Andy Connelly explains the chemistry behind the ethereal beauty of stained glass windows

Life, like a dome of many coloured glass, stains the white radiance of eternity – Percy Bysshe Shelley

I often find peacefulness in a soaring stone church, a cool open place to sit and contemplate. The giant trunk-like pillars and the gentle play of the light cast through the stained glass create a shaded garden of stone and multicoloured light.

Stained glass windows are never static. In the course of the day they are animated by changing light, their patterns wandering across the floor, inviting your thoughts to wander with them. They were essential to the fabric of ancient churches, illuminating the building and the people within, both literally and spiritually. Images and scenes leaded together into windows shed light on the central drama of Christian salvation. They allowed the light of God into the church.

The history of stained glass dates back to the middle ages and is an often underestimated technical and artistic achievement.

Glass itself is one of the fruits of the art of fire. It is a fusion of the Earth's rocks: a mixture of sand (silicon oxide), soda (sodium oxide) and lime (calcium oxide) melted at high temperatures. Glass is an enabling material used for more than just drinking vessels and windows. It also allows scientists to observe distant stars and the smallest biological cells, and colourful chemical reactions in test tubes.

The history of glass

The earliest evidence of human interaction with glass was the discovery of flaked obsidian tools and arrow heads dating from more than 200,000 years ago. Obsidian is a volcanic glass formed when hot volcanic lava is rapidly cooled.

The earliest manufacture of glass probably occurred in Mesopotamia during the early part of the third millennium BC. Early glass finds consist of relatively crude beads usually formed around a metal wire. They are blue and green suggesting that the earliest glass was used to replace or evoke semiprecious stones such as lapis lazuli and turquoise. This reflects much of the history of glass-making where glass was a surrogate for luxury, a man-made stone, glass being cheaper and softer and so easier to work.

Sheets of glass both blown and cast have been used architecturally since Roman times. Writers as early as the fifth century mention coloured glass in windows. Around AD 1000 Europe became less war-like, and church building and stained glass production began to flourish. However, these churches were Romanesque in style with massive walls and pillars to bear their weight and so had only relatively small windows.

But by the 12th century the pointed arch and flying buttresses of the Gothic style were allowing builders to insert "walls of light", giant windows that filled the church interior with the perfect light of God.

A common misconception is that the glass in these ancient cathedral windows has flowed over time, now being thicker at the bottom than the top. This is not true and the explanation goes to the atomic heart of glass.

The chemistry of glass

So what is a glass? Why can we see through it when other materials are opaque? Glasses exist in a poorly understood state somewhere between solids and liquids. In general, when a liquid is cooled there is a temperature at which it will "freeze", becoming a crystalline solid (eg. water into ice at 0C). Most solid inorganic materials are crystalline and are made up of many millions of crystals, each having an atomic structure which is highly ordered, with atomic units tessellating throughout. The shape of these units can be observed in the shape of single crystals (eg. hexagonal quartz crystals).

Glass is different: it is not crystalline but made up of a continuous network of atoms that are not ordered but irregular and liquid-like. This difference in atomic structure occurs because the liquid glass is cooled so quickly that the atoms do not have time to arrange themselves into regular, crystal-like patterns.

If cooled fast enough almost any liquid can form glass, even water. However, the rate of cooling must be very fast. Fortunately for us, liquids composed primarily of silicon oxide can be cooled slowly and still form a glass. They get gradually stiffer during cooling until they reach the "glass transition temperature" below which they are effectively solid.

This transparent silicate material is what we know as glass, and despite its liquid-like atomic structure it is to all intents and purposes solid, only flowing over billions of years – much too slowly to be noticed in the hundreds of years cathedral windows have been in place. Cathedral windows are sometimes thicker in one place than another because forming glass into perfectly flat sheets is a very difficult process that has only been possible in the last 60 years.

Glass's liquid-like structure is one of the main reasons it is transparent. However, transparent does not necessarily mean that all light passes through. For example, some obsidian glasses are so darkly coloured that they are effectively black and opaque. This is because electrons of some elements in the obsidian are arranged in such a way that their energy is the same as the energy of visible light. This means they absorb either some visible light, giving colour, or all visible light, making the material opaque. Clear glass does not contain elements that absorb visible light so it is colourless and transparent. However, it does absorb ultraviolet light, thus preventing you from getting sunburned through your car window.

Staining glass

This ability of certain elements to absorb light and give colour is used to great effect in stained glass windows. For example, adding cobalt oxide to the glass during melting will make it blue because cobalt absorbs wavelengths at the red end of the spectrum but does not absorb blue.

