Fiber Optic Association
- Tech Topics
Ever hear the
rumor that glass
is a "supercooled liquid" that flows over time?I've
heard it for years. It has been said that the uneven glass in
old windows "flowed" over centuries to look uneven.
I've even heard it about fiber that has only been made twenty
surfaced on a mail list
I monitor recently, so I tried to track it down. Below are articles
that debunk this rumor! At the flow rate of glass (well it does
flow, but very slowly), the glass in old windows would need longer
than the age of the universe to flow that much! (And I suspect
the "Big Bang" would break quite a few of those windows!)
News magazine. This is a great, readable article that tells the story
- And this
article was posted on the SCTE Mailing List in June, 1999:
Economist - Science And Technology
- EVEN so
rational a subject as science has its myths. And like the more
traditional sort, scientific myths are often used to illustrate a
general truth. One myth which weaves its way from textbook to textbook
is that the reason the glass in old windows (especially medieval
windows) is thicker at the bottom than at the top is because
glass, despite its apparent solidity, is actually a liquid. Given
time, therefore, it will flow from the top of a window pane to the
bottom, accumulating there as a perceptible bulge.
- In the
abstruse world of physics, glass is indeed classified as a liquid
(albeit a supercooled and therefore not very mobile one). The myth of
the ancient window pane has therefore been thought of as a good way to
show students that the everyday meaning of the word "liquid" is not
completely subverted by thinking of glass as a liquid too.
the myth to the test, several researchers have recently made a
stab at calculating how fast glass actually flows. Unfortunately for
textbooks, the latest estimate, made by Yvonne Stokes of the
of Adelaide, and which will be reported in the Proceedings of the
Society next month, is that it would take over 10 million years for a
pane to flow perceptibly.
Stokes's calculations, which use the equations of fluid dynamics,
also show that a thickening at the bottom of a pane of glass would not
result in a thinning at its top, as might naively be expected. Rather,
the flow of glass would cause a reduction in the overall height of the
pane. Even a 5% thickening at the bottom of a window a metre high
result in a shrinkage of the window's height by about a centimetre. In
other words, if the original myth were correct, old windows would have
gaping holes in them.
- That does,
however, leave the question of what the real reason is that
the glass in old windows tends to be thicker at the bottom than at the
top. Perhaps it is just that medieval glaziers preferred it that way.
Of Older Glass
- In October,
2006, we (JH) visited the Henry Wadsworth Longfellow House in
Cambridge, MA, which was built in the 1740s. Here the glass shows
typical circular unevenness from having been blown as a globe,
flattened to a disk and cut to shape. The glass is even obviously
thicker and more uneven at the bottom!
- Note the
second pane down on the left with the circular variations in thickness
and the uneven reflections from other panes.
- Here the
panes are quite uneven, as shown by the distortions.
- Now look
how the bottom of the pane has lots of distortion, but the rest is
fairly clear. Can you see how the rumors of glass flowing to the
of the pane got started?
The Fiber Optic Association,
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