How Glass Is Made
and Windows Are Crafted

Glass
Glass is a rigid liquid. It is made of sand, lime and soda, all melted together. The colors are created by adding various metallic oxides to the molten glass before it is formed into a sheet. The colors are permanent and do not alter with time. The colored glass is called Stained Glass.

Three Types of Glass:
1. Antique, or hand-made, glass. This type glass is characterized by its production method, not its age. It is produced by blowing, using a medieval technique. This glass is colored, transparent, and has an irregular optical quality, often with bubbles. This technique dates to the 8th or 9th century. Two methods are used in “antique” manufacture. The most common is the muff or cylinder method, where a long tubular bubble is blown, and while still molten, the bottom and top of the tube are cut off. The tube is called the muff. After cooling, the tube is cut lengthwise and the glass is reheated to allow the tube to open up flat. The second method is called the crown method. In this method a round bubble of glass is blown. The bottom of the bubble is cut open, then the blow pipe is spun rapidly, causing the glass to form a flattened, plate-shaped disc, called a table. Crowns are usually less than several feet in diameter or smaller. The center piece (called the bulls-eye) is attached to the blow-pipe and is very thick. The bulls-eye is usually discarded, but sometimes is used in decorative windows.

2. Cathedral Glass. This transparent glass is made by pouring the molten glass onto an iron table and rolling it flat. Some rollers have a design in them which is imparted to the glass, much as cookies are made with a special rolling pin. Machine-rolled glass was first made in the 18th Century, and were rolled on one side only. In the 1870s double-rolled glass was invented in England, so that both sides of the glass were textured. Cathedral glass is less brilliant than antique glasses because of its regular and somewhat dull surface texture.

3. Opalescent Glass. This glass has a milky opacity. Particles are suspended in the glass, rather than fully melted. The particles diffuse light, but do not allow it to pass straight through as in Antique or Cathedral glass. A sheet may have more than one color. Sometimes the sheet is manipulated before it sets to create drapery, mottled or catspaw glass. Opalescent glass was developed for windows in the late 1870s by John LaFarge and Louis Comfort Tiffany.

Paint
A monochromatic paint is used to create details (for instance facial features, folds in clothing, architectural details) and to control light transmission in stained glass since at least the 10th century. Silver stain (transparent yellow) was developed in the 14th century; colored enamels became popular in the 16th century. The “paint” or “glass paint,” as it is known, is made from glass-based powder, mixed in a liquid medium, and applied to the flat glass. Then the glass is fired in a kiln at temperatures between 800° and 1300° F. The painted features melt and fuse to the surface of the glass during firing, becoming permanently affixed. ( Most of us see this process at the grocery store every week on “painted” soda pop bottles, vinegar bottles, etc.)

Cames
After the glass is cut, painted and fired, it is held together with “I”- or “H”-shaped metal strips called cames. The pieces of glass fit between the flanges, or parallel legs, of the cames. The came is fitted around the shape of the glass and joined at the ends by soldering. A stained glass artist often uses the placement and soldering of the cames to complement and enhance the design of the window.

How Stained Glass Windows Are Made
1. A small-scale drawing is made for the architect and the patron’s approval
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2. When the drawing is accepted, a full-size drawing called a cartoon is drawn, usually in charcoal.

3. From the cartoon, two sets of tracings of the leadlines are made, one for a glazing guide and one to cut up as patterns for the individual pieces of glass. (The original cartoon is kept for reference).

4. A special, three-bladed pair of scissors is used which cuts a narrow strip of paper from between the pieces to leave room for the heart of the came.

5. The colored glass is selected, one piece at a time.

6. The pattern pieces are placed on the glass and cut, using a diamond or steel-wheel glass cutter.

7. Rough edges are smoothed with a tool called a grozing plier.
8. The paint is applied to the pieces and fired.

9. When all decorative pieces are completed and fired, the panel is ready to put together.

10. The glazing diagram is placed on the bench. The pieces are assembled on top of the diagram, inside a lath frame, which keeps the pieces from moving around.

11. All the joints are soldered on both sides. Unlike welding, the lead, zinc, brass or copper of the came or foil is not melted – only the solder is melted. It is melted and applied to the metal with a soldering iron. When the upper side is finished the lath is carefully removed and the piece turned over to solder the opposite side.

12. The piece is carefully cleaned, then waterproofed by forcing a putty made of boiled linseed oil, whiting, red or white lead, kerosene and lampblack beneath the flanges of the came.

A single window is usually made in several sections for ease in manufacture, handling and installation. Support bars are applied both outside and inside the window.

References

“How Stained Glass Windows Are Made” (Part One, October 2006) By Julie L. Sloan; Featured in: “Connick Windows,” The Charles J. Connick Stained Glass Foundation, Ltd.

“How Stained-Glass Windows Are Made” (Part Two, February 2007) by Julie L. Sloan; Featured in “Connick Windows,” The Charles J. Connick Stained Glass Foundation, Ltd.

Julie L. Sloan is a stained-glass consultant in North Adams, MA. She is the author of Conservation of Stained Glass in America and many articles about stained glass history and conservation.