The development of stained glass
by John Rattenbury


The origin of glass dates back to 3000 BC probably to the pottery makers of Mesopotamia.  The temperature required to create glass is greater than the ordinary cooking fire would create so it is most likely to have been created in a potter’s furnace.  This very early glass was used to create coloured beads for decoration.
Serious glassmaking really started with the Egyptians and involved the creation of beautiful moulded glass objects and colourful core formed vessels for storing perfumes and ointments.  With the core forming, a clay sausage shape is moulded on the end of a stick and a thick string of liquid glass is drizzled in a coil onto the clay.  When cool the stick would be removed and the clay core scraped out to create the finished vessel.  At this stage glass was not used in windows.
Glass is manufactured from three raw materials: Silica in the form of sand, with an alkali in the form of plant ash and lime to act as a flux to remove impurities from the glass.  In the early days of glassmaking, the lime came as part of the other materials in either the form of limestone mixed with sand or from the plant ash (particularly where aquatic plants were used).  Many impurities would still be present in the glass and although richly coloured glasses were created from the early days it was 400 AD before reasonably transparent and colourless glass became generally available. According to Pliny, the Roman historian, fully transparent and colourless glass  was “the most valuable product on the earth’s surface”
The first window glass appeared around 300 BC and consisted of small gemlike pieces of broken thick glass embedded in wooden, stone or cement panels.  This kind of window is still in use in the Orient.  The Romans used translucent sheets of alabaster, mica or marble as window materials.  Glass was still a very rare commodity in windows but was beginning to appear, such as in the bath house in the Plaza of Pompeii where two small flat panes of glass are mounted on a bronze frame to create a ceiling window.
At about 100 BC the process of glass blowing first appeared, at first using clay pipes (too short for use with anything but small quantities of glass) then later metal pipes which allowed much larger glass bubbles to be blown. Glass blowing caused a revolution in the production of glass vessels and glass sheet.  With glass blowing, the glass vessels could be produced in mass quantities with thin walled fine glasses created in a large variety of shapes.  Using a mould in combination with glass blowing allowed the standardisation and duplication of objects.
The two methods by which a flat sheet can be created from a blown bubble of glass are cylinder glass (or muff glass) and Crown glass.  With cylinder glass the bubble is swung at the end of the rod to produce a sausage shape which can be up to 7 feet long and 18 inches in diameter.  When the sausage is cold, the two ends are cut off and the cylinder is scored and cracked down the middle.  It is then placed in a furnace to allow the hot sheet to spread out flat.  With Crown glass, the bubble is opened up and spun so the hot glass spreads out into a circular flat disc.  The blob of glass at the centre of the disc which is attached to the rod is called the bull's-eye and can still be seen in old leaded glass windows are now highly sought-after.
Cylinder glass and crown glass were both used for many centuries, sometimes even together in the same window.  Cylinder glass allowed a larger sheet and had a more uniform thickness where as Crown glass was thinner and tapered from the central bull's-eye to the outside edge of the disc.  During the 18th century when a tax on glass was introduced, Crown glass was more popular as it was thinner.  All the 1,000,000 ft.² of glass for the Great exhibition (Crystal Palace) was made using the cylinder method to get larger more uniform sheets. You can identify which type of glass has been used in a window by looking at the ripples and imperfections in the surface.  Crown glass has subtly curved regular ripples and in many cases, bubbles.  Cylinder glass however has faint parallel ripples, or slight natural distortions without any particular pattern.
The leads in stained glass windows are called 'cames' and in mediaeval times were cast in sand moulds.  They frequently contained bubbles and impurities such as tin and antimony which caused the lead to be less malleable (less soft and flexible).  The lead sections were heavier and cut out more light but provided good support.  Victorian lead by comparison was much purer so it was more flexible and was rolled and stretched into shape.  As a result, it provided much less support so many stained glass windows that were restored in Victorian times now show signs of sagging as the lead slowly moves. Nowadays, conserving the cames is as important as conserving the glass.
To construct a leaded stained glass window, several stages are involved.  After the window has been designed a cartoon is drawn showing the shape and colour of every piece of glass.  In mediaeval times, when paper was very expensive, the cartoon would be drawn on a whitewashed table and then re-whitewashed when the window was complete.  As a result, early cartoons are very rare.  Each piece of glass is carefully cut to the correct shape then prepared with any necessary colouring or enamelling required.
All early coloured glass (and most modern coloured glass) is created using the pot metal method.  Metal oxides are added to the crucible of molten glass to create the desired colour.  So pot metal glass is coloured through the entire thickness of the glass sheet.  Onto the shaped pieces of coloured glass a black enamelling process could be used, even from the earliest days, to create lettering, the folds in clothing, facial features, etc.  Black enamelling involves the use of a low melting point glass which is ground into a powder and mixed with iron oxide (Rust) and gum and water to create paste which is painted onto the surface of the glass.  The glass fragment is then heated in a furnace just enough to melt the ground glass, creating a dark brown enamel.
