The work presents results of studies concerning the reological stability of water-clay pastes. The dynamic viscosity of bentonite pastes was assesed by an ultrasonic method. The studies were focused on kinetics changes at so called restoring period. Changes occuring in that period are the result of thixotropy phenomenon, which is common in dispersed system. It is shown that the degree of the viscosity change is distinctive enough to play an important role in the green moulding sand technology (it amounts to over 240 kPa centre dot s with paste concentration of 75 percent). The studies were original both in a methodology and obtained results. The dynamic viscosity of bentonite pastes was assessed in the function of their concentration (eta = f( percent of clay)). The empirical relationship between bentonite paste stretching durability and dynamic viscosity was assessed (R_m = f(eta)). The unmonotonous nature of the dependence of dynamic viscosity on temperature was found (eta = f(T)). The viscosity of concentrated water-clay systems reaches it's minimum in the temperature of about 30 deg C. The research of dynamic viscosity was conducted using an ultrasound method devised by the author.
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