Treatment of Unstable Glass Devitrification Removal of Sulfide Stains from Lead Crystal Reconstruction Summary Glass is usually the most stable of archaeological materials, but glass artifacts, and 17th-century glass in particular, can undergo complex disintegration. Soda glass is characteristic of southern Europe, where it is made from crushed white pebbles and soda ash derived from burnt marine vegetation. Soda glass, which is often used for the manufacture of cheap glass, is twice as soluble in water as potash glass. Potash glass is more characteristic of interior Europe, where it is made from local sands and potash derived from wood ash and burnt inland vegetation. A little salt and minute amounts of manganese are added to make the glass clear, but potash glass is less clear than soda glass.
Most early glass is green because of iron impurities in the materials.
Alkali lowers the melting point of the sand, and the flux facilitates the mixture of the components. As long as the original glass mixture was kept in balance, the resulting glass will be stable. Problems arise when an excess of alkali and a deficiency in lime are present in the mixture, for the glass will be especially susceptible to attack by moisture. If old glass contains 20-30 percent sodium or potassium, it may have 'glass disease,' where the glass weeps and begins to break down. The absorbed moisture and exposure to carbon dioxide causes the NaO 2 or NaOH and KO 2 or KOH to convert to sodium or potassium carbonate.