Stabilization of divalent chromium Cr(II) in soda-lime-silicate glasses

Trivalent chromium is a well-known and widely used coloring ion in the glass industry. When glasses are melted in very oxidizing conditions, hexavalent chromium Cr(VI) can also be stabilized, and is found in specific commercial applications for UV-protection containers. It is known that chromium can also exist in divalent state Cr(II), notably in highly reduced iron-free minerals. However, its occurrence in conventional soda-lime-silicate glasses at room temperature has never been reported. In this paper, we demonstrate for the first time the possibility to stabilize Cr(II) in soda-lime-silicate glasses containing iron. The specific optical properties of divalent chromium are also presented. These results show that the occurrence of divalent chromium has to be considered when melting soda-lime-silicate glasses in very reducing conditions.