of the core drill alternatively this castable or rammed patch is left complete to be drilled with the crown the crown is then drilled using a diamond core drill which generally must be water cooled in the case of zircon and zirconia burner blocks 24 these are pre heated to the maximum practical temperature before installation in order to prevent thermal shock in the case of using a bonded azs burner block 24 because of chemical incompatibility between the azs and the silica crown it is necessary to apply a chemical barrier that is a chemically compatible material such as a zircon cement and the like to the external surfaces of the burner blocks the barrier applied burner block is advantageously left overnight on top of the crown in order to set and to preheat all three types of burner block 24 are preferably installed with holes 25 28 in the top of the burner block plugged to minimise stack venting and to stop the flow of hot furnace gases through the block as shown in fig6 such holes include an aperture 25 for accepting the burner as well as apertures 28 for accepting fuel discharging means and oxidant discharging means this procedure minimises the risk of thermally shocking and breaking the burner block in most instances it is preferred to install the preheated burner blocks immediately following the drilling of the crown since the freshly cut crown has been cooled.