured to said furnace in heat exchange relation with respect to said furnace tube; and, a plurality of heat conducting baffles for contacting and warming raw ore, the baffles adapted to be removably coupled within said enclosure in vertically spaced-apart, generally inclined relation therewithin; means for injecting a stream of hot gas into said furnace tube to heat and thereby expand ore there within; and, means for selectively bypassing said raw ore preheating means to permit use of said furnace with ores of varying types.