In glazes: The most commonly used oxide in ceramics, both as a flux and as a colouring agent. Sets the melting point down. Colors honey yellow to dark brown with additives of 1-8%. In alkaline glazes red to violet, in reduction firings grey-green-blueish (seladon). An alkaline, supersaturated iron glaze forms crystals (aventurine). In bulk: Iron influences the burning colour of the body. It is reddish-brown and, when finely distributed, colours the mass in this colour. With lime content the colour changes to yellow. Is present in all yellow-brown-red burning masses. In engobes: Additions of 5-10% iron oxide to white engobe result in reddish-brown engobe.