Ultra-high purity alumina was sintered in hydrogen at temperatures ranging from 1325 degrees to 2020 degrees C. Normal grain growth was maintained in all samples and grain boundary mobilities were measured. The data represent the most complete measure of intrinsic grain boundary mobility for alumina grain growth currently available. There are two orders of magnitude of discrepancy between the calculated and measured intrinsic grain boundary mobility. It is proposed that this discrepancy results from the poor assumption of the Burke-Turnbull model, which suggests that grain boundaries are ideally disordered and atomically rough. Implications of these results on the mechanism of solid-state single-crystal conversion in alumina are also discussed.