A popular world-wide myth about inorganic glasses based on silica frameworks, such as glass windows in cathedrals, is that over hundreds of years noticeable flow is able to occur under the influence of gravity, as a result of which cathedral glass windows are thicker at the bottom than the top.

There are a number of useful references in the scientific literature where this is discussed at some length. In a letter entitled ‘Fact or fiction? Can cold glass flow under its own weight and what happens to stained glass windows?’ sent to the Editor of Glass Technology, 37, 143 (1996) R.G. Newton notes that at room temperature inorganic glass can flow if a large enough compressive stress is applied, so that for example hardness indents can be made in inorganic glass. However, irreversible flow can only occur if stresses of at least one tenth of the theoretical breaking strength of glass are applied. Such stresses are not achievable in inorganic glass because of the presence of surface defects, which will cause the glass to shatter well before such a stress can be applied.

Further discussions of this myth, with calculations to show conclusively why cathedral glass will not flow under its own weight over hundreds of years, are to be found in:

• F.W. Preston, ‘The post-instantaneous creep of long-loaded glass specimens’, Glass Technology, 14, 20-30 (1973)
• E.D. Zanotto, ‘Do cathedral glasses flow?’, American Journal of Physics, 66, 392-395 (1998)
• E.D. Zanotto and P.K. Gupta, ‘Do cathedral glasses flow?-Additional remarks’, American Journal of Physics, 67, 260-262 (1999)

A telling point made by Zanotto and Gupta from their calculations is that the timescale over which flow might be seen at room temperature (termed the relaxation time) of 10²³ years is well beyond the age of the Universe (10¹⁰ years). The explanation for why cathedral glasses are thicker at their bottom than at their tops lies in the manufacture of ancient glass. Such glass was made by blowing the glass into large cylinders that were split and flattened manually. This manufacturing process produced glass that was inherently not of a uniform thickness, unlike the glass made by the float glass process that we have today.

Instinct would make fitters of window panes in cathedrals put thicker sides of glass at the bottom, little realising that their actions would give rise to the urban myth that would sweep the globe hundreds of years later.

** Image from http://commons.wikimedia.org/wiki/File:Peterborough_Cathedral_glass_05.JPG

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