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Glass Conservation
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"FILE 5: GLASS CONSERVATION
IN THIS FILE :
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. Ideally, glass should
consist of 70-74 percent silica, 16-22 percent alkali or soda ash (sodium carbonate) or potash (potassium carbonate,
usually derived from wood ash), and 5-10 percent flux (lime [calcium oxide]). Soda-lime glass has been the most
common glass throughout the history of glass-making, and the modern equivalent is 74 percent SiO 2 , 16 percent Na 2 CO 3 , and
55 percent lime added as stabilizer.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.
In all glass, the sodium and potassium oxides are hygroscopic;
there"
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