色・発色
iron oxide
Japanese: 酸化鉄
A source of green to brown coloration. When combined with reducing agents and sulfur, it turns brown.
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Iron oxide is a metal oxide that imparts a variety of colors—such as green, brown, and black—to glass, and it is the most common impurity found in natural glass raw materials (silica sand). Ferrous oxide (FeO) produces a blue-green color, while ferric oxide (Fe₂O₃) produces a yellow to brown color; the valence changes depending on the firing atmosphere (oxidizing or reducing), which in turn alters the color.
In glass fusing, transparent green glass (clear green) containing iron is commonly used. During reduction firing (reduction-blackening), the iron may turn black. The higher the transparency of the glass, the lower its iron content (low-iron glass), and the better the reproduction of the transmitted color.
This entry is written in Japanese by Rei Chikaoka and translated for readers outside Japan. If something reads oddly, the original is the source of truth.