of the tube block in one embodiment in which a 73 4 34 by 73 4 34 by 0 750 34 tube block consists essentially of oxynitride or nitride bonded silicon carbide the weight of the tube block is only about 6 5 pounds in some embodiments having three spaced ridges the central ridge extends farther than the lateral ridges typically this extension is between 0 5 and 1 0 inches longer than the extension of the lateral ridges due to the extremely high temperatures generated in the primary combustion zone or first passage in which the tube blocks are used the tube block typically comprises silicon carbide preferably an oxynitride nitride or oxide bonded silicon carbide however other suitable refractory materials such as alumina zirconia and carbon may be employed in addition to the refractory material per se the tube blocks will further contain a high thermal conductivity bonding system a preferred tube block composition contains about 80 to about 95 parts silicon carbide and about 5 to about 20 parts bonding agent such as a nitride or oxide based material more preferably the block will be made from any of cn 163 cn 183 cn 127 or cn 101 each of which is available from the norton company of worcester mass or comparable refractories any conventional technique typically used in the manufacture of tubes blocks may be used to make the present invention in preferred embodiments a mixture comprising silicon carbide grain and binders is loaded into a dry press and pressed to form a green body the green body is then dried and fired in a tunnel kiln having an oxygen or nitrogen atmosphere to produce a fired refractory the refractory mortar used with the present invention.