The present invention relates to a method of making mineral fibers, comprising providing a circulating combustion chamber (1) which comprises a top section (2), a bottom section (3) and a base section (4), injecting primary fuel, particulate mineral material and primary combustion gas into the top section of the circulating combustion chamber and combusting the primary fuel thereby melting the particulate material to form a mineral melt and generating exhaust gases, separating the mineral melt from the exhaust gases wherein the exhaust gases pass through an outlet (8) in the circulating combustion chamber and the mineral melt collects in the base section of the circulating combustion chamber, injecting secondary fuel, which comprises liquid or gaseous fuel, and secondary combustion gas into the bottom section of the circulating combustion chamber to form a flame in the bottom section which heats the melt, and flowing a stream of the collected melt through an outlet (15) in the base section to a centrifugal fiberising apparatus and forming fibers. The present invention also provides an apparatus for use in the method of the invention.