Effect of the redox state on container glass colour stability

The effects of redox on the fining process, gas solubility, viscosity and final colour stability of container glass are well known. A constant redox state is important mainly for reduced coloured glass containers, such as amber, yellow-green ultraviolet absorbing glass (UVAG), dead leaf glass, etc. In order to ensure a stable and homogeneous final colouration strict control of several chemical and physical parameters is required. Redox state control of the final product can be undertaken by measuring the concentration ratios Fe2+/Fe-tot and S2-/SO3tot With appropriate analytical techniques (redox analysis). In this paper the results of about 200 redox analyses on commercial hollow glass fillies are statistically processed. The analyses Were performed by x-ray fluorescence spectrometry (XRF) and visible-near infrared range (NIR) spectrophotometry for the detection of Fe2O3tot, FeO, SO3tot, S2- and Cr2O3, with analytical procedures set up by Stazione Sperimentale del Vetro, (SSV). The analytical results made it possible to define the chromatic stability fields for the different glass container colours, based on the chemical composition and redox state. Particular attention was devoted to the analysis of colouring elements and compounds for the reduced UVAG and amber glasses. Finally a very limited area was defined for Which the colour stability of UVAG is ensured.