Abstract: The output of coke ovens is increased if the brick constituting the wall between the flues and the coking chamber has a thermal conductivity matched to the rate at which the coke accepts heat. Magnesite has been found to have appropriate thermal conductivity. This material at the lower end of the coking range, namely 1200.degree. C, is impractical due to the fact that it develops cracks and flakes off. However at the higher end of the desired coking range, namely 1300.degree.-1400.degree. C, these difficulties do not appear, and the use of the material leads to increased output of the furnace. More particularly according to the invention firebrick is used which has a thermal conductivity of 2.0-6.0kcal/mh.degree. C and is composed of 20-45% SiC, 23-57% Al.sub.2 O.sub.3 with the remainder consisting of SiO.sub.2 and traces of other oxides.