Making Stained Glass Windows

Creating sufficiently high temperatures needed to melt silica and fashion glass was difficult, but at some point we discovered that the addition of potash (potassium, from wood ash soaked in water), soda (sodium carbonate, from plant ash), and lead would lower the melting point, making production easier.

Now you have molten glass; how do you shape it?

The oldest stained glass windows were made from "muff" glass: a blob of molten glass, called a "gather," was placed on the end of the blowpipe. Using metal tools and wooden forms soaked in water, the glass was blown into a cylinder. The ends of the cylinder would be cut off, and the cylinder cut open so it could be flattened.

"Crown" glass (see illustration) started similarly with a gather of glass on the blowpipe; after introducing a bubble of air, the pipe would be spun back and forth quickly, causing the glass to spread out into a thin disk. This could also be accomplished by placing the molten glass on a contraption like a potter's wheel. The circle of glass could be cut to fit into square windows, or used round. Crown glass windows are recognizable because of the concentric circles formed during the spinning. They usually also have a "bull's-eye": the thicker blob at the center of the spun circle.

You may hear of "cathedral glass." This is rolled glass: molten glass poured onto a metal table and rolled thin with a metal roller. It was sometimes even put through a pair of rollers. This technique has nothing to do with medieval cathedrals, having been developed in the mid-1830s.

Coloring the glass was done by adding copper oxide (green or bluish green), cobalt (deep blue), or gold (wine red and violet). (Modern glass produces red using copper.)

For stained glass windows to survive wind pressure, it needed to be at least 3mm (1/8") thick. The production of red required a concentration of added material such that a 3mm window had so much coloring that the red was very dark; it could be mistaken for black without a very strong light source. This prompted the development of "flashed glass": adhering a thin pane of red to a thicker pane of clear.

Before we leave the subject of old windows, I want to tackle a common talking point that I have heard many times: "Glass is a liquid that flows very slowly; the proof is that medieval windows are thicker on the bottom than the top!" Sure, maybe. But let's remember Occam's Razor: common sense would tell the architect that a pane of glass that is not of even thickness—not an unexpected outcome considering the imprecise methods of manufacture—would be more stable if you put the heavier end as the base.

Being able to color panes and pieces of glass allowed creators of windows to offer elaborate pictures, such as stories from the Bible or history. Tomorrow I'll share a story of Theophilus, who was a common theme for church windows in the 13th century.