Why can iron in clay produce red, gray, or black ceramic ware?

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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 color of earthenware is determined by the composition of the clay and the atmospheric conditions within the kiln during firing. Iron is commonly found in earthenware clay and reacts to these conditions in the following ways:

Red: Under standard firing conditions, the iron in the clay undergoes oxidation. Depending on the concentration of iron present, this process produces colors ranging from buff to dark red.1
Gray or Black: In a reducing atmosphere—where a limited supply of air results in the presence of carbon monoxide—the iron produces colors varying from gray to black. Additionally, dark coloration can occur due to the action of smoke during the firing process.1

These variations are famously illustrated in the black-topped pottery of predynastic Egypt.1