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Pr‐doped Lu3Al5O12 (LuAG) transparent ceramic, a potential scintillator material, was fabricated by sinter/hot isostatic pressing (HIP). The specimens were subjected to various post‐densification heat treatments and the evolution of porosity and its relationship to transparency was monitored and studied. Annealing was necessary to remove discoloration and to restore stoichiometry, but when performed at too high a temperature it caused a severe decrease in transparency. Transparency was restored by reHIPing, indicating that some nanometer‐size pores remained even after the original HIP cycle, which expanded in size during annealing and contracted again during re‐HIPing. Annealing at a lower temperature restored stoichiometry without serious transparency degradation, due to a favorable difference in diffusion rates for mass transfer and O2− diffusion. This phenomenon illustrates a fundamental difference between residual porosity in ceramics consolidated by pressureless and pressure‐assisted processes.

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