How Are Stained Glass Windows Made?
In a church you often see windows decorated with colorful pictures and designs. The windows are made of many pieces of colored (stained) glass. The windows let light in and they decorate the church. In making a stained-glass window, colored glass is cut into pieces to match an original pattern.
The jigsaw puzzle of glass pieces is usually held together with strips of lead. Fine details, such as eyes and noses, are painted onto the glass with special enamel paints. Many stained-glass windows in churches show scenes from stories in the Bible. There are six basic steps involved in the production of stained glass windows.
Cartoon: An artist makes a sketch of the overall composition of a window. Then full-sized drawings for the whole window or for different sections (panels) of the window are made. These full-sized drawings are called cartoons. Generally, the shapes of the individual glass pieces, the details to be painted, and the colors of the glass are indicated on the cartoon. In the early Middle Ages these were drawn on whitewashed boards.
Cutting: Different colors of glass are chosen for separate parts of the design, and the outline of each piece is then painted on the surface with white lime wash. Pieces are cut into rough shapes using a dividing iron, the heated tip of which is applied to the surface of the glass, causing it to break.
The pieces are further reduced to the desired size with an iron bar with a slot at each end that is used to chip away at the edges of the glass until the exact shape is created.
Painting: After the glass pieces are cut and shaped, they are painted with a pigment formed by mixing iron oxide and ground copper with powdered glass.
Vinegar or wine is added to help apply the pigment to the glass. This vitreous paint ranges from brown to gray to black. Once painted, the separate pieces of glass are placed in a wood-fired oven called a kiln. The heat of the kiln causes the vitreous paint to fuse permanently to the surface of the glass.
What is vitreous paint? Glass painters use a special paint made of glass particles suspended in a liquid binder—vitreous means “consisting of glass”—to paint the side of glass that would face the interior of the building. During firing, the glass particles in the paint melt and merge with the glass surface to create a range of brown and black tones.
Lead: Pieces of glass are held together with narrow strips of lead to form a panel. These strips are referred to as “lead came.” Lead is used because it is flexible and provides the adaptability needed for fitting around the various shapes of the glass pieces.
Glazing: “Glazing” is the term for assembling a panel of stained glass that can then be set into a window. After separate pieces of glass are painted and fired, they are placed in position on the cartoon and joined together with lead came to form a panel. In the Middle Ages, a combination knife and hammer was used for this process. The knife edge was used to cut the pieces of came and the hammer end was used to secure nails to the work board to hold the edges in place during assembly.
The sections of came are then joined together with solder, an alloy of lead and tin that melts easily at a low temperature and sets quickly.
Cementing: The panel is then cemented to help secure the glass within the leads and to waterproof the window. Semi-liquid cement is applied with a brush and then is covered with a layer of chalk or sawdust to absorb excess liquid. The medieval recipe for this cement is not known, though the main ingredients were probably crushed chalk and linseed oil. The panel is then scrubbed down with a dry brush until the cement only remains under the lead.