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.

Eyeglasses

Back here we spoke about the manufacture of glass, but when and where did glass come to be used for enhancing and correcting vision?

Ptolemy (c.100-170CE) wrote in his Optics about glass shaped so that it enlarged an image. His ideas were expanded on by the "Father of Modern Optics," Hasan Ibn al-Haytham, whose Book of Optics (c.1021) spread through Europe at (coincidentally?) about the same time that we get records of "reading stones."

A reading stone is a glob of transparent substance, flattened on the bottom but with a curved top, making it in essence a convex lens. It was (still IS, actually) meant to be placed on and slid along a line of text, causing the letters below to expand and be more easily read. I call it a "substance" because it wasn't necessarily glass. Early ones were ground from quartz, and they can be had today in glass or plastic.

Several lens-shaped items of quartz have been found in Viking graves on Gotland in Sweden, some mounted in silver. They date to the 11th or 12th centuries. They might be reading stones, although several are not shaped to be good at focusing.

But when were lenses put into a frame for enhancing vision? In 1301, in Venice, there were guild regulations governing the sale of eyeglasses. The Dominican friar Giordano da Pisa delivered a sermon in February 1306 in which he said:

It is not yet twenty years since there was found the art of making eyeglasses, which make for good vision... And it is so short a time that this new art, never before extant, was discovered. ... I saw the one who first discovered and practiced it, and I talked to him.

 So far as we can trace, then, eyeglasses were first made in Italy around 1290.

The Name Glastonbury

Glastonbury is a small town in Somerset, England, that has been inhabited since Neolithic times. A recent post discussed the discovery of early medieval glass-making furnaces at the site of the now ruined Glastonbury Abbey. This prompted some to point out to me that Glastonbury "must have been known" for glass production—it is "right there in the name." Let us address that.

Remains of the nave of Glastonbury Abbey

In his book The Flowering Hawthorn, Hugh Ross Williamson tells the story of St. Collen. Collen was a 7th century hermit who took up residence at what is now Glastonbury. Williamson relates how the saint encountered Gwyn, King of the Fairies, in a magical glass castle on Glastonbury Tor. Rejecting the fairies' offer of food and drink, he cast holy water on them, causing all to vanish. Numerous versions of this story exist, but Williamson's 1962 book is the only version that introduces glass as the material involved. As a source for the site's name, this is not reliable.

William of Malmesbury refers to its earliest name as Ynys Witrin, which some translate as "Isle of Glass" based on the fact that English "vitreous" comes from Latin "vitrum" meaning "glass." "Isle of Glass" would more properly be Ynys Gwydr, however. "Witrin" is a puzzle, but no serious scholar thinks it is from Latin for "glass." (The "Isle" makes sense because, in earlier times, higher sea levels turned some hilly areas into islands.) Malmesbury does suggest that the place was named for someone named Glast. Since the first recording of the name is Glestingaburg, the place of Glestinga. No one knows exactly to what Glestinga refers.

But it's not about glass.

Glass and Recycling

In 1977-79, a shipwreck off the southern coast of Turkey was investigated. It was determined to have sunk about 1025. The ship's hold contained three tons of broken glass and chunks of glass. (This amount of glass would make about 12,000 Coke bottles.)

Jesse Tree at York Minster (1150-70); some of the oldest
stained glass of the Middle Ages

Glass requires extreme heat applied to a mixture of silica, soda, and lime. Silica was derived from sand; soda happens to reduce the temperature at which glass can form; lime makes the glass "chemically stable." Impurities—by accident or design—added color to the glass.

We know little about how glass-making came about; records do not explain the technique, but anecdotal mentions tell us a little. Pliny's Natural History tells us that the best sand for glass comes from the mouth of the Belus River near Akko, Israel. The shells in that sand provide the lime needed to make the glass stable. William of Tyre (1130-1186) and Jacques de Vitry (1170-1240) around 1200 both mention the same source. It is thought that the ancients did not understand why that source was the best, chemically, that they did not understand that the shells contained necessary lime.

In England, the Glastonbury Abbey Project has discovered evidence of a stone structure on the site of Glastonbury Abbey dating to c.700. They have also found evidence of early Roman and Saxon activity from before the Abbey's founding. Remains of glass-making furnaces have been uncovered, showing archaeologists that Saxons were recycling Roman glass brought from Europe. The Glastonbury site was possibly the first Saxon glass-making factory in England.

The remarkable thing about glass is that it is recyclable, like metals. Although it took a lot of energy to process, every bit of broken material (unlike wood or stone or pottery) could be re-smelted and re-cast. It is possible that the glass in windows like the example above was even older than our estimates, having been re-used from earlier glass objects that broke or had outlived their usefulness.