On Cobalt in Glass

JSGT 1948 V32 T224-T232

The colour and magnetic properties of some cobaltous compounds are discussed in relation to the electron arrangement around the cobalt nucleus, and the ideas developed are then applied to the colours of cobalt glasses. It is concluded that the red colour in cobalt glasses is due to the Co²⁺ ion, and that the blue colour arises from the superposition of a strong absorption band in the red, caused by a relatively small proportion of anion complexes of the type [CoO₄^B]^2- where OB represents an oxygen atom forming bonds to the cobalt atom on the one hand, and to a silicon atom on the other (in silicate glasses). In the alkali borate glasses studied by Brode, the cobalt is entirely in the ionic state in the region where the addition of alkali serves to convert boron from three- to four co-ordination; but addition of alkali to the point where this change in boron co-ordination ceases produces an increasing proportion of [CoO₄^B]^2- groups.

J. E. Stanworth