Iron is the most used colourant in ceramics and the cheapest. It is in the clay under your feet, in most bodies a supplier sells, and in a large share of the glaze recipes ever published, giving everything from pale celadon green to tenmoku black depending on quantity and atmosphere.
Red and black iron oxide
Red iron oxide is the common form, fine and easy to disperse. Black iron oxide is coarser and can leave speckles where particles sit, which is sometimes exactly the effect wanted. They contain slightly different amounts of iron by weight, so substituting one for the other shifts a recipe a little, and most recipes name which they assume.
Percentage decides everything
One to two percent gives the pale greens and ambers of a celadon; four to six gives honey and amber; eight to ten and above gives the saturated dark browns and blacks of a tenmoku, breaking rust where thin. No other single material covers that much ground, which is why iron appears in more glaze recipes than anything else.
Atmosphere shifts the colour
In reduction iron moves towards green and grey; in oxidation towards amber and rust. The same recipe in an electric kiln and a gas kiln gives noticeably different pots, and neither is a mistake. That is why every glaze page on this site ends up recommending a test tile from your own kiln rather than a photograph from anyone else's.
Cost and handling
Iron oxide is among the cheapest materials on a supplier's list and is used in percentages rather than fractions, so iron glazes are inexpensive to mix in quantity. It stains everything it touches, including hands, benches and any pale clay in the room, so keep it away from white bodies and wet-clean after weighing.
Questions people ask about red iron oxide
What is the difference between red and black iron oxide?
Red is finer and disperses evenly; black is coarser and can leave deliberate speckles. Their iron content differs slightly, so recipes name which they assume.
How much iron makes a black glaze?
Around eight to ten percent and above, which is a tenmoku. It reads black where thick and rust-brown where thin.
Why does iron look different in my kiln?
Atmosphere. Reduction pushes iron towards green and grey, oxidation towards amber and rust, from the same recipe.