Chun Plate

Plate with chun glaze (blue grey) over tenmoku, with wax resist decoration. Reduction fired to 1300C.
 
Reduction firing: This is where pots are fired using fuel that produces a flame - gas, wood, oil or even coal. When the temperature in the kiln is around 1000˚ C, the flame is starved of oxygen by closing off air vents. The flame must have oxygen to continue burning, and so it will take it from iron and other minerals in the clay and glaze, chemically reducing - in the case of iron - iron oxide to pure iron. 
This reduced iron has a wide range of effects in pottery. In celadon glazes it produces subtle greens and blues. Sometimes the neat iron will show up as a speckle. In darker glazes, it produces blacks and reds.
Glaze chemistry in reduction glazes is complex, and every glaze we use is the result of hours, days, weeks of testing and refining recipes.