Cerium is the most abundant of the rare earths.  It is characterized chemically by having two valence states , the +3 cerous and +4 ceric states. The ceric state is the only non-trivalent rare earth ion stable in aqueous solutions, making it strongly acidic and moderately toxic, as well as a strong oxidizer.  The cerous state closely resembles the other trivalent rare earths.      The numerous commercial applications for cerium include metallurgy, glass and glass polishing, ceramics, catalysts, and in phosphors. In steel manufacturing it is used to remove free oxygen and sulfur by forming stable oxysulfides and by tying up undesirable trace elements such as lead and antimony. It is considered to be the most efficient glass polishing agent for precision optical polishing.  It is also used to decolor glass by keeping iron in its ferrous state. The ability of cerium-doped glass to block out ultra violet light is utilized in the manufacturing of medical glassware and aerospace windows; it is also used to prevent polymers from darkening in sunlight and to suppress discoloration of television glass. It is applied to optical components to improve performance. Cerium is used in a variety of ceramics iincluding dental compositions and as a phase stabilizer in zirconia-based products. Ceria plays several catalytic roles: in catalytic converters, it acts as a stabilizer for the high surface area alumina, as a promoter of the water-gas shift reaction, as an oxygen storage component and as an enhancer of the NOX reduction capability of rhodium. Cerium is added to the dominant catalyst for the production of styrene from ethylbenezene to improve styrene formation. It is used in FCC catalysts containing zeolites to provide both catalytic reactivity in the reactor and thermal stability in the regenerator. ATT produces cerium as metal and compounds with purities from 99% to 99.999% (ACS grade to ultra high purity); metals in the form of foil, sputtering target, and rod, and compounds as submicron and nanopowder.