Origin of blue and green colouration in sulphur containing alkali borate glasses

Simple binary sodium and potassium borate glasses containing sulphur were prepared. Depending on the composition of the glass and the quantity of sulphur added the glasses produced were blue, green or yellow. The absorption spectra of these glasses were measured in the range of 300–700 nm.The blue colour is characterized by an absorption band at 585 nm which is attributed to S₃^?.The green colour is characterized by two absorption bands; the band at 585 nm observed in the spectra of the blue glasses and a more intense band at 390 nm. The latter is attributed to S₂^? and the green colouration to the presence of both S₂^? and S₃^? where the S₂^? state predominates.