Heat-resistant corundum materials are widely used in many fields of engineering because of the availability of the raw material and their excellent chemical resistance and good mechanical and dielectric properties [1].Corundum materials are the strongest in the A1203--SIO 2 system. However, because of their inadequate thermal-shock resistance they can only be used in assemblies which are not subjected to high thermal loading. The thermal-shock resistance of the normal corundum articles is equal to that of high-chamotte refractories [2]. The thermal-shock resistance of dense corundum materials is not very high. Corundum with coarse crystallization is 3-4 times more thermal-shock resistant than the corundum with a fine crystal structure [3]. High-alumina materials are used to improve the thermal-shock resistance [4]. The use of an ethyl silicate binder could be a promising method of improving the thermal-shock resistance of corundum products [5].

specimen breaks. The difference in the readings of the previously graduated micrometer also determines the load which breaks the specimen.The fragment of filamentary crystal attached to the lower quartz filament is studied under a microscope and its diameter is measured in different planes. The area of the cross section is calculated as the area of a circle with the average diameter of these measurements.The diameter is measured to an accuracy of • 0.5 #m which is 17, 10, and 7%, respectively, for diameters of 3, 5, and 7 ~m.Consequently, the area of the cross section is determined to an accuracy of 34, 20, and 14%, respectively.The measurement of the areas of the cross section of the filamentary crystals involves a fundamental error in the determination of their strength since the load is measured to an accuracy of + 10 mg which is 0.2-0.3%. The achievable accuracy of the determination of the strength of an MgO FC can be regarded as completely satisfactory since the majority of crystals are 5-7/~m and the large number of the measurements makes it possible when treating the results to use statistical methods with adequate reliability.The ultimate extensive strength of the crystals varied from 500 to 1500 MPa depending on their diameter.