Abstract: A bottom-up cocurrent combustion furnace for the production of synthetic microspheres by thermal expansion of glass particles is provided having improved characteristics with regard to anti-fouling, process efficiency, and yield. The disclosed furnace uses preheated combustion air to preheat the feed material and to convey the feed material in a dilute phase transport regime to a burner. The combustion air, fuel, and feed material are premixed prior to being injected though the burner. The feed material rapidly expands as it is ejected through the burner and through a flame and then rapidly cools to solidify the microspheres. Additional features are provided to prevent the furnace from fouling by keeping the feed material away from the furnace walls, removing feed material that adheres to the furnace walls, and collecting feed material that agglomerates or does not expand.