Provided herein are methods and apparatus to remove unwanted elements in commercial powders, and particularly in commercial powders that include one or more of a crystalline ceramic oxide. The methods involve treating powders in reduced pressure atmosphere, such as a vacuum, with or without heating, for a period of time sufficient to remove impurities. Impurities and contaminants, including anionic species, are removed from the powders without any undesirable changes in the physical characteristics of the starting material, such as particle size and particle size distribution, surface area, and volume, for example. The resulting purified powder starting material can be consolidated without the need for any sintering aids such as LiF to produce nearly colorless, extremely transparent polycrystalline articles that approach identical properties and performance of single crystal spinels.