21 December 2011

Regarding the "fluidity" of window glass

It's not a fluid.  Fuck Yeah Fluid Dynamics debunks the myth that because glass is an amorphous solid, over a prolonged period of time it can behave as a slowly-moving fluid.
The observation that old panes of glass tend to be thicker at the bottom is usually used as evidence that glass flows over the centuries, but this assumes that the glass was flat to begin with. However, glassblowers at the time usually made panes by spinning molten glass to create a round, mostly even flat, which was then cut to fit. Although spinning made the glass mostly flat, the edges of the disc tended to be thinner. When installed, the glass was typically placed thicker side down for stability purposes. One researcher even calculated the time period necessary for glass to flow and deform at ordinary temperatures as 10^32 years—longer than the age of the universe. 
The most extensive discussion I've found of this subject is at this link.

Photo - 12th century window at Chateau de Beynac.  Credit Vincent van der Pas.


  1. I learned this from Wikipedia's List of Common Misconceptions :)

  2. Adding to Unknown's comment the following would be great required Wikipedia reading as part of a public education curriculum:

    List of common misconceptions

    List of fallacies

    List of cognitive biases

    Probably a good amount of TYWK material in that mix (cognitive biases are utterly fascinating!)

  3. Thanks for the links Constantine. I'll definitely include this reading in preparation for my (home-schooled) daughter's logic and problem solving class next semester :)

  4. I agree, Z. Cognitive biases are fascinating. I listed some a year ago -


    - but should do individual ones in detail (when I get time....)

  5. ... and you've already begun by seizing that opportunity to demonstrate cryptomnesia with "my" idea to list lists...

    Well played.

  6. My mother worked in stained glass and was convinced it was fluid. I'll have to send this along to her for her reference.

  7. I often wondered about this one. I mean, arguing along these lines would have you believe anything made in a similar fashion (ie. bricks, ceramics) would also be fluid. Sure, the process doesn't actually MELT the material like it does with glass; but there is some kind of physical/chemical change taking place, along much the same line. I haven't seen any old brick buildings slowly oozing down any streets! ;-)

  8. You haven't been around long enough...

  9. I've heard this from a chemistry prof and I'm convinced by his arguments. The debunking only says that it would take longer than people think.

    The transparency (of glass) or a fluid is a result of lack any kind of microscopic plain surfaces which result from symmetrical arrangements of atoms (crystallization).

    The difference to other materials is that glass REALLY freezes solid, while most materials also transform: partially crystallize/ symmetrically arrange.


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