Hexavalent chromium is used in chrome plating, chromate coatings and primers, and in chromic acid. Chromium trioxide is the anhydride of molecular chromic acid. Oxidation with chromic acid yields propionic acid. Hexavalent chromium compounds (including chromium trioxide, chromic acids, chromates, chlorochromates) are toxic and carcinogenic. Catgut Chrome (B Braun) suture is a variant treated with chromic acid salts. The oldest anodizing process uses chromic acid. Catgut treated with chromium salts, known as chromic catgut, is also used in surgery. These coatings are free of pores, relative to the sulfuric and chromic acid processes. In solution, the chrome exists as chromic acid, known as hexavalent chromium. It is formed by the action of chromic acid on ammonium hydroxide with subsequent crystallisation. Very thin coatings similar to those produced by chromic anodizing are known as Type IIB. However, chromic acid oxidizes it to carbon dioxide. Typical etching processes involve chromic acid at elevated temperatures. When heated with a chromic acid solution it is oxidized to acetone. Unlike chromic acid solutions, piranha does not contaminate glassware with heavy-metal ions.