The Colour of Iron-Containing Glasses of Varying Composition

JSGT 1951 V35 T332-T406

The colour changes caused by systematic composition variations in iron containing silicate and silica-free glasses have been evaluated in terms of visual colour, spectral transmission and chromaticity. In alkali–lime–silica glasses the replacement of silicon by an alkali element changed the colour from green towards yellow as the short-wave absorption attributed to Fe³⁺ was broadened into the visible spectrum. The replacement of silicon by boron changed the colour from green to yellow as the short-wave-absorption edge was shifted into the visible and the infrared band of Fe²⁺ was decreased. The replacement of sodium by potassium favoured a decrease, that by lithium an increase, in Fe²⁺ absorption resulting in higher colour purity in the potassium, and lower in the lithium glass. Little change in colour was noted on replacing calcium by the other alkaline-earth elements. The zinc and cadmium replacements of calcium sharpened the Fe²⁺ and Fe³⁺ bands, resulting in lower purity. The replacement of calcium by titanium resulted in a dark amber colour with a considerable shift of the short-wave-absorption edge into the visible range. Neutral coloured glasses were made with reduced zinc potassium borosilicate base glasses containing fluorine. Heat absorption with high visible transmission was demonstrated as possessed by a reduced magnesium potassium zinc phosphate glass. The replacement of aluminium by iron in an aluminium metaphosphate glass produced brown-coloured glasses, whilst pink colours with high visible transmission were realised in sodium fluoride–aluminium phosphates. A similar type of light amber colour was produced in glasses as different as sodium fluoride–titanium germanate and calcium aluminate glasses. Theoretical concepts are employed in this paper only as an aid in the understanding of means for controlling the colour of iron-containing glasses.

Gordon F. Brewster & Norbert J. Kreidl