The scope of the paper is to present the result of tests on high-temperature samples of "Surmin" type boehmit kaolin upgraded by means of the hydrothermal method. The obtained results of test baking trials indicate that, in comparison with crude samples, the upgraded grogs have considerably higher density, lower porosity and absorbability. A high level of sludge sedimentation within the range of 55.7-60% obtained in the samples subjected to hydrothermal treatment constitutes an important result, especially as compared with crude grog sludge sedimentation only at the level of 49.0%. The authors specify the feasibility of applying the hydrothermal method in upgrade treatment of raw kaolin materials.