It was shown in an Investigation of mixtures of fine-ground electrofused corundum with orthophosphoric acid that at 20–25°C the degree of the reaction of the components is highest during the first four days. During this period only water-soluble acid aluminum phosphates are formed and a significant proportion of the orthophosphoric acid remains uncombined. The reaction rate is low and falls still further with an increase in the size of the corundum grains.