色・発色
cobalt
Japanese: コバルト
A typical blue-colored metal. Depending on the alloy composition, it can also turn purple.
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Cobalt is the most common metal oxide used to impart a deep blue color to glass. It is added to glass in the form of cobalt oxide (CoO, Co₂O₃) and produces a vivid blue color even in trace amounts of just 0.001 to 0.5%. It is characterized by its high tinting strength, allowing for deep coloration with only a small amount.
Cobalt blue retains a stable blue color in both oxidizing and reducing firing atmospheres. While it is relatively non-reactive with other metal oxides and easy to work with, a reaction line may appear at the interface when it comes into contact with silver. When viewed under transmitted light, you can see its beautiful blue color (transmitted-light-check).
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.