A furnace, and a method of firing it, wherein part of the fuel supplied to the furnace is produced from waste plastics by a depolymerisation process, waste heat from the furnace being used to promote the depolymerisation process. The furnace is equipped with regenerators for waste heat recovery and is fired alternately in first and second opposed directions, with the direction of firing periodically reversing between the first direction and the second direction. The supply of fuel to the furnace is temporarily interrupted while the direction of firing is reversing, means being provided to accommodate the fuel produced during the temporary interruption. The furnace may be used for producing glass.