A method for estimating the thermostability of refractories by the change in the frequency of natural oscillations of a specimen after a single thermal shock is presented. The method has been tested in estimating the thermostability of silicon carbide plates on silica binder produced by the Volzhskii Abrasive Plant. At the same time, a direct dependence of the apparent density on the frequency of natural oscillations of a specimen is established, the method is quicker and less laborious than the conventional one.The operating ability of refractory products is determined to a large extent by their therrnostability, because of which the determination of this characteristic in the production of refractories and in the development of new kinds of refractories is indispensable. The technique for determination of thermostability is specified in the State Standard GOST 7875 -83. It is laborious and requires a large number of tests, and the result depends on an overly large number of the factors associated with destruction of specimens. Because of this the thermostability of produced refractories is tested only irregularly, and often with violation of the rules of testing.A rapid method for determination of thermostability is proposed below. It is based on the hypothesis of "equivalent crack formation," which assumes a dependence of the mechanical properties of the material on the formation of microcracks resulting from sharp cooling of a specimen without its destruction [1].In particular, a method is known by which the thermostability and the associated rated modulus of elasticity are determined by the change in the velocity of propagation of ultrasound in a specimen. It has been established that the ultrasonic method provides an objective estimate of the destruction of the specimen and makes it possible to determine the thermostability in a single loading cycle [2]. This method, however, relies on the use of rather intricate equipment.The proposed rapid method is based on using known acoustic devices of the "Zvuk" series [3]. Noting that the velocity of propagation of ultimate oscillations is proportional to the natural oscillation frequency, the thermostability can be evaluated by the ratio of the frequency of natural oscilla-1 Volzhskii Research Institute for Abrasives and Grinding, Volzhsk, Russia. 129tions of a specimen after a thermal shock to its value before the thermal shock.In the method proposed, bar-shaped specimens of size 20 x 20 x 120 mm or 25 x 25 x 120 mm are cut from refractory products. A specimen is placed vertically in the measuring stand of a "Zvuk-107" instrument arid the frequency of forced oscillations is measured up to the onset of resonance. Four or five measurements are made with change in the fastening point of the specimen, and the arithmetic mean value is then determined. The specimens are then placed in a basket made of heat-resistant wire and subjected to a thermal shock in a furnace at a temperature of 850°C with a hold for 30 min. After that the specimens are cooled in a flow of cold w...