Gold turns glass Ruby red

When you visit medieval churches in Europe, be sure to look at the stained-glass windows, which are works of art in themselves. Somewhere in there are minute nano-sized flecks of gold that give the red glass its tint. Nanotechnology is a relatively recent science, and its principal concepts have developed in the past half century; but humans have been employing nanotechnology for over a thousand years.

One of the best documented examples of nanotechnology in history is that used by medieval stained-glass makers. They were the first nanotechnologists, as they, trapped gold nanoparticles in the 'glass matrix' in order to generate the ruby red colour in the windows.

For centuries, cranberry glass or 'Gold Ruby' glass was created by adding gold salts or colloidal gold to heated, liquid glass (molten glass). The presence of gold in the coloured glass made it a luxury item used for expensive decorations. The effect of gold on the glass is quite significant, even though it needs to be used in very low concentrations (around 0.001%) to produce the rich ruby-coloured glass. In even lower concentrations, it produces a less intense red, often marketed as "cranberry glass."

The cost of gold and the glass was generally so high that manufacturers make it in craft production, typically hand blown or moulded, rather than in large quantities. Legend holds that gold ruby glass was first discovered when a European noble tossed a gold coin into a mixture of molten glass. Unfortunately, this is almost certainly not true, as the gold needs to be first dissolved in aqua regia, a mixture of hydrochloric and nitric acids.

Today, gold continues to be used in innovative ways to not only add colour to glass, but also as a reflective coating to reduce the harmful UV ray from sunlight getting through. Manufacturers embed gold particles into the window film or directly into the glass to help reduce ultraviolet or UV radiation and glare, while still allowing heat to pass through. The decrease in UV radiation is especially helpful for home-owners as the reflective glass helps protect furniture, floors, upholstery and artwork from fading.

Gold in glass has been known to be used in hot climates, such as many parts of India, to control solar heat gain and maintain lower internal temperatures.