AbstractIt is a matter of common knowledge that borax (Na2B4O7) is a very efficient agent for retarding the set of plaster of Paris. Its effect on the setting time is shown by the typical curve given in Fig. 1 (after Gibson and John‐on1). One of the explanations offered2for this retardation phenomenon is that borax is physically adsorbed on the surface of the crystal, thus reducing the rates of solution of the hemihydrate and the formation of gypsum nuclei. Thin short investigation was initialed to test and the validity of this postulatio