A borax bead containing a trace of a metal salt is green while hot and blue on cooling in the oxidising flame. In the reducing flame it becomes red and opaque. The metal ion is

Copper forms $\mathrm{Cu}(\mathrm{BO}{}_2){}_2$ in the oxidising flame, green hot and blue cold, and is reduced to red opaque $\mathrm{Cu}{}_2\mathrm{O}$ or Cu in the reducing flame. Iron gives a yellow-brown bead, chromium a green bead in both flames and manganese an amethyst bead. The ion is $\mathrm{Cu}{}^{2+}$.