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Bentonite mixtures belong and will always belong among the most widespread sand mixtures for the “green sand” technology of castings production. This technology’s essential advantage is bentonite mixtures application reversibility in the closed circulation after composition modifications and circulation losses replenishment. After the casting of a mould, the surrounding sand mixture is strained by the solidifying casting heat and bentonite degradation occurs. In case of appropriate conditions the oolitization occurs. This phenomenon is specific only for bentonite-bonded mixtures. The oolitization of opening material silica grains brings a number of negative as well as positive features with it. It is not only a technological problem but economical and ecological as well because for minimization of mixture regeneration with a help of new sand it is necessary to know bentonite mixtures quality control tools even in term of the oolitization rate. This paper deals with the description of undemanding physical method of the oolitization rate evaluation with a help of powder density assessment and its examination with actual sand mixtures which were obtained from the Czech Republic foundry plants. There are foundry plants of heavier weight castings and in one case there was an operation where bentonite mixtures regeneration was applied. Moreover, the oolitization effect on metal penetration in test castings was verified.