The main constituents of glass are silica, lime, and soda. If the glass is surrounded by moisture, the soda and lime will be dissolved out, turning the glass surface into a water-rich gel. When the soil dries out for a season, this gel turns into a paper-thin film of silica that wraps loosely over the vessel's shape. Repeated wetting and drying, year-in and year-out, creates a crust consisting of dozens of these silica wrappings.
The appearance of the crust depends largely upon the chemistry of the soil that surrounded the vessel over the centuries. The higher the acidity of the soil, the more rapidly the weathering process will occur. In the case of the most common green-tinged glass, the outer part of the crust often hardens into an enamel-like shell that covers the entire vessel.