Ferric EDTA is the coordination complex formed from ferric ions and EDTA. With trivalent metal ions, alums such as ferric ammonium sulfate are formed. Ferrous chloride is used in the manufacturing of ferric chloride. Chemical etching is usually done with ammonium persulfate or ferric chloride. Examples include decomposition of hydrates such as ferric hydroxide to ferric oxide and water vapor. The teeth are coated with magnetite, a hard ferric/ferrous oxide mineral. It works by converting the ferric iron in hemoglobin to ferrous iron. Ferrous iron combines with thioglycolate to form red-violet ferric thioglycolate. Black dyes and gold produced by inorganic means (ferric ammonium oxalate) are more lightfast. The best known is a process using ammonium ferric citrate and potassium ferricyanide. The paper is impregnated with a solution of ammonium ferric citrate and dried. It forms water-soluble salts of calcium and ferric iron. Sulfuric acid is commonly added to solutions to reduce oxidation to ferric iron. In addition to ferric chloride, other oxidizing agents.