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How does the kiln atmosphere affect the color of fired earthenware?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Stages in the slip casting of a thin-walled whiteware container. Clay powder is mixed in water together with a dispersing agent, which keeps the clay particles suspended evenly throughout the clay-water slurry, or slip. The slip is poured into a plaster mold, where water is drawn out by capillary action and a cast is formed by the deposition of clay particles on the inner surfaces of the mold. The remaining slip is drained, and the cast is allowed to dry partially before the drain hole is plugged and the mold separated. The unfinished ware is given a final drying in an oven before it is fired into a finished product.
traditional ceramics Stages in the slip casting of a thin-walled whiteware container. Clay powder is mixed in water together with a dispersing agent, which keeps the clay particles suspended evenly throughout the clay-water slurry, or slip. The slip is poured into a plaster mold, where water is drawn out by capillary action and a cast is formed by the deposition of clay particles on the inner surfaces of the mold. The remaining slip is drained, and the cast is allowed to dry partially before the drain hole is plugged and the mold separated. The unfinished ware is given a final drying in an oven before it is fired into a finished product.

The atmosphere within a kiln during the firing process is a primary factor in determining the final color of earthenware, alongside the specific composition of the clay body.1

Because iron is naturally present in most earthenware clays, the kiln atmosphere dictates how this element reacts:

  • Oxidizing Atmosphere: Under standard firing conditions with sufficient air, the iron in the clay oxidizes. This typically results in colors ranging from buff to dark red, depending on the concentration of iron present.1
  • Reducing Atmosphere: In a reducing atmosphere—where a limited supply of air leads to the presence of carbon monoxide—the iron produces colors ranging from gray to black. Additionally, the presence of smoke during firing can also contribute to these darker tones.1

These variations are historically significant, as evidenced by the black-topped vases produced in predynastic Egypt, which showcase the distinct color shifts achievable through controlled firing atmospheres.1