Glass defects are typically unavoidable, but their occurrence must be minimised during the production of high-quality glasses. Common glass defects include bubbles, stones, knots, cords and cat scratches. One unusual stone defect in a pattern glass was recently discovered, and a scrupulous methodology was necessary to investigate its source. Numerous plate glass samples with white-coloured stone defects were received for analysis. These defects were generated in an end-fired furnace that pulls around 240/tpd soda-lime pattern glass. The relatively new furnace had only been in operation for 18m. The hot spot temperature was reported in the range of 1540-1570 deg C, while the refiner temperature was in the 1240-1250 deg C range. The stone defects were unusually large; greater than 2mm, and showed a "wispy" milky-white pattern.
