Stained Glass Window (12th century)
Chartres Gothic Cathedral.
How Stained Glass is Made
the Glazier's Art
HISTORY OF VISUAL
STAINED GLASS DESIGNERS
STAINED GLASS ARCHITECTURE
STAINED GLASS ICONOGRAPHY
the Glazier's Art
Stained glass is usually designed for a particular setting, with a specific light and an expected audience. Both setting and audience can change radically over time. New buildings may block the original light; corrosion and dirt may obscure the details; vandalism and poor maintenance may cause loss of glass. In addition, stained glass has always been an expensive medium: the materials are costly and the fabrication of a window is time-intensive. It is the intrinsic beauty of the materials and the exceptional skill of its practitioners that have ensured its secure place in the history of art.
Although Theophilus discussed both the
manufacture of sheet glass and its use in the making of windows, stained-glass
artists rarely made their own glass. While obviously connected, these
are two very different and highly specialized disciplines. Some stained-glass
workshops did have glassmaking facilities on site, but glassmakers were
usually located near the raw materials they needed for their work. Many
medieval accounts detail the cost of transporting finished sheets of glass
to workshops. Modern workshops have the same separation.
Glass can be made into sheets in several
ways. Early sheets were cast onto a flat surface, such as sand or wet
wood. With the invention of the blowpipe, in the first century BCE, glass
could be formed much more quickly. Two methods have been used for blowing
sheets: the cylinder/muff and the crown method. In the first,
a bladderlike shape is blown. First one end is opened up and then the
other end is removed, forming a cylinder (or muff). This is then split
down one side and flattened into a sheet in a re-heating process. In the
crown method, a bubble is blown, transferred to a metal rod called a punty
(or pontil), pierced, and spun out, yielding a round sheet with no need
for flattening, but with a pontil mark remaining in the centre. A third,
less commonly used technique called 'Norman slab' was developed in the
19th-century and involves blowing the bubble into a mould to form a hollow
block that is later separated into small sheets. Modern hand blown glass
is referred to as mouth-blown or antique glass.
For rolled sheets of glass, the molten glass passes between rollers set about 3 mm apart, thereby determining the thickness of the glass. Textures can be imprinted on one of the rollers and pressed into the molten glass as it passes through. Tugging or pulling the glass also affects the surface, producing a rippling effect. Machines began to be used in the 19th-century to make both machine-blown and rolled glass in a variety of textures and colours. In some factories the glass is a continuous ribbon from the batch melting in the furnace through the rollers and then onto a long annealing lehr. At the end of the lehr, sufficiently cooled, it is finally cut into sheets.
For unusual or unique effects, hand-rolled
sheets are sometimes preferred. For this, a ladle of one or more colours
is poured onto a steel table, mixed to a desired integration and pushed
under rollers. For drapery glass, the puddle of glass is first rolled
and then manipulated to produce three-dimensional folds. It is finished
through a roller, but, after manipulation to produce folds, is left in
three-dimensional relief. This must be done quickly, while the glass is
Artist's Designs: Sketches and Cartoons
Before paper was readily available, the
full-size drawing was made on a whitewashed table that was used for cutting
and painting the glass, as well as for putting the finished window together.
Remarkably, one such medieval table has survived, if only because it was
later used to make the door of a cabinet. Two fourteenth-century windows
made on it also survive in Gerona Cathedral in Spain. Examination under
ultraviolet light has revealed several layers of drawings on the board,
which contain lead lines, symbols indicating colours and some of the dark
trace lines that were to be painted on the glass. There are also nail
holes from the glazing, or leading-up, of the panels.
In the Middle Ages sheets of glass were first split into smaller pieces using a hot iron. Heat, aided by water or spit, was used to initiate a break and the sheet was split in two. This rough shape was then refined using a grozing iron, which was a metal slot or hook into which the edge of the glass was slipped. By pulling the iron down and away, the glass edge was nibbled into shape. Skilled craftsmen could make difficult and intricate shapes.
At some point in the history of the craft, an observant artisan realized that a deep scratch or score made on the glass surface would give better control of the breaking. One can sometimes see these scratches coming off grozed edges on medieval pieces and even find, on occasion, ungrozed edges from the period. Diamonds set in handles are known to have been used for scoring by the 14th-century and were probably used earlier, although edges were normally still grozed, and diamonds are still used to score glass today, mainly in Europe. The steel wheel cutter was developed in the 1860s. As the wheel is rolled across the glass it focuses a tremendous amount of pressure just at the point where the wheel meets the glass, creating a surface fissure. The scored glass is then snapped apart, using the hands or a pair of pliers as a fulcrum.
Early scores were somewhat haphazard. To
be a success, a score must travel from one edge of the glass to the other
in an unbroken line. If the wheel has a nick in it, there will be skips
in the score and the break will go awry at the skip. Glass is an amorphous
material; it has no grain to guide a break. The score is the starting
point, but the artisan must decide where else to place scores in order
to break away extraneous glass without putting too much stress on the
pieces that are to be kept, for the glass will break at the point of least
resistance. Some shapes, such as inside curves, are difficult to cut and
require some skill to make. Different types of glass cut differently.
Even with relatively simple shapes, one always needs extra glass that
will end up as cut-off, even more so when shapes are difficult.
