We’ve all heard the story that glass is a supercooled liquid that flows slowly. The “evidence” often cited is that centuries-old cathedral window panes are thicker at the bottom, so obviously the glass must have flowed downwards under gravity. But the real explanation is that panes of glass weren’t actually made uniformly thick in those bygone days, and was almost always mounted thick side down. Wikipedia says so, and there are plenty of other pages on physics blogs and sites giving the same debunking of this pervasive urban myth.
It’s a myth, right? Right?
Maurizio Vannoni, Andrea Sordini, and Giuseppe Molesini of the CNR-Istituto Nazionale do Ottica in Florence have published an article in Optics Express: Long-term deformation at room temperature observed in fused silica. You need to pay to see the full article, but in summary, they have interferometrically measured the flatness of fused silica optical flats over a period of 10 years, as a routine part of their optical calibration work. (Fused silica is essentially an ultra-pure optical glass.)
The flats are circular, and stored in a clean room under controlled temperature (19-21°C) and humidity (40-50%). They are stored horizontally, supported by circular mounts. And, over 10 years, they have sagged measurably in the middle. The sagging is of the order of a nanometre, is greatest in the centre, and is least at the three points of the mount where the silica is clamped with elastomer pads, which supports the hypothesis that the sagging has been caused by gravity.
Now, a nanometre is not much. You’d have to let the silica sag for roughly 10 million years before the deformation was the order of a millimetre and therefore easily detectable with an unaided eye. It’s certainly not enough to account for the medieval windows, by a factor of hundreds of thousands or more. And the researchers are slightly hesitant to declare that this is a case of the silica “flowing” under gravity – they propose other possible explanations, but admit the gravity flow one seems most likely. They conclude that the calculated viscosity of the silica – assuming it has flowed under gravity – is some 23 orders of magnitude less viscous (i.e. more flowy) than previously quoted and assumed by the optical community.
This is not even enough of an effect to materially affect, say, the working life of a space telescope mirror or something like that. For all non-ultra-ultra-ultra-high-precision-optical purposes, it’s still safe to say glass doesn’t flow. But, as is often the case in the real world, things are never perfect!