In coastal countries, seawater has probably always been used in the preparation of ceramic paste. The bleaching of the clay which occurs when seawater is used is seen only in clays rich in calcium. The experiments which have been made indicate that the presence of calcium carbonate (in the clay) together with chlorides (in seawater) is the cause of this discoloration. The experiments also reveal the influence of seawater on the chemical composition of calcareous pastes -- there is an increase in the proportions of sodium and magnesium and a decrease in the percentage of potassium. It has been shown that the amount of calcium lost is related to both the initial amount of calcium and chlorine in the paste and the firing temperatures. These results explain some anomalies of composition which were either dimly discerned or wrongly interpreted until now.