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Connecting the Pieces of Medieval Stained-Glass Windows

Although stained-glass windows were first seen in ancient Rome, the art of using stained glass for windows reached its height during the Middle Ages, where this art form was used for the windows of the magnificent cathedrals of Europe. Much of what we know of the process of making medieval stained-glass windows comes from a text written by a German monk called Theophilus in the thirteenth century who carefully studied the techniques of the various craftsmen who played a role in creating these masterpieces of stained glass.

The glass used for the stained-glass windows of the Middle Ages was composed of wood ash, called potash, and sand. The mixture was first melted into a liquid under extreme heat. The glassmaker then gathered the molten glass onto the end of a blowpipe. By blowing through the pipe and constantly rotating it, the molten mixture was formed into a cylindrical shape. The cylinder was then scored lengthwise and cooled. The cylinder was placed on a stone and reheated in an oven, causing it to flatten into a sheet. The sheet was then removed from the oven and cooled slowly.

Early stained-glass windows were composed of pot-colored glass – glass that was colored while still in its molten state. The addition of various metal oxides – iron for red, copper for green and cobalt for blue, created the palette of colors that provided the necessary variety for executing designs. Pot-colored glass was of uniform color.

By the dawn of the Renaissance, however, several new methods of coloring glass had come into use. Flashed glass was created by “flashing” a very thin layer of colored glass to a layer of glass of another color while in its molten form. Various gradations of color could be created in flashed glass by either etching with acid or abrading the surface to remove various amounts of the thinner layer and exposing the color under it. Flashed glass was also useful in creating uniform colors that were otherwise too intense if made from a single sheet of colored glass; this was often the case with ruby-colored glass. Silver stain (also called yellow stain), composed of chloride of silver or silver nitrate that was fused to the glass by heating at low temperature, was most often used for crowns and halos since it produced shades ranging from yellow to tawny brown. Colored enamel paint was also applied to clear stained glass and left to dry, allowing the artist to create various color intensities needed for shading and highlighting the images of the design.

To create a particular window design in stained glass, the design was usually drawn onto a large wooden board or table but sometimes cloth, parchment or paper was used. Then the pieces of sheet glass were arranged over the design and cut to the desired shape. To cut the glass, a thin line of sheep or cow urine was applied to the glass, and a hot iron was placed along the line, causing the glass to crack. Rough edges were then removed with tools resembling pliers. During the Renaissance, however, diamond cutters came into use, enabling the creation of far more complex designs. After cutting, the artist used black or dark brown enamel, composed of ground glass and metallic oxides in a binding medium such as vegetable oil, to enhance the image on the colored glass. The enamel could be applied thickly or as a wash, allowing it to be used for both fine details or shading. The enamel was painted on the glass and then fired, fusing it onto the surface of the glass.

Once the pieces of glass were appropriately colored and detailed, they were jointed together by fitting the edges of each section into pieces of lead called cames that were H-shaped in cross section. The cames were then soldered together, forming one sheet of stained glass. Putty was applied to any gaps between the glass and the cames to make the window waterproof. The entire design was then placed in an iron frame and mounted in the opening in the building wall.

To see the magnificent stained glass windows of Chartres Cathedral in France, please click here. These windows date from 1200 to 1240.

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Diana Blake is a professional artist and art history enthusiast. Her fascination with art history began when she encountered European art firsthand during several trips abroad as a young adult. As she began to compose a portfolio for her own art career, she called upon what she had seen in Europe and extended her knowledge to other styles of art through profuse reading and exhaustive research. As a result, Diana has written more than one hundred articles in which she delves into a variety of art history topics and she has compiled a list of links that she believes to be invaluable for art history enthusiasts. In addition, she also reviews books and movies on the topic of art history and has assembled an extensive list of online stores that sell books, movies and gift items related to art history.
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