Inhomogeneous glass surfaces resulting from rapid forming operations – evidence from differential corrosion

A chemical compositional heterogeneity develops on glass surfaces which have experienced rapid expansion/stretching – as during the press or press/blow process used in forming. A differential etching technique using HF and HBF4 is developed to demonstrate that such heterogeneities occur on the surface of commercial articles such as soda-lime silicate tumblers. Comparison with articles exposed to aggressive conditions in a well-characterized dishwasher reveals that similar corrosion patterns appear in both cases – as alternatively corroded/non-corroded regions arranged as striations running vertically along the tumbler. Analysis of these striae by imaging ellipsometry and interferometry indicates that they consist of ridges 50–150 nm high in relief with a width and repeat distance of 0.05–0.15 mm. This pattern is in qualitative agreement with a recent physical modeling study and suggests that the ridges arise from durable low alkali glass which is at the surface of the gob/parison before the press/blow and which has been striated by less durable, high alkali glass from the interior of the gob. Under corrosive conditions, the alkali-rich glass dissolves away to leave behind the durable ridge.