If you've ever felt overwhelmed by the games of light and colour of stained glass, then you've felt how well chemistry connects with art and beauty.
The use of stained glass to embellish basilicas and cathedrals has developed since the early Middle Ages, but the origin of the coloured glass is much earlier, as the Egyptians and Romans were already experts in its production.
The chemistry of stained glass overlaps with the very chemistry of glass: glass is obtained from the fusion of silicon dioxide sand or silica. Silica is a crystalline solid, i.e. a solid with regular and repetitive packaging of its atoms, which in this case, are arranged in pyramids containing a silicon atom surrounded by four oxygen atoms.
Glass is formed by cooling the fused silica, under conditions which do not allow it to recover this regular packing, giving place to an irregular and no repetitive arrangement, i.e., an amorphous solid.
Of course, glass is also a chemical recipe! The most common glass is a mixture of silica, sodium carbonate and calcium oxide, with smaller portions of aluminum oxide, sodium chloride and other salts each with their own role: "network formers, "network modifiers" and "melting agents".
For this programme on stained glass it is important to highlight the role of the "melting agents." "Melting agents" are substances added to silica to reduce its melting point. In fact, the melting point of pure silica is about 1700 ° Celsius, a temperature very difficult to achieve, even more in times when ovens used wood as fuel. Therefore, the first glass producers used "melting agents "as plant ash and sodium carbonate.
These materials contained several impurities and coloured glasses resulted of these impurities.
The first craftsmen could not control the colour of the glass they produced, but when they began to understand the origin of colour, there was a true explosion of experiments to discover the most suitable substances to get each specific colour.
A search involving artisans and alchemists, which had many aspects of real scientific research in chemistry!
Investigation in chemistry bringing us the beauty of stained glass.