The third ingredient charged to the electric furnace of the present invention is carbon, such as calcined petroleum coke or graphite, in an amount from 1 to 10 times the theoretical amount necessary, based on the amount of TiO2 and Na2 O added, to reduce the TiO2 and Na2 O. As used herein, the amount necessary to reduce the TiO2 and Na2 O is considered according to the following formulas: