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How do iron oxide impurities affect the color of bricks and tiles?

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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.

Iron is a ubiquitous component in the clay used to manufacture bricks and tiles. The final color of these products is primarily determined by the iron content and the atmospheric conditions within the kiln during the firing process.1

Under standard oxidizing conditions, the iron in the clay oxidizes, resulting in colors that range from buff to dark red, depending on the concentration of iron present. Conversely, if the firing occurs in a reducing atmosphere—where a limited supply of air leads to the presence of carbon monoxide—the iron produces colors varying from gray to black.1

While these natural colors are inherent to the clay body, manufacturers can also influence the final appearance through techniques like "flashing," which alters surface color, or by adding other metals, such as manganese, to the clay to develop specific hues.2