One look at this month’s photo reveals the subject – stained glass. Like many of our topics, this one has a history. Unlike many, this history is impressive. Some things come and go with time, but the appeal of stained glass has remained. Many craftsmen today are dedicating their labors to restoring old stained-glass windows so that generations to come can enjoy their beauty.

Glassmaking dates back 5,000 years with stained glass being used in European Christian churches by the third or fourth century A.D. One of the oldest known examples of a stained-glass window, dating to 686 A.D., was unearthed at St. Paul’s monastery in Jarrow, England. The 12th and 13th centuries were known as the Golden Age of Stained Glass. Unfortunately during the Renaissance period, painted glass replaced stained glass, and by the 18th century the early methods of manufacture were all but lost.

The second half of the 19th century, in both Europe and America, brought about a renaissance in the art of stained-glass making. William Jay Bolton made his first window for a church in New York in 1843, although no other American professionally practiced the art until Louis Comfort Tiffany and John La Farge began working with stained glass in the late 19th century. Thanks in part to Tiffany’s influence, only 10% of stained glass produced today is used in churches. The rest is used in residential and industrial architecture as well as ornamental applications.

Like blacksmithing, stained-glass techniques have changed little since the Middle Ages. The color of the glass is a result of adding metals or metal oxides to the raw materials. Copper oxide can produce ruby, blue or green colors in glass under different conditions. Cobalt is usually added to produce most shades of blue. Iron oxide and chromium are added to produce green shades. “Flashed glass” is made by dipping a ball of molten white glass into a bath of colored glass, which results in a less-intense color.

Large manufacturers of stained glass mix the raw materials – including fluxes and stabilizing agents – and heat the mix at 2500°F (1370°C). For cathedral glass, the molten material is ladled into a machine that rolls the glass into 1/8-inch-thick sheets. These sheets are cooled in a furnace called an annealing lehr. A typical factory will produce eight to 10 different color runs each day.

As in bygone days, a stained-glass window is made by first creating the design. A traditional narrative window has panels that tell a story. Others are simply created by use of a replicated design. Once the pattern is determined, the true art of the stained-glass process begins. The design even incorporates the exact position of the lead, which holds the glass in place, as a visual effect. The glass can be cut to basic sizes, but exact size and shapes of individual panes are created by grozing the edges to nibble off small pieces.

Once the fine details are created, the pieces are assembled into H-sectioned lead cames. A strip of narrow lead is fitted around the exposed edges. The joints are then all soldered together, and the window is waterproofed by forcing a soft oily cement, or mastic, between the glass and the cames. This also keeps the small glass panes from rattling. The process for making an entire stained-glass window can take two to three months.

The structure, pieces and parts complicate the maintenance of large stained-glass windows. The need for restoration companies has grown in Europe as well as the U.S. The window you see in these photos was thoroughly restored last year. Originally built in the early 1930s, time, gravity and the elements had taken their toll on this beautiful window gracing Harbison Chapel on the campus of Grove City College in western Pennsylvania.

The life span of windows like this is typically 80 to 100 years. The restoration project involved removal and documentation of each piece of glass followed by reassembly and re-leading. The rest of the process is as previously described. This project was scheduled to take eight months, but the outcome is worth the wait and will be enjoyed by students and faculty for the rest of this century.

Stained glass is just one more way thermal processing is involved in making our world more beautiful. IH

Red PresentThis article is featured in Industrial Heating's Top-10 Heat-Treated Holiday Gifts.  Click on the present to find more gift ideas and to learn how these gifts benefit from thermal processing.