A study of the process conditions that lead to an unusual frosted glass

A remarkable frosted glass finish, showing unusual scale pattern, has been used since 1988 for innovative glass bottle decoration. To achieve this kind of frosting, the glass flasks have to be immersed in a frosting bath, which has a limited working lifetime. This study deals with the determination and understanding of the key parameters required to induce this type of frosting. The frosting bath has to be unstirred to allow a diffusion phenomenon giving the scale pattern, whereas standard processes lead to a homogeneous finish. The composition of the stripping bath and the drainage time between the stripping bath and the frosting bath has also to be carefully controlled. The resulting finish could also be explained by the role played, in the chemical mechanism of frosting, by the three species HF, NH₄ ⁺ and NO₃ ⁻ present in the frosting solution. Therefore in the frosting bath, the concentrations of critical chemical species must be kept within rigorous limits to achieve a high quality frosting. To complete this study, different compositions of frosting bath have been considered, wherein nitric acid was substituted by other strong acids; this confirmed that nitrate ions are required in the frosting bath to obtain the desired finish.