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.
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.
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.
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.
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
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!