These colours were discovered by the ancients through trial and error, adding different minerals to the melting pot and melting for different periods of time, giving an incredible array of colours. Copper-bearing minerals can produce a red or sky-blue glass, manganese pink or purple, and iron various greens or a bright yellow glass. These colours were used to great effect by ancient glaziers, even though they had no inkling what caused them. Minerals often made their way into glass as impurities in sand, giving faint colours such as green (iron) and purple (manganese) that can often be seen in "clear" cathedral glass.

The term "stained glass" telescopes three different processes: colouring, staining and painting, each one complex and requiring the application of many skills. The glaziers who made these windows did not themselves make the glass, this was the job of the glass-makers. It was hot and dangerous work that required great skill and knowledge. Glass-makers knew and jealously guarded the glass recipes and furnace conditions needed to make a myriad of colours. They would mix the raw materials in clay pots heated with wood fires and then manipulate the resulting viscous liquid with metal and wooden implements.

Glass-making was a difficult and unpredictable process: it required just the right proportions of ingredients and controlled furnace conditions. Any small variations could lead to imperfections such as uneven tinting, odd colour hues, bubbles, or embedded impurities.

Glass-makers would supply sheets of coloured glasses to the glaziers to create their windows. The process of making a stained glass window begins with the artist's sketch, known in medieval times as the vidimus (Latin for "we have seen"). The vidimus was then drawn to full scale (known as a cartoon) on a whitened table top. The panes of coloured glass would then be cut to shape, placed on the cartoon, and joined with strips of lead.

The use of lead is thought to have emerged in the middle ages. It was the ideal material to join the pieces of glass because it is flexible yet strong and durable enough to support a great mosaic of glass against extremes of weather and temperature. The panels would be made weatherproof by rubbing a putty-like mixture of lime, lead and linseed oil into the joints. The panels could then be mounted in the window space.

Complex patterns of different coloured glasses could be produced to stunning effect. However, if stories were to be told human images were required, including details such as hands, faces and the folds of drapery. These were added to the surface of the coloured glass sheets using a black enamel pigment based on copper or iron oxide. This mixture was painted onto the glass with different thickness and textures to give different shading effects, allowing control of light and providing artistic detail. After painting, the pieces were fired to fuse the paint to the surface of the glass.

From the 13th century a second pigment in the form of a "stain" of silver chloride or sulphide was painted onto the glass. Traditionally, this would have been the only true "stain" in stained glass. After painting the stain onto the glass it was heat-treated in a furnace. During the treatment the silver ions migrated into the glass and were suspended within the glass network, rather than sitting on the surface like glass paints and enamels.

Silver stain can give colours ranging from pale yellow to a deep red, depending on glass composition, stain composition, the number of applications, the temperature of the furnace, and colour of the background glass. It was the perfect way to depict yellow hair, halos and crowns along with faces on the same piece of glass, so reducing the amount of light-blocking lead. The stain could also be applied to blue glass to give green, making possible depictions of blue sky and green fields.

Enamels

The variety of colours and effects that ancient glaziers could achieve with these simple facilities was incredible, but in the mid-16th century different coloured enamels began to be used. Like glazes for pottery, these used either ground-up coloured glass or clear glass with a metallic oxide in a binder, which was painted onto the glass then fired. Methods such as these continued to be used in the 17th to early 19th centuries as they allowed windows to be painted like easel pictures on clear rectangular glass. This reduced lead usage with the metal often being used merely to hold the large panes together, the designer's aim being to conceal the lead rather than integrate it.

However, it has been said that the emergence of enamelling was the death of the great stained glass window artists. I'm not sure if this is true, but I do know that to stand in front of a great leaded stained glass window is a magical experience. Next time you are in a soaring cathedral take a moment to contemplate them.

Notice the aesthetic beauty of the biblical stories depicted in vivid simplicity. As you get closer, see the amazing use of lead and coloured glass to form these images. Closer still and the individual colours, staining and shading start to come into focus. Can you see the cobalt blues and the yellow silver stain? Then, as you get really close, you can see the way the uneven handmade glass distorts the light, giving a natural organic quality with bubbles trapped forever in the heart of the glass – bubbles that were frozen in as the hot liquid cooled.

Stay awhile in that shaded garden and marvel at the conjoining of ancient stories, art and science.

Andy Connelly is a cookery writer and former researcher in glass science at the University of Sheffield. He is training to become a science teacher