The Colours of Cobalt in Glass

JSGT 1955 V39T351-T384

The colours given by cobalt in boric oxide, alkali borate, alumino-borate and silicate glasses have been investigated by spectrophotometric methods, and the absorption bands in the visible spectrum have been compared with those given by solutions of cobalt salts, and by sintered masses of cobalt with silica and with alumina, some of which contained small proportions of alkali. The absorption bands of glasses containing small proportions of the halogens have also been investigated. In boric oxide glass the colour given by cobalt is pink, but a new absorption band in the visible region is developed when alkali is added in sufficient amount, causing the glass to become blue. This new absorption band in the visible spectrum is associated with the development of two new absorption bands in the near infra-red region, centred at approximately 1·25 and 1·75 µm. Magnetic measurements were made on a number of glasses ranging in colour from the full pink given by cobalt oxide in boric oxide glass to the full blue of high-alkali borates and alkali silicate glasses containing cobalt. These showed that in all the glasses the cobalt was present as divalent cobalt ions. The main conclusions are: (1) The pink colour is due to the Co²⁺ ion 6-co-ordinated with oxygen. (2) The blue colour is due to the Co²⁺ ion 4-co-ordinated with oxygen as a (CoO₄’)^2- group which exists in the glass as a structure-building unit. (3) The green and yellow colours given by the glasses containing the halogens are caused by absorption due to the "free" halogen in the shorter wavelength region of the visible spectrum superposed on absorption in the longer-wavelength region due to groups of the (CoBr₄)^2- and (CoI₄)^2- type. (4) The blue colour given by acidified cobalt chloride solution is due to (CoCl₄)^2- ions, and is not the same as the colour given by cobalt in blue cobalt glasses.

M. A. Allan & H. Moore