Abstract: Small articles such as springs are heat treated as they flow downward by gravity through a straight cylindrical tube. The tube need not be cylindrical or straight and means to assist gravity may be provided. The articles are heated by hot air or other gas introduced to said tube through apertures in its walls and which flows through portions of the tube at high velocity. Hot air thus introduced is withdrawn through other apertures in the walls of the tube in a manner to prevent heat loss by exhaust of the hot air at either end of the tube or by introduction into the tube of ambient cool air at either end of the tube.