Glass is not a naturally occurring substance. It is formed when quantities of silica, sand, soda ash, and lime are heated in a furnace to a molten state at a temperature in excess of 2000°F. Colors are produced by adding metal oxides to the molten batch. Cobalt and chromium produce blue, add manganese to cobalt blue for purple, add copper for greens and blacks, selenium and cadmium for red and yellow, and gold oxides for gold-pinks. Sheets of stained glass are then produced using one of three methods: mouth-blown antique, machine rolled, or hand cast.
Some of the most beautiful sheets of stained glass are produced by using the traditional technique of mouth blowing. Teams of glassblowers pick molten glass up out of the furnace on the end of a blow pipe. A cylindrical shape is blown and then rotated in a mold to even the surface and create striations unique to antique glass. The ends are cut off and the cylinder is cut lengthwise and allowed to flatten and form a sheet of glass. The sheet is then gradually cooled in a large temperature-controlled oven, called a lehr. This process, called annealing, allows the sheets of glass to cool at a controlled rate to relieve unwanted stress within the glass. The cutability of a glass is usually determined in the annealing stage. Rich, translucent hues are characteristic of glass made using this early method of glassmaking.
Machine-rolled glass is known for its consistency in color and thickness and is generally easier to cut. Molten glass is continuously fed through sets of metal rollers and onto a slow-moving conveyer-type line. As the glass passes through the roller, patterns and textures can be embossed on the surface before the ribbon of glass reaches the lehr.
Hand-cast glass is produced one sheet at a time. Using a large ladle, molten glass is poured onto a metal table and rolled by hand to the desired thickness. Interesting sheets of glass can be produced by mixing in swirls of different colors, thin glass rods, and shards of colored glass before the sheet of glass is transferred to the lehr and allowed to gradually cool.
