Barium carbonate (BaCO3) is used to create a unique glaze color known as barium blue. However, the ethical nature of using barium carbonate for glazes on food contact surfaces has come into question. Barium poisoning by ingestion can result in convulsions, paralysis, digestive discomfort, and death. It is also somewhat soluble in acid, and can contaminate water and soil for long periods of time. These concerns have led to attempts to substitute Strontium carbonate (SrCO3) in glazes that require barium carbonate. Unlike Barium carbonate, Strontium carbonate is not considered a safety hazard by the NIH. Experiments in strontium substitution tend to be successful in gloss type glazes, although there are some effects and colors produced in matte type glazes that can only be obtained through use of barium. To reduce the likelihood of leaching, barium carbonate is used in frit form and bound to silica in a 1:1 ratio. It is also recommended that barium glazes not be used on food contact surfaces or outdoor items. Chromium(III) oxide () is used as a colorant in ceramic glazes. Chromium(III) oxide can undergo a reaction with calcium oxide (CaO) and atmospheric oxygen in temperatures reached by a kiln to produce calcium chromate (). The oxidation reaction changes chromium from its +3 oxidation state to its +6 oxidation state. Chromium(VI) is very soluble and the most mobile out of all the other stable forms of chromium. Chromium may enter water systems via industrial discharge. Chromium(VI) can enter the environment directly or oxidants present in soils can react with chromium(III) to produce chromium(VI). Plants have reduced amounts of chlorophyll when grown in the presence of chromium(VI). Uranium(IV) oxide (UO2)  Urania-based ceramic glazes are dark green or black when fired in a reduction or when UO2 is used; more commonly it is used in oxidation to produce bright yellow, orange and red glazes Uranium glazes were used in the 1920s and 1930s for making uranium tile, watch, clock and aircraft dials. Uranium dioxide is produced by reducing uranium trioxide with hydrogen. Chromium oxidation during manufacturing processes can be reduced with the introduction of compounds that bind to calcium. Ceramic industries are reluctant to use lead alternatives since leaded glazes provide products with a brilliant shine and smooth surface.