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Glass Flowing [Was Opal Discoloration]

    is that glass flows and you don't need heat to start it
flowing. Gravity alone will accomplish that task with only
gravity as a master. Check out old glass with a wavy
appearance. It wasn't made that way originally. It takes many
years (decades in fact) but it does it in the end. 

I do recall reading a report in the New Scientist on a study (an
issue within the last 12 months) on Medieval Stained Glass
Windows. The finding was that the oft quoted phenomenon of old
glass having flowed over time, the result of which was that
their bottom increased thickness, wasn’t quite what it seemed to

They were deliberately made that way.

I can’t recall what tests were used but one of them was computer
analysis of using different configurations of thicknesses to
simulate the mechanical strength and stability of the windows
and, you guessed it, the medieval craftsmen, through trial and
error must have arrived at the most stable and strongest design.
There are an amazing number of seemingly large fragile stained
glass windows which survived WWII bombs while structures around
them collasped from the blasts. One must also realise the fact
that you don’t hear of medieval stained glass windows leaking in
bad weather being a problem and they didn’t use caulking. I may
have stated an error of fact on caulking (fuzzy detail) here
but our modern caulking materials couldn’t quite match their
hands-on methods.

The elegant solutions produced by medieval craftsmen just
through observation, working with their hands and a good feel of
their material, are certainly amazing.

Do read up a recent report in the Scientific American on
medieval Japanese pagodas. These seemingly beautiful and
fragile structures survived even the strong earthquakes that are
endemic in Japan because of an architecture design not suspected
by modern architects - the use of a single central pole (a tall
tree log) to anchor the pagoda so that the rest of the structure
can slide harmlessly around this pole. By the way, no nails
were used to build these pagodas.

“The best way to tell if it is that old is to check both the
side where it is wave and the side where it seems smoother”.

The explanation for this (another book) is they way medieval
craftsmen made plate glass without modern machines. Large glass
cylinders, not spheres, were blown from an elevated platform
with a helper at the floor level to keep the sides of the
cylinder as straight as possible. The ends of the cylinder were
then removed, the cylinder scored lengthwise and annealed so
that the scour line separated and the sides of the cylinder
folded flat on the ground to make plate glass This accounts for
one smooth side and a not so smooth (wavy) side.

Back to the original allegation of old glass flowing slowly
under gravity. I remember coming across a picture of an
excavated tomb from imperial Roman times. This showed a couple
of small blown bottles which collasped upon themselves much like
a plastic bottle would have done under low heat in an oven. I
accepted the explanation given and filed it away in my memory.

I am no expert in any of the above subjects. Its just that
factoids like these stick in my mind. Thought you might find
them interesting.

Kelvin Mok (

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