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Glass decay

August 13, 2011 , , , ,

Topette with emaille doses

The picture above shows a topette that has glass decay. Unlike glass that is cloudy because of calcium deposit (caused by milk or hard water) decayed glass cannot be cleaned. Many people may think that glass cannot decay at all, but this is not true. Especially old glass is prone to deterioration. In this post I would like to explain why.

Basically only two materials are needed to produce glass: sand as the primary material and flux which is an agent that lowers the temperature that is required to form the glass. The Romans used plant ashes as flux. In the early Middle Ages the prevalent fluxes were sodium carbonate and potassium carbonate which are better known as soda and potash.

But time showed that glass made with potash was prone to decay. On the other hand lime-rich sand proved to make a durable glass. Thus the combination of soda, lime and silica became prevalent in the Middel Ages while potash was widely abandoned. Still, in Southern Europe potash was used in glass making until to the late 19th century.

Today we can look at the atomic structure of a material. Glass has the molecular structure of a frozen liquid, meaning it does not have the ordered structure of for example a crystal. The modern glass making uses better formulations and better technology which results in very fine yet hard glass that is not very likely to deterioriate.

But what causes decay in old glass like the topette in the picture? There are two causes: Surprisingly one cause is water, be it as a liquid, as condensation or as humidity. The scientific explanation is the following:

“The deterioration process which moisture initiates is complex but it usually involves the diffusion of hydrogen ions from water within the glass network. One hydrogen ion (H+) from the water molecule displaces the sodium (Na+) or potassium (K+) ions from the network, leaving an hydroxide ion (OH-). The byproduct of this displacement is sodium or potassium hydroxide, both highly reactive alkalis which leach from the surface of the glass, depleting the body of the glass as a result. The accumulation of water on the glass surface will continue to produce a build-up of alkaline corrosion products which attack the silica network of the glass causing surface decay. Alkaline corrosion products react with atmospheric gasses to form sulphate and carbonate crystals. […] The net result is a gradual deterioration of the glass, usually accompanied by the simultaneous formation of a crust, normally composed of carbonates and sulphates and other opaque weathering products. As the alkalis build up, so the corrosion will increase, taking the form of pitting or crusting. Pitted glass surfaces are often strikingly uniform in size across a corroded area. These pits, depending on a number of factors, may range from 0.1 mm in diameter to much larger, perhaps around 2.0-4.0mm. Pits can become linked together and crusts form within them so the difference between pits and crusts become slightly ambiguous. In the most severe cases, pitting and crusting can reduce the glass to a very fragile state often depleting the thickness of the glass, and in extreme cases, pitting may even produce holes.”

(Stained Glass and Its Decay by Drew Anderson)

The second cause for glass decay is fungus. Fungi not only trap moisture which deteriorates the glass as described above, they also can produce acidic byproducts that further reduce the transparency of the glass.

Finally, how should you treat your old glass to keep it free from decay or prevent further decay? The answer is, don’t let moisture sit too long on the glass. Keep it rather dry and clean to prevent fungal growth.


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