Copper red is not difficult to mix — this recipe has nine materials and one of them is the colourant. It is difficult to fire. The red is metallic copper and cuprite held in suspension in the glass, and it only stays there if the kiln is reduced hard at the right moment and then cooled in a way that does not re-oxidise it. Most failed copper reds are not a recipe problem.
H 18 · Ø10 / Ø14 / Ø5 cm
Base
A copper red for cone 10 reduction, built on a low-alumina, high-alkali base with a small amount of boron to widen the window. Copper red is the least forgiving glaze in this library: it is either right or it is a muddy grey-green, and the difference is often two or three minutes of firing.
| Material | Parts | Share |
|---|---|---|
| Custer Feldspar | 38 | 37.4% |
| Silica (SiO₂) | 24 | 23.6% |
| Whiting (CaCO₃) | 14 | 13.8% |
| EPK Kaolin | 8 | 7.9% |
| Bone Ash (Ca₃(PO₄)₂) | 6 | 5.9% |
| Gerstley Borate | 6 | 5.9% |
| Talc | 4 | 3.9% |
| Tin Oxide (SnO₂) | 1 | 1.0% |
| Copper Carbonate (CuCO₃) | 0.6 | 0.6% |
| Total charge | 101.6 |
Fired surface
A deep, blood-red to oxblood surface with a slightly softening rim where the copper oxidises back to a green-grey. Where the glaze is very thick, the red deepens toward a liver colour; where it is thin, the copper burns out entirely and the surface is white.
The reduction window
Copper red needs heavy reduction starting between 1000 and 1100 degrees Celsius, held for 20 to 40 minutes, and then a return to a lighter but still reducing atmosphere for the rest of the firing. Reduced too early, the copper volatilises and the surface is a weakly coloured green-grey; reduced too late or too briefly, the copper stays as oxide and the result is the same green-grey with a different cause.
The practical marker is the flue flame: during the heavy reduction window you want a flame of 40 to 60 cm at the flue, and the kiln should be producing visible smoke that is not black. Black smoke means you have gone past reduction into incomplete combustion, which deposits soot on the glaze and produces blisters.
Cooling is half of it
A copper red that is reduced perfectly and cooled fast will still come out muddy. The copper particles need time to grow to the size that scatters red light, which happens between about 1100 and 800 degrees on the way down. A slow cool — four to six hours from cone 10 to 800 degrees — gives a deep red; a free fall gives a thin, patchy red or none at all.
In practice this means a copper red firing takes about two hours longer than a celadon firing in the same kiln. Plan for it, and do not schedule a copper red load before a load that needs to be out on time.
Thickness and the burn-out zone
Apply at 1.0 to 1.4 mm. Under 0.8 mm, most of the copper is lost to volatilisation during the reduction and the glaze fires white or a very pale pink; over 1.6 mm, the red goes liver-brown and the glaze often runs onto the shelf. The 1.0 to 1.4 mm band is narrow and worth checking with a fired thickness test on a curved tile, not a flat one.
Rims and other exposed edges will always show some green-grey, because the copper there is exposed to more oxygen and to more of the flame. This is not a defect; it is how a copper red tells you it was fired in reduction. If you want no green at all, apply thinner at the rim, which is the opposite of what every other glaze wants.
Bone ash, tin and the disappearance of the red
Bone ash at 6 percent and tin oxide at 1 percent are both there to stabilise the red. Bone ash (calcium phosphate) makes the copper particles form in a narrower size range, which produces a more uniform red, and tin oxide acts as a nucleating agent and a mild opacifier that keeps the red from going transparent and grey.
If you remove the bone ash, the same recipe gives a red that is patchy and often partly green; if you raise the tin above 3 percent, the glaze turns opaque pink. Change one variable at a time and fire a test tile for each change; a copper red testing program is slow by nature.
Steps
- Weigh all nine materials and combine them dry, breaking up the copper carbonate lumps first.
- Add water and mix for eight minutes, then adjust to 1.48 specific gravity.
- Sieve through 100 mesh and let the bucket stand for an hour before use.
- Dip test tiles over one second and dry them fully.
- Fire a test tile alone in the first firing to confirm the reduction window for your kiln.
- Bisque to cone 06 and glaze at 1.0 to 1.4 mm using a thickness gauge on a curved tile.
- Begin heavy reduction between 1000 and 1100 degrees, holding 20 to 40 minutes with a 40 to 60 cm flue flame.
- Return to light reduction through 1260 degrees, then damper down for a four to six hour cool to 800 degrees.
- Open the kiln only at room temperature; copper reds are read final only when cold.
Where it goes wrong
- Cooling freely. The copper particles never reach the size that scatters red light.
- Reducing with black smoke. Incomplete combustion deposits soot and blisters the glaze.
- Applying at 0.6 mm. Most of the copper volatilises and the glaze fires white.
- Removing the bone ash to simplify the recipe. The red becomes patchy and partly green.
- Firing a copper red in a shared load with a white you need to stay white.
| Symptom | Cause |
|---|---|
| Glaze fired muddy grey-green | reduction started too early (below 1000) or too late (above 1150). |
| Red is thin and patchy with green areas | the cooling was too fast for the copper particles to grow. |
| Glaze fired a pale pink or white | applied too thin, under 0.8 mm, so the copper burnt out. |
| Surface is bubbled with black specks | heavy reduction continued into incomplete combustion, depositing soot. |
| Glaze ran onto the shelf and the red went liver-brown | applied above 1.6 mm. |
Recipe and schedule numbers here were recorded on the workshop’s own kiln. A different kiln, a different thermocouple position or a different clay batch will move the result, so treat every firing schedule as a starting point and keep your own log beside it.