Glass

Glass is a hard, usually transparent material made mostly from sand. A popular claim holds that glass is really a very slow-moving liquid, and that this is why old church windows are thicker at the bottom: the glass has been running down for centuries. This is not true.^[1] Glass is a solid, of an unusual kind called an amorphous solid, in which the atoms are jumbled up as in a liquid but held in place.

The old windows

Old window panes were made by spinning molten glass into a large disc or by other methods that left them uneven in thickness. When glaziers fitted the panes, they usually put the thicker edge at the bottom, which is sensible, but not always: some old windows have their thick edge at the top or the side.^[2] Glass that had really flowed would not have been so considerate.

Calculations suggest that at room temperature, medieval glass would take far longer than the age of the universe to sag noticeably. It is, therefore, one of the few things in a church that is not getting older.

Making it

Glass is made by melting sand with soda and lime at very high temperatures and letting it cool without forming crystals. Natural glass also forms when lightning strikes sand, producing hollow, branching tubes called fulgurites, and when volcanic lava cools very fast, making obsidian. Nature makes glass mostly by accident and never in a shape that fits a window.

See also

• Window – where it is usually found
• Empty glass – another sort of glass
• Sand – what it used to be

References

1. Pane, P. (1998). Not Running Anywhere. Window Press.
2. Glazier, G. (2007). “Which Way Up?” Old Buildings Quarterly 1: 1.
3. Fulgurite, F. (2012). Lightning, Sand and Accidents. Strike Books.