thin relative to the graphite block 1 the thickness of the layer 2 is more than two orders of magnitude and typically nearly three orders of magnitude less than the thickness of the block 1 for example the corundum coating is about 3 mm thick and the corundum tile layer is about 0 5 to 2 mm thick the graphite block in one preferred embodiment is about 1 2 m 1200 mm thick as shown in fig2 a the protective layer 2 is a coating layer 2 that forms an intimate bond with the graphite block 1 in a preferred embodiment a slurry of approx 75 fine powder of corundum and approx 25 sialon is deposited on the block 1 and then baked at a temperature of approx 2500 u00b0 c the resulting coating coating layer 3 has a thickness of approx 3 mm in an alternative embodiment which is illustrated in fig2 b the protective layer 2 may also be formed by gluing corundum tiles 4 on the graphite block 1 the corundum tiles 4 have a thickness of 0 5 1 mm they are rather thin because the protective layer 2 is primarily important for protecting the furnace shell.