Causes and prevention of soda bloom in glass industry

Abstract

The study is cooperation project with a local table ware glass producer. The causes of a persistent quality defect due to hydrolytical interaction of the glass surface (“soda bloom”) are investigated. The investigations comprise the role of glass composition, forming and annealing, as well as the storage conditions. Temperature and relative humidity in the stores were recorded over a period of 8 months. Factory samples were taken from different positions of the production route and submitted to different surface analysis tests. As a supplement, laboratory samples with an optimized composition bases on the original formula, however, with decreased sodium content were prepared and submitted to the same tests. These are : step-wise acid etching (0.5% HF + 2% HNO₃, 45℃, 15 s per step) and consecutive wet-chemical analysis of the Na content in the etching solution; leaching in D.I. water (98℃ , 4 h) and consecutive determination of the electrical conductivity of the leachate ; morphological inspection by scanning electron microscopy combined with energy-dispersive X-ray analysis of manifested bloom. The freshly produced glass surfaces were less resistant towards HF attack than the bulk glass. Accumulation of high Na levels in the surface occurred during the forming steep, however, decreased again during annealing. The optimized glass compositions were lower in soda release than the original formula. The conditions in many areas of the warehouses were such that the stored glass products were exposed to condensation of atmospheric water for extended periods of the year. A major reason is the decreased inside temperature during daytime. The manifested bloom contained all elements from the original glass composition, however, at an increased Al:Si ratio. Sulfur was not found. In a very distinct way, the bloom occurred at surface flaws only, which were most certainly generated during the forming process. Countermeasures are suggested with respect to 1) the mould sticking problem 2) the atmospheric conditions in the store and 3) the overall glass composition.