Little has changed since Medieval times and we use essentially the same techniques today.
In the first quarter of the twelfth century, a German monk, ‘Theophilus’, wrote a description of the techniques of making stained glass which we now know had been made like this since the 8th Century. The basic methods have hardly changed. The glass was made by melting sand, potash and soda lime together in clay pots. lt was coloured by the addition of metallic oxides – copper for red, iron for green, cobalt for blue and so on. This is called pot-metal glass. Pot-metal glass, particularly red glass, was often too dark to transmit a lot of light. To overcome this, ‘flashed’ glass was made by dipping a lump of white glass (transparent) on the blowpipe into a pot of red glass and then blowing. As the bubble expanded the red glass on top got thinner and therefore lighter making a glass which could then be removed by grinding with an abrasive wheel; this produced two colours, red and white, on the same piece of glass. The combinations of colours available today have increased to around 30 available to the glass artist.
As paper was scarce in the middle ages and parchment very expensive, the full scale outline of the design for a stained glass window was drawn out on a whitened table top. The designer would indicate the principal outlines of his drawing, the shape and colour of the individual pieces of glass to be used, and the position of the lead strips (calmes) that would eventually hold all the pieces of glass together. The panes of coloured glass were scored with a diamond to give a line to work to and was then cut to shape with a ‘grozing iron’ which was simply a rod of iron bent over at the edge of the glass and nipped of and was then laid on top of the drawing.
Through the glass, details of the drawing – faces, hands, drapery etc. – could be seen and these details were traced with an iron oxide pigment on the surface of the glass. After painting, the pieces were fired in a small wood fired furnace for sufficient time to fuse the paint to the surface of the glass thus becoming part of the glass, and then relaid on the table and assembled by the glazier, using strips of lead H-shaped in section, which allowed the glass to be slotted into the grooves on each side. The lead provided a strong but flexible bond.
The intersections of all the lead strips were then soldered, and an oily chalk (like putty) was rubbed into the overlap of lead to make the glass watertight. The panels were then held in place in the window openings by a grid of iron bars set into the masonry. From the early fourteenth century a further range of colours varying from a pale lemon to a deep orange could be achieved on one piece of glass through the discovery of ‘silver stain’, a silver compound painted on the back of the glass and then fired in a kiln. This is where the name “Stained Glass” came from. Now this term is used describe many glass techniques. By the mid sixteenth century many different coloured vitreous paints were being used. As a result, windows began to be painted like easel pictures on clear glass of regular rectangular shape, with lead calmes no longer an integral part of the design. These methods prevailed from the seventeenth to early nineteenth centuries. However, the earlier techniques were revived in Victorian times.
These days every window starts as a full-size cartoon drawn in the studio on tracing paper, some use computer print outs. The coloured glass is then selected from a sample range of over 1500 types of glass to conform with the designer’s conception and the position and purpose of the window. The glass is cut to size with a glass cutter. Awkward curves can be nipped (‘grozed’) with a pair of grozing pliers or diamond machinery can give greater accuracy.
The design is traditionally applied as a black or brown paint which is a mixture of metal oxides, powdered glass and gum arabic although there are other mediums. The painter mixes it with water on a thick glass tile. Solid lines are painted thickly, carefully tracing the design from the cartoon. Thinner washes are left to dry and then dusted with a badger hair brush to give fine shading effects. Finished pieces are then stored in glass racks to await firing in the kiln. The painted glass is laid on a heat resistant material and fired in a gas or electric kiln at a temperature which fuses the paint to the glass. If silver stains are required these are applied to the back of the glass.
With the recent development of large electric kilns it is now possible to make pieces of painted and enameled glass 2 meters by 1 meter which can then be toughened to conform to safety regulations. You can see examples of this on the website.