The critical concentration of Ca required for the onset of abnormal grain growth in alumina was determined by controlled doping of Ca in ultrapure alumina (>99.999%), by sintering under clean contamination-free conditions, and by microstructural characterization. As in the case of Si, the excess concentration of Ca beyond its solubility limit was inversely related to the average grain size at the moment of first appearance of abnormal grains, which corresponds to the moment of sufficient enrichment of Ca in grain boundaries to form stable intergranular liquid films. However, the critical concentration of Ca was found to be in the range of only a few tens of ppm, which is lower than that of Si by almost 2 orders of magnitude. The equivalent silica concentration to form such a stable intergranular calcium aluminate glass film and its minimum thickness were estimated from the inverse relationship with the assumption that the glass composition is close to calcium hexaluminate.