An investigation of ancient opal glasses with special reference to the Portland
JSGT 1959 V43 T285-T288
The nature of the opacifier in a fragment of opal glass from the Portland Vase and in other specimens of cameo glass of the same historical period has been investigated. Chemical analysis shows the base glass to be a soda–lime–silica type containing more than 8% of antimony oxide and x-ray diffraction proves that a precipitate consisting of the calcium pyroantimonate Ca2Sb2O7 is responsible for opacification. The crystal structure of Ca2Sb2O7, although of relatively low symmetry, has affinities with that of the cubic pyrochlore class of compound.
H. P. Rooksby