Colour of Heavy Lead Silicate Glass

Abstract

IT has been generally accepted in the optical industry that heavy lead flint glasses are yellow in colour. The Schott catalogue¹ on Jena optical glasses marks most of the heavy flints they produce as being yellow, and it is further pointed out that noticeable yellow colour would result even by using chemically pure materials. The question whether the yellow colour is intrinsic to the composition or is due to the presence of colouring oxides as impurities has been the concern of many ceramists and glass technologists^2,3. Based on the fact that for heavy lead glasses the colour of the glass becomes browner as the temperature is raised, it was considered obable that the yellow tinge at room temperature might be due to the residual effect of thermal broadening and shifting of the characteristic ultra-violet absorption band towards the longer wave-lengths of the spectrum. Thus it has been suggested by several workers as being due to the thermal dissociation of lead silicate into unbonded lead oxide². W. Weyl³ further suggested, on the modern view, that the deepening in colour might be due to the influence of the PbO bond in the glassy structure. On the other hand, it is also well known that colouring oxides such as iron, copper, etc., all produce more intense colours in heavy lead glasses than in ordinary lead-free glasses. In fact, it is not impossible that the thermal deepening in colour is partly due to the presence of such oxides.