Glaze and Fire
Glaze Colour in Plain Language
Glaze colour comes from a handful of metal oxides behaving differently in different melts. The same oxide can produce green, brown, red or black without changing.
The colours on stoneware are not dyes and not pigments in the ordinary sense. They come from metal oxides dissolved or suspended in a thin layer of glass, and their appearance depends on what else is in that glass and what the kiln atmosphere was doing while it formed. This is why a potter cannot simply choose a colour the way a painter chooses a tube.
What a glaze is made of
Every glaze is a balance of three things. A glass former, almost always silica, provides the glass itself. A flux lowers the melting point of the silica to something a kiln can reach. A stabiliser, usually alumina supplied by clay, stops the melted glaze from running straight off the pot.
Those three produce a clear, colourless glass. Everything beyond that is adjustment: opacifiers to make it milky, matting agents to make it satin rather than shiny, and colouring oxides added in small quantities, often well under five percent of the dry recipe.
Iron, the most versatile
Iron oxide is the workhorse of stoneware colour and accounts for an enormous share of traditional glazes. In small amounts in a clear glaze fired with plenty of oxygen it gives honey and amber. In larger amounts it gives brown and, where it is thick, a saturated near-black that breaks to rust at the edges.
Fired in reduction, where the kiln is run with insufficient air, the same iron produces the grey-greens and pale blue-greens of the celadon family. Nothing has been added; the oxygen has been taken away, and the iron has changed form.
One oxide, one recipe, two atmospheres, and the results look as though they came from unrelated glazes.
Iron is also responsible for the way a good stoneware surface varies over a single pot. Where the glaze runs thin over a raised edge, less iron sits in the light path and the colour lightens dramatically. Potters shape pots with that behaviour in mind, cutting facets or throwing ridges specifically so the glaze will break over them.
Copper, cobalt and the rest
Copper oxide gives green in an oxidising atmosphere and, in reduction and in a narrow range of recipes, the deep reds that are among the hardest results in the field to repeat. Copper is volatile at stoneware temperatures and can migrate as vapour onto neighbouring pots, which is a real consideration when a kiln is packed.
Cobalt gives blue and gives it powerfully. A fraction of a percent produces a pale blue; one percent produces a strong one. It is stable across atmospheres, which makes it reliable and also makes it obvious, since cobalt blue looks much the same wherever it appears.
Manganese produces browns and purples and is often combined with iron. Rutile, a titanium-bearing mineral, produces creams and tans and encourages crystals and streaking as the kiln cools. Chrome gives greens and, with tin, pinks.
Why thickness changes everything
A glaze layer is typically less than a millimetre thick, and the colour depends on how much of it light passes through. Doubling the thickness does not simply darken the colour; it can change it entirely, because a thicker layer stays molten longer, dissolves more of the material beneath it, and holds more of whatever crystals form during cooling.
This is the single most common cause of an unexpected result. A pot dipped a second too long, or dipped where the bisque was slightly more absorbent, carries a different thickness and fires differently. It is also why a test tile is glazed with a deliberate gradient, so that the whole range is visible on one piece.
Why the cooling matters as much as the heat
A glaze becomes glass on the way up and then has to become a solid surface on the way down. Many effects depend entirely on the cooling curve. Crystals need time in a particular temperature band to grow, so a kiln that is held on the way down produces visible crystal formations that the same glaze would not show in a fast cool.
Matt surfaces are usually the result of microscopic crystals grown during cooling rather than of anything added for the purpose. Cool the same glaze quickly and it stays glossy.
What this means when looking at a pot
A surface that varies across a single piece, breaking lighter on the edges and pooling darker in the hollows, is showing the interaction between a glaze and the form underneath it. It is the normal behaviour of a stoneware glaze on a thrown pot, and it is a reasonable thing to look for.
A surface that is perfectly uniform edge to edge, on the other hand, is usually the result of an opaque glaze with a high opacifier content, which is a legitimate choice with a different character. Neither is better. They are different intentions, and knowing which one is in front of you makes the pot easier to read.