exfoliative manner to fill or partially fill the pores thereby reducing the cross sectional space into which slag may penetrate notwithstanding the fact that the size of the pores is reduced thereby impeding penetration of the pores by slag it may still be possible for some slag to penetrate the pores and this is undesirable accordingly after the heating step the block is subjected to a second impregnating step wherein the block is soaked in a second liquid comprising water containing very fine particles of alumina e g less than 2 microns minus 600 mesh this impregnates the block with the second liquid and deposits the very fine particles of alumina within the pores to further decrease the penetrability of the pores by the slag it is important that the second liquid contain very fine particles of alumina in the size range set forth above this is because if the particles in the second liquid were too large these particles could not enter the pores as the latter have a reduced size resulting from the formation therein of aluminum ortho phosphate formed after the first impregnating step to retain the very fine alumina particles in the pores and keep them.