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  1. #11

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    Quote Originally Posted by edp View Post
    Also consider glass shelves with piles of books on, and very old, heavy mirrors in big telescopes. Shelves don't bend, and very heavy mirrors don't deform.
    They certainly do.
    So do the panes in your windows when it's blowing outside.
    Glass is flexible.

  2. #12
    Vaughn's Avatar
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    I have always liked the idea of glass slowly flowing, but alas, it does not. Finding out it did not flow was sort of like finding out my parents were "Santa"...disappointing while oddly satisfying. Old glass is wavy and varies in thickness because that was the way it was made in the good old days. If one end was thicker than the other, it made sense to put the thick end on the bottom. This was the case in a building I use to live in that was built on the west coast in 1885. I would think the best glass was reserved for photographic plates!

    If you put a nail into a tree 5 feet above the ground, twenty years later it will still be 5 feet above the ground. I worked with a fellow I could not convince that this was true. His proof -- he had seen trees with fences nailed to them -- and the fences were a foot or two above the ground. The idea that the ground may have erroded down over the last 50 years did not enter his head because the "reason" was obvious -- the fence was lifted up by the tree as it grew.

    Vaughn

    PS -- When the wind (60+MPH) would hit the old building I mentioned above -- the glass would deform a tremendous amount. One pane had a crack and a gust blew that window out like a shotgun (it was very thin on top!)
    At least with LF landscape, a bad day of photography can still be a good day of exercise.

  3. #13
    Marco B's Avatar
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    Quote Originally Posted by holmburgers View Post
    The classic example given is "look at old window panes, they're thicker at the bottom!" But I don't know; it's possible that the manufacturing techniques of the day were just that imperfect, or perhaps the thickness at the bottom was for better weight-bearing.
    Techniques certainly weren't perfect and old window pane glass not perfectly flat, this is something now even considered of "monumental" value here in the Netherlands, meaning any old building with monumental value and such old pane glass, needs to have it maintained during restorations. It may not be replaced by modern glass, or they may even go as far as buying specially made "old type" window glass. I think there is / was still one factory in Eastern Europe using these outdated techniques and capable of delivering such glass new.

    Quote Originally Posted by Q.G. View Post
    They certainly do.
    So do the panes in your windows when it's blowing outside.
    Glass is flexible.
    Quite a number if not all modern telescopes even use this flexibility to allow for active correction of the atmospheres turbulence using actuators on the bottom side of the glass for a better view on the universe.
    My website

    "The nineteenth century began by believing that what was reasonable was true, and it wound up by believing that what it saw a photograph of, was true." - William M. Ivins Jr.

    "I don't know, maybe we should disinvent color, and we could just shoot Black & White." - David Burnett in 1978

    "Analog is chemistry + physics, digital is physics + math, which ones did you like most?"

  4. #14
    Marco B's Avatar
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    This page seems to answer your questions: Is glass liquid or solid?

    Quote from that page:
    "It is sometimes said that glass in very old churches is thicker at the bottom than at the top because glass is a liquid, and so over several centuries it has flowed towards the bottom. This is not true. In Mediaeval times panes of glass were often made by the Crown glass process. A lump of molten glass was rolled, blown, expanded, flattened and finally spun into a disc before being cut into panes. The sheets were thicker towards the edge of the disc and were usually installed with the heavier side at the bottom. Other techniques of forming glass panes have been used but it is only the relatively recent float glass processes which have produced good quality flat sheets of glass."
    Last edited by Marco B; 10-18-2010 at 01:32 PM. Click to view previous post history.
    My website

    "The nineteenth century began by believing that what was reasonable was true, and it wound up by believing that what it saw a photograph of, was true." - William M. Ivins Jr.

    "I don't know, maybe we should disinvent color, and we could just shoot Black & White." - David Burnett in 1978

    "Analog is chemistry + physics, digital is physics + math, which ones did you like most?"

  5. #15
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    Take a trip to Corning, NY and visit the glassworks museum if you ever get a chance. It's worth the trip.

    Per my experience there, I learned that old time glassmakers used to put a blob of molten glass on the end of a pole and spin it so that centrifugal force caused it to flow into a disk shape. Then, while it was still hot, it was laid flat on a table and polished till it was smooth. That is why you see ripples in glass windows in old churches and historical buildings. Later on, it was made by pressing or rolling hot glass. The technique produced a better product but it was still not perfect.

    It wasn't until "float glass" was invented where a thin sheet of molten glass was poured on top of a pool of molten metal that the smooth glass we use in windows, today, was possible.

    I have heard that glass is amorphous and flows over time, too but I don't think the time period in which that occurs... if it does, indeed, occur... would be observable in a human life time. It is my guess (only a guess) that this effect would take thousands of years to be observable by the best scientific means we have available, today. But that means that we would have to have a tested a sample for control purposes more than a thousand years ago. Back then, we didn't have the technology for that.

    So, to find out if glass really does flow, we would have to take very precise scientific measurements then store that glass under scientifically controlled conditions for a couple-few thousand years and take the same measurements again. It would be unlikely that the equipment, the sample and the data would all be preserved for the future.

    That all assumes that anybody in the future would really care to repeat the experiment.

    Like I said, glass MIGHT flow but it won't be in my lifetime that we ever find out the truth, if ever.
    Randy S.

    In girum imus nocte et consumimur igni.

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    http://www.flickr.com/photos/randystankey/

  6. #16
    Sirius Glass's Avatar
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    Quote Originally Posted by edp View Post
    No it's not, and no it won't.
    If you take a thin glass rod and lean it against a wall over a long period of time [read: years] the rod will bend. The same thing will happen with a thin sheet of glass. This can be a problem when stain glass pieces, not in a window, is stored almost vertically will over a few years bend. This has happened to me, so every few months, I lean the glass in the other direction.

    However, a glass lens mounted in a lens barrel will not flow because it is constrained and it is thicker.
    Warning!! Handling a Hasselblad can be harmful to your financial well being!

    Nothing beats a great piece of glass!

    I leave the digital work for the urologists and proctologists.

  7. #17
    Vaughn's Avatar
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    I believe a definition for something to be "liquid", its molecules must be able to move pass each other -- this does not seem to be the case with glass rods, etc, bending. Still intersting stuff!

    Vaughn
    At least with LF landscape, a bad day of photography can still be a good day of exercise.

  8. #18
    AgX
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    Quote Originally Posted by pgomena View Post
    I rotate my lenses on my lensboards 90 degrees every 3 months just to be sure.

    Peter Gomena
    When the issue of glass being a fluid come up at school, our teacher chemistry told us us that he lays down his glasses at night on his bedside table up side down each night... with a wink.



    For further thinking one might start at this discussion:
    http://www.phys.ncku.edu.tw/mirrors/...ass/glass.html
    Last edited by AgX; 10-19-2010 at 12:02 AM. Click to view previous post history.

  9. #19
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    Quote Originally Posted by Sirius Glass View Post
    If you take a thin glass rod and lean it against a wall over a long period of time [read: years] the rod will bend.
    Seriously? You believe this?
    Stop worrying about grain, resolution, sharpness, and everything else that doesn't have a damn thing to do with substance.

    http://www.flickr.com/kediwah

  10. #20
    Steve Smith's Avatar
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    It's easy to prove or disprove the theory about old glass being thicker at the bottom after many years. Assuming that when it was installed the glass was a good fit in the frame (or stone surround) if it has flowed and thinned out at the top then there will be a gap in the frame at the top tapering down towards the bottom. If it has not flowed it will still be a good fit.


    Steve.
    "People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.

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