a process is disclosed for the removal of water and water derived impurities , e . g . oh - , substitutionally or interstitially incorporated in the structure of crystalline and amorphous materials , more specifically , in metal oxides , e . g . fused silica or aluminum oxide , wherein the material is exposed in powdered form to a gaseous mixture of halogen and carbon monoxide at a predetermined elevated temperature . the mixture of halogen and carbon monoxide reacts to cause the water and oh - ion concentration in the processed material to be reduced to an extremely low level . materials purified by the process can be used to produce optical fibers and laser windows of excellent mechanical , thermal and optical properties .