On 5/04/2016 10:17 PM, Karl Auer wrote: > On Tue, 2016-04-05 at 22:10 +1000, Paul Brooks wrote: >> this 'old glass flows/deforms and gets thicker at the >> bottom' is an urban myth that has been debunked. The glass doesn't >> flow, the thickness gradient is a byproduct of the pouring process, >> and panes have been found that were installed >> upside-down, with the thicker part still at the top. > I did not know that. Thank you. > > I did know about the cylinder and disk production methods, but never > put two and two together properly. Amazing the things you learn from Dr Karl on JJJ podcasts :-) Turns out its human nature, the most comfortable way to carry something is with the heavier section to the bottom, as its more stable - even when the weight imbalance is very subtle. Same if it needs to be stored by leaning it up semi-vertically against a support. So when the stained-glass (and normal glass) window panes were being carried to the cathedral, in the vast majority of cases the thicker edge with the excess weight would have been carried and installed as the lower edge - leading to the 'discovery' a century or two later 'hey look - most of these glass sections are thicker at the bottom than the top - they must have flowed/slumped over the years' - a classic case of 'correlation isn't causation' if ever there was one.
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