The bibliographical study shows that the principal aim of research hitherto has been to reproduce under heat conditions the instantaneous breaking tests usually conducted at ordinary temperatures. The theoretical study of the deformation of ceramics products under the combined action of a stress, temperature or time, proves the necessity of investigating first elastic deformations: modulus of elasticity and elastic limit, and second, plastic deformations, such as creep. An attempt is made, using experimental data to represent hot deformation under load. Theoretical considerations are adduced to demonstrate the difficulty of carrying out a hot breaking test with significant physical results.