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Thread: Melting Glass...

  1. #1

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    Melting Glass...

    I live in New England. I've had the privilage of living in two 300+ year old homes. In my opinion one of the greatest charms of an old home, one that cannot be replicated, is its windows. They're always warped in the most interesting ways.

    Assuming the disfigurment is indeed a product of time, and not of the poor production standards in the 18th century (I could be wrong), I'm fairly sure the reason windows warp is because glass is in fact a supercooled liquid, or some sort of amorphous solid. I'm a little patchy on the chemistry but you get the idea. Under the pull of gravity and over a long period of time, the molecules in a pane of glass move perceptibly.

    If glass changes shape in a window, wouldn't it also change shape in a lens?

    I ask this question because I wonder if the old lenses so desired by modern
    wet-plate and alternative process photographers have changed their properties over time. Obviously many photographers enjoy vintage lenses, and use them often. But Eventually won't all lenses become merely nostalgic, and have no real value to a working photographer? Won't the characteristics of a certain lens from a certain period change over time?

    -Alex

  2. #2

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    Re: Melting Glass...

    While I had not thought about it before, of course you are right. You can measure a thickness change from top to bottom in old windows. Since lenses depend on close tolerances, they should also be affected through time. Maybe they should be rotated in storage.:-)

  3. #3

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    Re: Melting Glass...

    On second thought this would probably only be problem if a lens were in the same position for a long, long time, since through active use it's orientation would be constantly changed. And even if it were in storage, like you said, 100 years of melt could be corrected by turning a lens upside down for another 100.

  4. #4

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    Re: Melting Glass...

    Alex,

    It takes an extreme amount of heat to reshape glass, the production standards in the 18th century, although were good for their time, we still very poor in comparison to what we have now a days, baring some massive fire over the last 300 years, the windows would not have changed structure over that amount of time, glass is pretty well inert once it cools back into a solid, one of the most devastating things that happen to glass over the years, it the drying out of the oils and moisture that is inherently in the glass when it is formed, hence it becomes brittle and subject to breakage. Another thing, is glass is actually quite a hard substance..on the Mohs scale it is a little over 5 on the hardness scale, Diamond being the hardest, but it is in line with many minerals that are used for gems in jewelry..the melting point of common glass is approximately 1723C or over 3000 degrees F depending on the chemical make up of the particular type of glass heated...this can be varied by mixing different types of minerals to create the particular type of composition that you want.

    I would suspect your waves and bubbles in your windows is just due to the wide variances in the productions methods of that particular time period.

    Dave Parker
    Satin Snow Ground Glass

  5. #5

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    Re: Melting Glass...

    There is an urban legend that windows that have been around for hundreds of years are thicker at the bottom than at the top because of the flow of the glass. It turns out that this is not true. The glass will move very little because of gravity. The explanation is more mundane. The old techniques of making glass were not very precise and the plates they made were not uniform in thickness. It turns out that as they assembled the windows that the craftsmen put the thick side down for extra strength. The heaviest part of the glass would be supported by the frame and not the thin part of the glass itself. I think our lenses are safe in any orientation.
    Cheers,
    Dave B.

  6. #6
    Greg Lockrey's Avatar
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    Re: Melting Glass...

    At any rate, you aren't going to live long enough to care.
    Greg Lockrey

    Wealth is a state of mind.
    Money is just a tool.
    Happiness is pedaling +25mph on a smooth road.



  7. #7

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    Re: Melting Glass...

    What you were seeing in the old houses was drawn glass. The variations in thickness were a result of the manufacturing process. Here is what wikipedia has to say about it.
    Sheet glass (sometimes called window glass or drawn glass) was made by dipping a leader into a vat of molten glass then pulling that leader straight up while a film of glass hardened just out of the vat. This film or ribbon was pulled up continuously held by tractors on both edges while it cooled. After 12 meters or so it was cut off the vertical ribbon and tipped down to be further cut. This glass is clear but has thickness variations due to small temperature changes just out of the vat as it was hardening. These variations cause lines of slight distortions. You may still see this glass in older houses. Float glass replaced this process.
    I think our lenses are safe.

    Cheers,
    Mark

  8. #8
    All metric sizes to 24x30 Ole Tjugen's Avatar
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    Re: Melting Glass...

    Drawn glass was a transition between the current float glass and the older blown glass.

    The old way of making window panes was to blow a big "bottle" which was then cut open while the glass was still soft. This gave at best reasonably flat glass, and a bottle end which was called "crown". That's the origin of the "crown glass" designation, too.

    Glass, once it's cooled below red hot, doesn't "flow". Nor does it "outgas" or "dry". But it does have a tendency to devitrify (crystallise) with age. That makes it liable to splinter into thousands of tiny sharp needles when it breaks, instead of just breaking up into shards.

    The "flowing windows" is an urban myth.

  9. #9
    Jim Jones's Avatar
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    Re: Melting Glass...

    The distortion of large glass lenses due to gravity can indeed be a problem. Traditionally it limited the diameter of telescope lenses to about 40 inches diameter, as in the Yerkes telescope, finished by Alvin Clark and son in 1895. While mirror lenses for telescopes made of more stable glass or other material are now much bigger, the Yerkes telescope is still the largest refractor.

  10. #10
    All metric sizes to 24x30 Ole Tjugen's Avatar
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    Re: Melting Glass...

    Yes, but that's just because glass is flexible: It sags under gravitu, but "bounces back" when the orientation is changed. It doesn't flow, which is a form of inelastic deformation.

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