I really loved putting together a handling table about glass for National Science and Engineering Week, because I discovered so much about the history of glass and the science behind glassmaking. The enjoyment levels had nothing to do with looking through lovely artefacts at all…really!
|My handling table and me...|
It was the Romans who revolutionised glassmaking through the invention of glassblowing. This technique works because atoms and molecules move around freely in molten glass, and do not form a crystalline structure when it cools, as they do in most other materials. Cooling glass stays flexible and can be manipulated by blowing a bubble into it through a long tube, and pulling it into various shapes before it hardens.
The technique of glassblowing is still used today, and the composition of modern glass is also very similar to Roman examples, consisting of:
• Sand (silica), the ‘former’. 65-70% of the overall mixture
• Soda, the ‘flux’. 15% of the overall mixture, which allows it to fuse more efficiently
• Lime, the stabiliser. 10% of the overall mixture, which makes it more durable and chemically resistant
• Pieces from malformed, broken, or old vessels, the ‘cullet’, which lowers the temperature of fusion
• Various impurities or colorants make up the remaining 5-10%
These colorants were usually powdered metals, and once again those used to colour ancient glass are still well known. Some of the most common include:
• Iron (Fe), which gives a pale blue, bottle green, amber or black. This is often used to colour wine bottles, although Chromium (Cr) is more commonly used today
• Manganese (Mn), used to create purple or yellow, and sometimes as a decolourant – clear glass was highly prized
• Copper (Cu), usually used for blue, green, ruby red or opaque red, but most recognisable as a turquoise colour
• Cobalt (Co), also used for rich deep blue, particularly in glass containing potash (burnt wood and bracken)
There were also more unusual colorants, including Uranium (U), which was used by 19th Century glassmakers who were unaware of the dangers of radiation, and creates an almost fluorescent yellow!
|Roman Glass from Museum Collection BATRM 2003.22.8|
Zosia - Collections Intern