Paradoxical crystalline morphology of frosted glass

Abstract

Hydrofluoric acid solutions are used to dissolve glass or silica surfaces. Depending on the parameters smooth or frosted surfaces can be obtained. The surface of frosted glass exhibits a peculiar morphology with a crystal-like aspect characterized by well facetted lattice planes. To understand this effect, we have studied the dissolution of glass slides in an acidic solution of ammonium fluoride (NH4HF2) by chemical analysis, kinetics studies and in situ optical microscopy. The frosting effect can be explained by the competition of two reactions. The dissolution of the silica network by hydrofluoric acid leads to the formation of H2SiF6. Because of the presence of ammonium ions in the solution, the precipitation of the weakly soluble ammonium fluorosilicate (NH4)2SiF6 forms a passivating layer that blocks the dissolution. In appropriate concentrations, the nucleation of the ammonium salt is rapid and leads to a bidimensional growth of a well crystallized layer at the surface of the dissolving glass. The uncovered glass surface continues to dissolve and feeds the protecting layer that progressively extends and masks the rest of the glass. Dissolution is stopped when all crystals merge to cover the whole glass surface. After washing the ammonium salt with pure water, the roughness of the frosted glass is the negative shape of the crystal growth.