Glass Flow in Old Windows
Old Building Window Glass
In many old buildings, window panes are noticeably thicker at the bottom than at the top. It was once commonly taught that glass behaves like a very slow‑flowing liquid, and that over centuries it gradually “sags” downward, causing the lower edge to become thicker.
Current scientific understanding shows that no such flow occurs within the time span during which these panes have been installed. The explanation lies instead in historical manufacturing methods: early sheet glass was uneven in thickness, and when a pane was cut to size, the thicker edge was deliberately placed at the bottom to create a visually stable and balanced appearance.
In older buildings it was therefore natural to position the thicker part of the pane at the lower edge. Because glass was an expensive material, ordinary households often received panes of lower and more irregular quality. As a result, the thickest portion of a pane could sometimes end up in the middle or elsewhere, not necessarily at the bottom.
During my examination of old churches in the summer of 2007, I found that the thickest area of the glass was not always located at the bottom but could appear in various parts of the pane. This supports the view that the variations are due to manufacturing irregularities rather than long‑term flow.
The Romans began using glass in windows in the 4th century AD. Even after nearly two thousand years, there is still no evidence that window glass has flowed perceptibly; the observed thickness differences are far more convincingly explained by the production techniques of the time.
1.6.2026*9:50 (969 - 281)
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