Materials And Techniques
Contrary to popular belief, the glassmaker and the stained-glass artist could seldom have been the same person even in the earliest times; in fact, the two arts were rarely practiced at the same location. The glassmaking works was most readily set up at the edge of a forest, where the tremendous quantities of firewood, ash, and sand that were necessary for the making of glass could be found, whereas the stained-glass-window-making studios were normally set up near the major building sites. The stained-glass artist, thus, has always been dependent upon the glassmaker for his primary material. Coloured with metallic oxides while in a molten state—copper for ruby, cobalt for blue, manganese for purple, antimony for yellow, iron for green—sheets of medieval glass were produced by blowing a bubble of glass, manipulating it into a tubular shape, cutting away the ends to form a cylinder, slitting the cylinder lengthwise down one side, and flattening it into a sheet while the glass was still red hot and in a pliable state. It was then allowed to cool very slowly in a kiln so that it would be properly annealed and not too difficult to cut up into whatever shapes might be required for the design. Since these sheets of glass, with the exception of a type known as flashed glass, were intrinsically coloured with one basic colour throughout, changes from one colour to another in the design of a window could be effected only by introducing separate pieces of glass in each of the requisite colours.
Whether by accident or by deliberate intent, the glass made in the 12th and 13th centuries had almost the ideal combination of crudity and refinement for stained glass. The sheets, 10 by 12 inches (25 by 30 centimetres) in size, were both flat enough and thin enough to be cut very accurately into the necessary shapes, yet still variable enough in thickness (from less than 1/8inch [3 millimetres] to as much as 5/16 inch [8 millimetres]) to have rich transitions in the depth of their colours. With the progress of glass technology in the Middle Ages and Renaissancecame the ability to produce larger, thinner, and flatter sheets of glass in a considerably larger range of colours than had been possible in the 13th century. At each distinguishable stage in this development, however, the glass became less visually interesting as an aesthetic element in its own right. The Gothic Revivalists later recognized this effect, and in the mid-19th century they initiated a return to the earlier methods of producing glass. They developed the so-called “antique” glass, which is remarkably similar in colour, texture, and shading to the glass that was used in the 12th- and 13th-century windows. “Antique” glass remains the basic material used in stained-glass windows to this day.