Structure and durability of opal crystallized glass plates

Abstract

An opal crystallized glass plate, obtained from the addition of fluorine to a soda-lime base, has been structurally characterized and altered in food contact like conditions. The investigations on the pristine glass evidenced the nature of CaF₂, BaF₂, and NaF crystalline phases. Overall a continuum of fully vitreous to glass-ceramic material was noticed with different morphology: the top surface that resembles a soda-lime glass which was prepared as a slab and the highly crystallized bulk using powder. Powder and slab were altered together at 70°C in acetic acid 4% (v/v) imposing a pH of 2.4 for 231 days to 3 years. The bulk powder alteration was characterized by a predominant hydrolysis mechanism impacting the crystals and the glassy matrix, leaving no remaining altered layer at the surface whereas a 1.25 µm thick alteration layer was observed on the top surface of the plate after 231 days of alteration. The mechanisms for the formation of this altered layer as well as the differences between the powder, representative of the bulk opal crystallized glass, and the slab that remains the actual surface in contact with edibles are discussed in the article.