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[Killietalk] Old Glass

Part of the myth of flowing glass comes from the old glass still in the
windows of old buildings.   Often it appears to have flowed because of the
way it diffracts light so that it has a wavy appearance. Sometimes it does
appear thicker at the bottom.

A friend of mine is a serious history buff and a fountain of 18th and 19th
century trivia. I'll take a chance and repeat his explanation because it
does seem to make some degree of sense.

Before the mid to late 19th century glass was hand blown into cylindrical
tubes similar to those elongated balloons circus clowns use to make animals
out of.  The ends were cut off and then they were slit down lengthwise the
glass was then opened to form a flat pane. Naturally, like a bubble blown in
chewing gum one end of the original tube was slightly thicker than the end
most distant from the blower. The process was naturally accomplished with
the glass heated until soft. The finished product was a glass tile.  Now I
am going to take a leap of faith here and assume that glass installers would
have installed such tiles in their window frames with the thick side down.
If you had to choose a convention, it was as good as any other and it
probably made more structural common sense to have the base thicker than the
top.  It would have also made all of the panes diffract light in a similar

As best as I recall near the end of the 19th century a process was developed
to inexpensively flow form large flat panes of glass. You might note that
homes built from this period onwards usually had single window panes rather
than the tiled variety. Hand blown windows are pretty uncommon these days.
Most have long since been replaced due to wood rot. The process with which
they were made has for the most part also been forgotten. When modern people
come across the old hand blown windows with their vertical streaks and
varying thickness usually occurring at the bottom, they make the assumption
that the glass is flowing. In fact the hand blown glass does have this
appearance as at one time it had flowed into its current form. So much for
glass mythology.

Regarding cutting or drilling old vs. new glass: Pretty much all of the
glass we come across in fish tanks whether they are metal frame or all glass
is modern glass that was made by 'modern processes' What we forget is how
much abuse a fish tank or pane of glass takes during its life time. Every
time the glass is struck, banged on or exposed to unusually high pressures
due to water stress or incorrect storage, micro fractures and tiny scratches
occur in the glass. As we all know glass is stronger than it appears and
usually survives most abuse. I have an ancient 55 Gal fish tank that has
countless scratches and I have seen the 3/8" front pane struck with
significant force on several occasions during moving etc. It holds water
fine and looks good when filled with water. When it is dry lots of scratches
are visible. The glass even has a slight bow from decades of faithful
service. When you try to cut old glass you run your score mark across the
previously stressed areas and scratch marks and hope that the ensuing break
is clean and in the intended location or direction. I would hazard to guess
that if someone were to try to recut any of the panes of my old 55 gal tank
they would be in for a very rude awakening.

Having replaced a few car windshields in my time I have observed the effects
of hidden stress cracks. I have gently removed a few perfect looking
windshields that have split without cause minutes after they were removed
while resting comfortably. I have also seen one used windshield of the same
variety dropped twice onto pavement before it was reinstalled into the car
where it still resides years later. I know that windshields are tempered
glass which is way more funky than regular glass, but I believe the concept
of the hidden flaw remains the same.

When it comes to old glass, I usually have a local glass shop do the
cutting. They have a very sharp and expensive cutting tool that makes an
unusually deep score mark. So far their track record has been excellent.
With old used glass 1/4" or thicker my track record has been OK, but I have
had  a few panes crack haphazardly even with good scores. Who knows who
dropped what on it while it was laying in the basement or wherever I found



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