How the Windows Were Made

Stained glass windows are composed of pieces of colored glass held together by a framework of lead and solder. Some of their functions are identical to those of ordinary windows. They admit light, protect the building interior from the elements, and help regulate temperature. But unlike ordinary windows, they also have aesthetic and intellectual functions. They create atmosphere, grant beauty and decoration, provide instruction and illustration, and commemorate and memorialize. They are a unique medium because they use transmitted light for these purposes. (Lee et al., 1982)

Glass is the most obvious component of the many substances that compose stained glass windows. Glass is a non-crystalline, amorphous material composed of silicon and oxygen, two of the most abundant elements of the earth's crust. It is usually formed by heating a mixture of silica-rich sand, potash, and lime or lead oxide to the melting point and then rapidly quenching it. The sand is the source of the silica and oxygen that constitute most of the atoms composing the glass. Potash or some other alkali material is used as a flux to remove impurities from the melted sand. The lime or lead oxide regulates the melting point temperature of the mixture. Rapid quenching causes the melt to solidify before crystals can grow, thus giving glass its clarity. The growth of crystals in the liquid would interfere with or possibly preclude the transmission of light.

The most common method of coloring glass is to add various metal oxides to the melt. Glass colored this way is known as "pot-metal" glass. This name comes from the "pot" which holds the melt. Examples of the elements and compounds that give pot metal glass particular colors include cobalt producing rich blues; manganese producing purple; chromium and iron oxide producing greens; and copper oxide producing ruby, blue, or green. A golden color may result from the addition of cadmium sulfide or titanium. The introduction of selenium can be used for colors ranging from canary yellow to brilliant red. Color differences from the same chemical are produced by creating either an oxidizing or a reducing environment in the pot.

Once created and impregnated with color, glass is made into flat sheets by one of several methods. "Antique" or "muff" glass is produced by blowing the glass into a cylindrical shape, removing the top and bottom of the cylinder, cutting the cylinder down one side, then re-heating it and unrolling it into a sheet. Because this kind of glass is blown by hand, it has a variable thickness that results in shading of color. In addition, it is characterized by elongated bubbles caused by the spinning of the molten glass during the blowing process.

"Norman slabs" are produced when glass is blown into a rectangular form and then cut at 90 degree intersections. This technique produces glass sheets that may vary by as much as one-quarter inch in thickness from edge to center. This variation in thickness also causes variation in shading because more light is extinguished in thick sections and less in thin sections; thus, the thicker areas appear darker in color.

"Crown" glass is made by spinning molten glass into a sheet. It is characterized by a "bulls eye" pattern in the center of the sheet where the rod it was spun around was attached. Also, it has an arched pattern of curves within it as a result of the spinning used in its manufacture.

"Flashed" glass results from the laminating of two colors of glass together in layers. One layer can then be locally removed by dissolving it in acid or abrading it with grit, thus revealing the color underneath.

All of these techniques create imperfections both on and within the glass that result in the glass acquiring special aesthetic qualities. The windows incorporating glass made in these ways begin their existence conceptually as small colored drawings called "sketches." These sketches, in turn, serve as guides for drawing the "cartoon," which is a finished charcoal depiction made to the full size of the window. It is an exact rendering of the window in black and white. Several copies or "patterns" are made from the cartoon by tracing it through numerous layers of paper interleaved with carbon paper. One copy is cut into pieces to serve as templates for glass cutting. The other copies are used to lay out the individual glass pieces and serve as a guide for leading the glass together ("glazing").

The glass may be stained or painted prior to glazing. "Tracing" is a process that involves applying glass paint by tracing along the line of the cartoon. The lines are usually opaque. "Matting " occurs when paint is applied to glass in order to create shading. The function of matting is to vary the amount of light coming through the glass. Matting is used to provide emphasis and counterpoint and to control tone and color. Both of these paints must be fused onto the surface of the glass by heating them in a kiln to 1250 degrees F. "Staining" with silver nitrate or silver chloride is a treatment where, unlike paint, the silver compounds penetrate the surface of the glass when fired in a kiln. The presence of these salts within the glass causes the glass to absorb yellow light. Depending on the amount of stain applied, the temperature, and the length of the firing process, staining can produce colors varying from intense orange to a subtle canary yellow. "Enameling" involves the application of enamel to glass. Enamels are transparent colors that are usually applied to clear glass.

The colored glass pieces are assembled in the process of glazing by using strips of lead came ("cames" are strips of lead with grooved edges into which glass is inserted and held). The glass fits into a grooved channel in the lead. The pieces of lead are held together with solder made of tin and lead. Once a window is leaded it must be "cemented." Cement is a waterproof putty that is brushed into the void between the glass and lead came. Any excess is then cleared from the surface of the glass. Finally, the window is "installed" when it is fixed into its architectural opening.

Go to: next section / previous section /main index