Almost every brown glaze in ceramics is an iron glaze, and iron is the most versatile and cheapest colourant on the shelf. The same oxide gives a pale honey at low percentages, a deep tenmoku black-brown at high ones, and rust and amber in between, with the firing atmosphere shifting all of it.
Iron at different percentages
At one or two percent iron gives the pale greens and ambers of a celadon. Around four to six it reads as honey or amber. At eight to ten and above it goes to the dark saturated browns and blacks of a tenmoku, often breaking rust-red where the glaze is thin over an edge. That single-material range is why iron appears in more recipes than any other colourant.
Why browns flatter most bodies
Iron browns are close in colour to the iron already in most clay bodies, so where a glaze thins over a rim or a throwing ridge and shows the clay through, the transition is gentle rather than jarring. That forgiveness is why iron glazes are recommended to people whose throwing is still uneven: the glaze does not draw attention to the parts that are.
Atmosphere changes it
In reduction, iron shifts towards green and grey; in oxidation towards amber and rust. A tenmoku fired in an electric kiln can read browner and less black than the same recipe in a gas kiln. Neither is wrong; both need a test tile from your own kiln before a batch, which is the recurring advice on every glaze page here for the same reason.
Cost, honestly
Iron oxide is among the cheapest materials a supplier sells and it is used in percentages rather than fractions, so a brown glaze mixed from raw materials is inexpensive per bucket. That makes iron browns the natural first glaze for anyone starting to mix their own, alongside a transparent.
Questions people ask about brown glaze
What makes a glaze brown?
Iron oxide, in almost every case. The percentage and the firing atmosphere decide whether it reads honey, rust, tenmoku or near black.
Why does my brown glaze look red on the edges?
The glaze is thinner there, and iron reads rust-red when thin over an edge. It is a characteristic of iron glazes rather than a fault.
Is tenmoku a brown or a black?
Both, depending on thickness. A high-iron tenmoku is black where thick and rust-brown where thin, which is what gives it its depth.