It was the 1400s before a new technique of glass colouring was discovered.  This was silver staining in which silver salts were painted onto the glass and carefully heated in a furnace when the silver salts would chemically bond with the glass and create yellow and amber colours.  This was a difficult technique as the temperature, concentration of the salts and the composition of the glass all affected the colour.  Another technique that was introduced into glass window making (but that had been known to glass vessel makers for hundreds of years) was glass flashing.  The bubble of coloured glass would be dipped into clear glass to create a two layer sheet of glass with two distinct colours.  Clear glass improved the transparency of the window.  The use of two different colours such as yellow and blue would create a third colour such as green.  Also, one layer of glass could be ground away in small areas showing different colours on the same piece of glass without having to use lead.
Once all the glass pieces for the window are ready, cames (lead strips) are cut and shaped and the window is assembled.  Long nails are used to hold the window together as it is assembled and finally all the joints are soldered.
So why was stained glass so popular in mediaeval churches and cathedrals?  Up to 11th century, church windows were very small as the walls had to be tremendously thick and heavy to support the outward push of the roof.  Many churches at this time had beautifully decorated walls with pictures of the Saints and other religious motifs.  In the 12th century, with the development of the new Gothic architecture involving flying buttresses and wonderful new stone masonry techniques, windows increased in size as the area of wall reduced.  It would be a natural development to fill these new open spaces with even more dramatic images in light, and mediaeval man would have experienced a window rather than read it.  In the early and middle ages only churches were able to afford expensive stained-glass windows.  Wealthy patrons would sometimes commission stained glass windows for their local church, Cathedral or abbey and in the later Middle Ages windows were commissioned to commemorate a notable person or group from the community.  On some windows, the donor family would be displayed as a public symbol of their generosity to the church (and a ticket to heaven).
By the 15th and 16th centuries, stained glass as we commonly know it was rapidly going out of fashion.  Religious attitudes were changing and in many cases windows were being actively removed from cathedrals and churches as the images they displayed were no longer in keeping with the new doctrines. The humanism and realism brought into art by the Renaissance marked changes in style and a decline in stained-glass.  The mysterious and spiritually uplifting effects of light streaming through coloured glass was no longer so relevant to religious worship.  Stained-glass was linked to the power and rituals of the Catholic Church so was inappropriate for Protestant churches of northern Europe.  Henry VIII dissolved the monasteries and in 1547, Edward VI made a Royal injunction decreeing that "they shall take away.... all other monuments of fained miracles, pilgrimages, idolatry and superstition; saw that there are all remain no memory of the same in walls, glass windows, or elsewhere in their churches or houses". The fanatical William Dowsing, who held the office of Parliamentary visitor, gives details in his diary of a journey through parts of East Anglia undertaken for the express purpose of stripping churches of ornament and smashing the stained glass "like a bedlam". Fortunately, replacing church windows was very expensive so in many cases the stained glass remained perhaps with just the face of a saint defaced.
Much of this destruction was aimed not at the stained glass itself but the images that were portrayed.  The stained-glass makers looked for new sources of work and found it in the homes of noble families with the construction of heraldic stained-glass windows.
It was the Victorian era before leaded stained glass windows came back into fashion with Victorian passion for mediaeval and Gothic art.  In the 19th century, with the new church building programme involving the construction of over 600 new churches, stained glass was very much back in fashion.  Unfortunately, with the Victorian enthusiasm for restoration, many windows were reconstructed with Victorian leading and many of the corroded glass pieces replaced with new Victorian glass.  Stained-glass windows can deteriorate for many reasons.  Lack of support can cause the window to distort and individual glass panes to crack.  The smoky acidic atmosphere associated with many city and town environments corrodes both the glass and the leading.
Nowadays, the key word is conservation rather than restoration, so instead of trying to return a window back to its original immaculate state, we try to preserve the window and prevent any future deterioration.  However, there are situations where a window is in such a bad condition that restoration or even complete replacement is the only solution.  In Kilmaurs there is a company called the "stained-glass design partnership" that has undertaken such work on some of the finest windows in the country.
A wonderful example of restoration can be seen on the work the stained-glass design partnership undertook to the west window of the Scottish Parliament building in Edinburgh.  Wherever possible, the original glass was retained and with modern cleaning and restoration techniques, returned to its original beauty.  Creative ways of adding support to the windows were developed that are all but invisible to the public.
Where damage to the window rules out even restoration, such as at the window of a church destroyed in a fire, and the design of the original window was not of special importance, the opportunity for a new contemporary stained-glass window emerges.  In this example, the stained glass design partnership created a design that is new and exciting and incorporates some of the best features of the original window.
There is something very special about the way light flows through a stained-glass window, whether that window is mediaeval, Victorian, contemporary, expensive or utilitarian.  We see stained-glass effect double glazed window panes and coloured glass panels made by pouring on coloured resins and stick-on lead strips.  Modern safety regulations don't look kindly on traditional stained-glass windows and modern toughened and laminated glass panels cannot be cut into intricate shapes.  But whatever age the window is or whatever technique was used to create it, take a second look!




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