The German term "Glasmalerei", or glass painting, most aptly captures what enabled stained glass to move beyond its obvious decorative and practical functions to develop into a powerfully expressive medium. Painting on glass gave artists the opportunity to construct large-scale imagery using light, colour and line. With stained glass, unlike other graphic media, the artist must be sensitive to translucency as well as line and form. The modulation of light animates the image.
Paint is used both to control light and
to provide details. It can be applied in washes, mats and dark trace lines
to both the front and the back of the
The vitreous paint for stained glass is
composed of a low-firing, essentially clear glass-flux and opaque metallic
oxides, generally iron or copper. It comes in powdered form, allowing
the artist to mix it with water, vinegar or oil and layer it, depending
on the desired effect. A binder, such as gum Arabic with water or Venetian
turpentine with oil, is used to temporarily hold the paint to the glass.
The paint must be built up in a series of thin coats, either using different
binders or firing between applications. A variety of brushes, sticks and
other tools are used to apply and remove the paint before it adheres
The Use of Lead Cames
The alloy used is important. Pure lead,
while technically possible today, is not a good choice, since it is more
susceptible to fatiguing and attack by acids than an alloy containing
trace amounts of antimony, tin and silver or copper. Old leads and cut-offs
from new cames are recycled. Cames may also be made from other metals,
such as brass and zinc, although their lack of malleability makes them
much less useful.
In an architectural setting, windows are subjected to extremes of weather - to wind pushing in and pulling out, to rain, sleet, hail, snow and so forth. The malleability that makes lead ideal for the freedom the artist needs also makes windows vulnerable to gravity and wind, so support bars must be anchored into the frame and attached to the stained glass. Panels are often set individually on T-bars set into the frame in order to support the weight of glass and lead.
Preservation, Cleaning and Repair of Stained Glass
Glass is it self vulnerable to decay. Seemingly
innocuous water is the first to attack. Its presence, in liquid or vapour
form, promotes the leaching of the alkalis from the glass, weakening the
network and building up corrosion products on the glass surface, although
rainwater can actually be beneficial to a window, as it can wash away
leached alkalis before they become concentrated on the glass surface.
The way each glass ages depends on its
Protective glazing, an external layer on the exterior of a window, is now a conservation possibility and, in Europe, has been one of the most effective ways to protect medieval glass. Such treatment is valid only if the space between the stained-glass window and its protective glazing is vented to allow a moving column of air. Otherwise, the window traps moisture, which actually accelerates deterioration of the historic glass. Nineteenth- and twentieth-century glass is less vulnerable than medieval glass, owing to its different composition, and rarely needs such protection. In modern buildings with climate control, an architectural choice may be to station the artistic glass on the interior, almost as a screen in front of the functioning window. This solution has also been used for conservation of historic windows, thus removing them from the stress of functioning as a weight-bearing element.
Just as glass paint is a film on the surface of the glass. so too are dirt and corrosion, which obscure the painted details and cut down significantly on the light passing through the glass. Since stained glass relies on transmitted light, these foreign materials must often be removed so that light may shine through. However, such cleaning is not always easy, nor is it simple to do under even the best of circumstances. It must be approached with great care. Glass paint can be fragile and hard to distinguish from the film of dirt. Opalescent windows are often several layers thick. Soot, dirt and old putty trapped between the layers can be very difficult to reach and they considerably diminish a window's effect.
Traditionally, there were only two reliable
waterproof methods of repairing breaks or losses in architectural glass:
the replacement of the broken glass with a new piece - sometimes a good
match, sometimes not - or the insertion of repair leads. The former means
that original material is completely removed. The latter is unsightly,
compromises the panel's legibility and usually results in partial removal
of original material, as the break line is grozed to make room for the
heart of the repair lead.
Copper foil is another alternative,
although it reads as an opaque black line, albeit a narrow one. It consists
of a thin copper strip with adhesive on one side, which is wrapped around
and pressed onto the edge of the glass. The pieces are then soldered together,
so the glass being repaired must be able to withstand heat. It has the
advantage of being a mechanical, reversible repair.
Deterioration of Lead Cames
Lead cames perform the very important function
of supporting the glass, but in addition to that they provide a rhythm
and scale to the composition. Deciding where the lead lines should be
placed is an important step in the realization of any sketch. When releading,
it is important to establish the location of the original lead lines so
that the conservator or artisan does not end up reinterpreting the composition
according to his or her own preferences, since the choices were made by
the work's original creators. Put another way, who are we to argue with
the choices made by the work's original creators?
There is no absolute dated time failure
for leads. Decisions about releading need to be made case by case. If
releading is warranted, it is important to duplicate the original leads
in width and height. It is not uncommon to find medieval pieces releaded
in the nineteenth century with wide flat leads, which cover important
painted details. Leading is not just a mechanical operation; it requires
skill and attention. A plodding or sloppy releading destroys the energy
of the original line.
References: we gratefully acknowledge the use of materials from The History of Stained Glass (Thames & Hudson, 2003) written by Virginia Chieffo Raguin. This magnificent work contains a wealth of wonderful pictures of stained glass windows together with a masterly explanation of the evolution of stained glass art: essential reading for any students of medieval architecture and Christian art.
For more information about stained glass materials, methods & crafts, see: Art Encyclopedia.