Short answer
A glaze is a thin layer of glass fused onto clay (or, in vitreous enamel, onto metal). Its colour comes from metal oxides dissolved or suspended in that glass — iron, copper, cobalt, chromium, manganese and others — and the same oxide can give quite different colours depending on the glaze chemistry, the firing temperature and above all the kiln atmosphere: copper is green in an oxygen-rich firing and red in a reducing one. The clay body beneath shows through translucent glazes, so a red earthenware and a white porcelain make the same glaze look different.
Relative to other materials in this section, not a measurement of a particular product.
| Process | What you see | Driven by | Slowing it |
|---|---|---|---|
| Crazing and staining | Fine crack network that picks up tea, dirt or grout colour | Glaze–body expansion mismatch and moisture | Well-fitted glazes; vitrified bodies |
Colours that work with ceramics, glazes, porcelain and vitreous enamel in a room
Ceramic colour chemistry is glass chemistry. Transition-metal ions dissolved in the glaze absorb particular wavelengths: cobalt gives strong blue even at low concentration, copper gives greens and turquoises, iron gives ambers, browns and, in reduction, celadon greens and blues, chromium gives greens, manganese gives purples and browns. Their colour depends on their oxidation state and on the surrounding glass — alkaline glazes push copper towards turquoise, for example. Modern commercial glazes often use manufactured stains, pre-fired pigments that are more stable and predictable than raw oxides. Opacifiers such as tin or zirconium oxide scatter light to make a glaze opaque and white; without them a glaze is transparent and the body shows through.
In an electric kiln or a well-ventilated fuel kiln, the glaze fires in oxygen and metals stay in their oxidised states. In a reduction firing, fuel-rich flames starve the kiln of oxygen and pull oxygen from the glaze and body. Iron then acts as a flux and gives celadon greens and blues and deep earth browns rather than ambers; copper, which fires green in oxidation, turns red, the basis of copper-red and sang-de-boeuf glazes. Iron particles in stoneware bodies melt and bleed through the glaze as dark speckles, a look many potters seek. Because reduction varies across a kiln, pieces from one firing can differ, and the same recipe fired elsewhere may not match.
Clay bodies carry their own colour. Iron-rich earthenware fires terracotta red in oxidation and is porous; stoneware ranges from buff to grey and is fired hotter; porcelain uses kaolin with very little iron and titanium and fires white. Fired close enough to melting to become partly glassy, thin porcelain turns translucent, and even small increases in iron or titanium reduce both whiteness and translucency. Reduction-fired porcelain has a bluish-white cast; oxidation porcelains are often slightly creamier, and makers add a touch of blue stain to imitate the reduction look. Transparent glazes over these bodies look warm on earthenware, grey on stoneware and clean on porcelain.
Vitreous (porcelain) enamel is the same glass-and-oxide system fused onto steel or cast iron: signage, baths, cookware, cladding. Because the colourants are inorganic and locked in glass, fired ceramic and enamel colours are among the most lightfast surfaces available — they do not fade in sunlight the way dyes and many organic pigments do. What changes them is physical: crazing (a network of fine cracks from a mismatch between glaze and body expansion) lets tea, dirt and grout stain the cracks; chipping of enamel exposes dark metal; and wear dulls matt glazes. When matching replacement tiles or sanitaryware, the challenge is firing variation between batches, not ageing of the originals.
| Colourant | Oxidation firing | Reduction firing |
|---|---|---|
| Iron oxide | Amber, honey, brown | Celadon green or blue (low %), deep brown (high %); body speckle |
| Copper oxide | Green, turquoise in alkaline glazes | Red (copper red, sang-de-boeuf) |
| Cobalt oxide | Blue | Blue |
| Tin or zirconium oxide | Opaque white (opacifier) | Opaque white |
Why: Firing variation between production batches.
Fix: Buy spare stock from the original batch; match against physical samples from the new batch.
Why: Crazed glaze lets grout colour and dirt into the cracks.
Fix: Seal crackle tiles before grouting and use a grout close to the tile colour.
Why: Insufficient or uneven reduction in the kiln.
Fix: Adjust firing atmosphere; accept variation as part of reduction glazes.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactModerate evidence
In reduction firing iron acts as a flux and gives celadon greens and blues and earthy browns, and copper fires red rather than the green it gives in oxidation.
FactModerate evidence
In reduction-fired stoneware, iron particles in the clay melt and create dark speckling through the glaze.
FactModerate evidence
Porcelain whiteness depends on keeping iron and titanium low in the kaolin, and reduction firing gives a bluish cast that oxidation-fired porcelains may imitate with added blue stain.
FactStrong evidence
Mineral colourants fired into glass are in the lowest light-sensitivity category used by conservators, far more stable than most dyes.
Source: Agent of deterioration: light, ultraviolet and infrared
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.