Not all solid inclusions in glass products are due to incomplete melting of batch ingredients, refractory erosion, corrosion or spalling, air surface volatilization from the melt, or "cold glass" devitrification. Raw materials may contain parts-per-million levels of "refractory" mineral grains and there may be unwanted materials present in the cullet,especially if externally sourced cullet is used. Comprehensive analysis of the inclusions by petrographic methods using transmitted polarized light microscopy (PLM) and/or sometimes augmented by qualitative elemental analysis via scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDX) can reveal the true root cause of the problem. This paper will present several examples illustrating the analytical techniques and some of the revelations that can be provided by those analyses.
