Abstract: Heat softenable material, typically glass, is heated in a furnace from above its surface, as by fossil fuel burners, and below its surface, as by Joule effect heating by electrical current flowing between electrodes immersed in the melt where predetermined relationships of the amount of heat supplied from each source is automatically established. One form of coordinated control employs ratio control where the relationship of fossil fuel burner heat supplied is maintained in constant proportion to the Joule effect heat supplied. Melter temperature is a primary control for a combustion air supply. A combustion air-fossil fuel ratio controller proportionally controls Joule effect heat applied in one embodiment. In other embodiments the electrical controller provides the master signal to a combustion air control proportionate to the requisite change of heat input dictated, or the combustion air control issues a ratio signal to the electrical controller for proportional changes.