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During the heating of glass mulling furnaces with a combustion chamber (8) and with regenerators for preheating oxidation gases and with port necks (1) that open into the combustion chanter (8) mid with primary burners (4, 5.6) and with secondary burners (13) that are installed in a cascade arrangement relative to the primary burners (4. 5,6), said secondary burners (13) are operated as cascade burners with a relatively low proportion of the fuel, the secondary fuel. Flames are thereby created In over- and conditions and the flame gases formed are mixed wilt one another so that the complete combustion process in the combustion chanter (8) is more or less stoichiometric. The secondary fuel is supplied by the secondary burners (13) to a step (7), installed in the port neck (1). In order to prevent or reduce soot and graphite deposits in the step (7) without increasing the nitrogen oxide content hi the furnace waste gases a secondary oxidation gas is blown into step (7) dazing the firing phase of each port neck (1) in addition to the primary oxidation gas preheated in the regenerators that flows over the step (7). If significant deposits of carbon materiel and/or temperature problems occur then in a similar way an oxidation gas can be blown in during the exhaust phase for the furnace waste gases of each neck (1) when the secondary burner (13) is switched off. (FIG.1 )