The formation of alpha alumina in gamma alumina obtained from ammonium alum was studied at 1150°C. The transformation occurs by a nucleation and growth process, preceded by an incubation time. Mechanical treatments of the gamma alumina markedly increase the density of nuclei and hence the transformation rate. The alpha alumina forms in porous colonies consisting of a single crystal with an interconnected wormy morphology.
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Dynys et al. (1982) studied this question.