For melting an inorganic fiber which is mixed with an organic substance, the inorganic fiber material and organic substance are heated in a furnace to a temperature sufficient for the organic material to combust, and sufficient oxygen enriched gas is fed to the furnace that the combustion of the organic substance produces sufficient heat to melt the inorganic material. The oxygen enriched gas is preferably oxygen enriched air having an oxygen content of at least 40% by volume. It should be fed in sufficient quantity to heat material having an adiabatic temperature of at least 850.degree. C., and preferably 1200.degree. C. which permits the melted inorganic material to freely flow out of the furnace. The oxygen enriched gas is preferably fed to a location adjacent a boundary layer between the melted material and unmelted material on the top thereof.