When people see iridescence on glass, they almost always ask: “Is this decoration?” The answer is no. It is actually a sign that the glass is slowly dying, its surface deteriorating as the chemicals in the glass react to the chemicals in the environment.
The Composition of Glass
Glass is made from a mixture of silica (commonly from sand), flux (alkali – either soda or potash), and a stabilizer (non-alkaline base – either lime or lead). The type of glass will vary depending on the amounts of flux and stabilizer in the mixture. The main types of glass are soda-lime, potash-lime, potash-lead, and lime glass.
Once exposed to the air, archeologically excavated glass can rapidly deteriorate. Because glass is hygroscopic, when buried it will absorb moisture from the soil. While it also absorbs moisture from the air, glass can lose up to 23% of its water by weight once unearthed and exposed to the atmosphere. This loss of water can cause the glass to deteriorate.
The absorption of water can also alter the chemical makeup of glass. Pollutants mixed with the water can create acids or glass can absorb salt through water. Both can have an adverse effect on glass.
Types/Signs of Deterioration
Dulling: the glass has lost its original clarity and transparency. Note – some glasses is naturally opaque
Iridescence: This is also referred to as patination. When the iridescence develops thick layers, these can flake off and weaken the glass, even to the point of collapse.
Cracking: This occurs when the glass becomes dehydrated, putting increased strain on the glass. These cracks can occur on the surface or penetrate deeper into the glass.
Milky/enamel-like surfaces: Usually opaque white, but can be mottled brownish-black in color. May flake away in small crystals, leaving pits in the glass.
Black discoloration: Opaque blackened layer. Only occurs in glasses with a high lead content buried in anaerobic conditions
Solarization: Glass made with manganese (c. 1890-1915) will develop a purple tint if left in the sun for an extended period of time. It is often seen in window panes. Note: Some collectors irradiate glass, giving glass from other time periods a solarized appearance. This is an irreversible process and not encouraged.
What to do when you notice signs of deterioration
Treating deteriorating glass is not easy, and I do not recommend trying to conserve glass unless you are a trained glass conservator. The best thing you can do is monitor the temperature and humidity where the glass is being stored. A stable environment should help to slow the deterioration. In some cases, depending on the cultural significance of the object and the availability of funds, you may consider contacting a glass conservator.
Can you identify the signs of glass deterioration in your collection? What other enemies of the curator do you want to learn more about?