Old Glass

In old castles and houses in Europe can be found windows with old glass in which many of the panes are slightly thicker on the bottom than at the top. What are some possible reasons for this result, and what is the most likely reason?

Answer

Many people have suggested that the glass experiences some flow downward in response to the gravitational pull of Earth. Contrary to popular conjecture, there is no evidence that any of this old glass could flow enough during the time interval of centuries to create the difference from top to bottom.

Another factor against the flow hypothesis is the actual profile, which is essentially a linear relationship of thickness to vertical distance. As a simple model, assume that the properties of the glass are identical at each vertical position along the pane. If a fixed amount of glass material flows from position 10, say, the same amount would replace this amount from position 11, slightly higher up the glass. The major changes over a long time interval would be a thick buildup at the very bottom and a depletion at the very top, with practically no thickness change between, in contrast to the linear dependence of glass thickness to height.

In the old days, window glass production made panes that varied slightly in thickness from one end to the other because the flat support surface had a slight tilt. The installers simply put the thicker end on the bottom. Quality control must have been marginal in some areas of the world, because we have seen some large differences in glass thickness between the two ends!

Glass is normally elastic at temperatures below about 1000 K, and glass may break but never deform permanently because the solid is crystalline. Delicate telescope and camera lenses would reveal such creep by changing their optical characteristics in obvious ways.