a method and an apparatus for heating glass sheets in a tempering furnace . the tempering furnace includes rollers for conveying and oscillating the glass sheets during heating , and resistors for heating the glass sheets from above and from below . the lower part of the tempering furnace is cooled at the initial stage of the heating period , and the lower surfaces of the glass sheets are heated at the final stage of the heating period by forced convection . this ensures good control over heat equalization in the tempering furnace , increases furnace power , prevents curving of the glass sheets , and also reduces heat shock from ceramic rollers at the initial heating stage .