FeO (Ferrous Oxide)

Notas

Ferrous oxide (FeO), also known as wüstite, is a black inorganic compound that functions as a highly potent flux within ceramic glazes and clay bodies. While often existing as a non-stoichiometric compound in practice, its behavior in high-temperature processing is driven by the reduction of ferric oxide (Fe2O3) in the presence of carbon monoxide. In ceramic systems, this reduction can initiate as early as 900°C. Once iron is converted to its ferrous state and becomes integrated into the glass matrix, it is resistant to re-oxidation, meaning that oxidation cycles at the end of a firing have minimal impact on the resulting color. Due to its significant fluxing capabilities, FeO can be used as a replacement for fluxes such as calcium oxide or lead. However, it must be managed carefully in clay bodies; if carbonaceous matter is not fully oxidized between 700°C and 900°C, the carbon will reduce iron oxides prematurely, resulting in 'black coring,' where the internal structure of the ceramic is compromised by localized melting. FeO is highly solubility-limited in molten glass, often leading to the precipitation of iron crystals upon cooling, a phenomenon that can produce decorative effects like sparkling, rust-colored surfaces or iron-speckled stoneware. Beyond its fluxing role, FeO is essential for achieving specific palettes in reduction firing, ranging from greens and blues in celadons to yellows, maroons, and deep crystalline blacks or browns in tenmoku and kaki glazes. The transition between these color states is sensitive to temperature and oxygen levels, allowing for mottled color effects as the iron shifts oxidation states.