temperature indeed such melting normally results in the presence of zirconia in cubic form the ratio r zr b f si is between 2 and 80 it is preferably greater than or equal to 4 preferably greater than or equal to 5 preferably greater than or equal to 7 or even greater than or equal to 9 and or preferably less than or equal to 50 preferably less than or equal to 40 preferably less than or equal to 30 or even less than or equal to 20 an elementary weight ratio r which is too low results in a product that is insufficiently densified and potentially porous if the elementary weight ratio r is too high the product with equivalent density is more sensitive to thermal shock and to corrosion with respect to certain aggressive agents such as alkali metals the content of silica sio 2 must not exceed 0 5 as it may impair the corrosion resistance in particular with too high a content the silica could combine with the impurities to increase the glassy phase harmful to corrosion resistance or react with alumina to form mullite yet the presence of mullite is prejudicial to the resistance to corrosion by papermaking black liquors in a product according to the invention the oxides na 2 o and k 2 o are considered to have similar effects the oxides na 2 o and k 2 o have an unfavorable effect on the resistance to the formation of beta alumina and tend to increase the content of the glassy phase the weight content of na 2 o k 2 o must therefore be less than or equal to.