A special fire-viewing glass is blue cobalt glass. It is mainly used in chemistry labs to see the flame color reaction of potassium. Blue cobalt glass is often produced in places like cement plants, steel mills, and kilns. Its production process is quite unique. In the manufacturing process, they add “cobalt blue” to make this special glass. It mainly contains cobalt aluminate: Co(AlO2)2. Cobalt blue adds a unique blue color to the glass. That’s why it’s called blue cobalt glass.
Cobalt can’t absorb blue light. Instead, it reflects or transmits it. But cobalt fully absorbs yellow light, which has a different wavelength. Due to the presence of cobalt ions, blue cobalt glass can filter out yellow light. When doing the flame test for potassium, use blue cobalt glass. Potassium compounds are important. They can mix with impurities like sodium. This mixture is tough to separate. In the flame test, sodium’s yellow flame can hide potassium’s purple color. So, someone must filter out the yellow light. Yellow and blue are complementary colors. This is due to the principle of complementary colors of light. Blue cobalt glass filters out yellow light. This lets the purple flame color of potassium shine through clearly.