Thursday, March 24, 2016

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.

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