Chemical Frosting of Glass

The frosting process is used in the bottle, cosmetics, lighting and wine & spirits industries as a method of decorating glass packaging. It is also used on flat glass as an opacifying process for interior and exterior applications. Frosted parts have a more or less matte satin finish, a soft feel and a distinctive aesthetic appearance. While various glass surface treatment techniques exist, chemical frosting is the most common and effective in terms of efficiency and simplicity. The principle consists of immersing glass parts in acid baths containing concentrated hydrofluoric acid and alkaline bifluoride salts, for very short periods of time (in the order of minutes). During this phase, the parts to be treated are covered with a semi-passivating white layer, formed from the reaction products of the hydrofluoric acid attack on the glass. Rinsing with water removes this layer, revealing the satin finish. Different satin finishes can be obtained, depending on the nature of the satin-finishing solution, its homogeneity and the process parameters applied, not to mention the know-how and technical expertise of each depolishing workshop.

Although the chemical satin-finishing of glass has been mastered experimentally and the associated know-how is available in the vast majority of depolishing workshops, little or nothing is known about the chemical mechanisms governing this process. The content of this article is based on a description of the chemical satin-finishing mechanism, the different aspects that can be obtained and, finally, the parameters influencing satin-finish appearance and their variations. The aim is to better control the phenomena that occur during the satin-finishing process, in order to optimize management and the quality of the satin-finish obtained, and possibly to develop innovative satin-finishes. The glassware satin-finishing industry is dependent on fluctuations in fashion, and offering innovative satin-finishes would help build loyalty in this market through recurring novelties.

This article is aimed at methods and production engineers in charge of acid depolishing lines, who need to understand how the satinizing process works, so as to know which lever to pull in the event of a non-conformity problem.