Glass deformation

Discussion in 'Large Format Cameras and Accessories' started by michael_r, Oct 18, 2010.

  1. michael_r

    michael_r Subscriber

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    Since glass is a fluid and gravity will eventually deform it, how long does any kind of lens have before it deforms enough to actually have a measurable (or later visible) impact on image quality? Is this something that might impact a 100 year old lens, or are we talking more like thousands of years or more? Just curious, not that this is a pracitical problem.

    Thanks
     
  2. edp

    edp Member

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    No it's not, and no it won't.
     
  3. holmburgers

    holmburgers Member

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    Interesting question. I have always heard that glass is an amorphous solid as well, but I'm not sure if that's true or kind of an old wive's tale.

    Judging by all the old lenses that are still in use, I'd say it's 100% not a concern.

    Here, these are almost a thousand years old and still doing fine.... http://en.wikipedia.org/wiki/Visby_lenses (granted, they're made of quartz, but cool nonetheless)
     
  4. michael_r

    michael_r Subscriber

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    Is that correct? I don't think so.
     
  5. michael_r

    michael_r Subscriber

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    edp's comment about glass not being a fluid. My understanding is glass is fluid, and will "flow" over time.
     
  6. holmburgers

    holmburgers Member

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    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.

    Why do you ask, by the way?
     
  7. Marek Warunkiewicz

    Marek Warunkiewicz Member

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    The glass flowing thing is another urban legend. If it was true, you would be seeing nothing but blobs when you look at ancient Roman (and older) glass. 5,000 years old and still holding up.
     
  8. edp

    edp Member

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    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. Roman glass is as beautiful today as it was 2000 years ago, and hasn't deformed into blobs. The glass transition temperature of ordinary window glass, or of the types used in lenses, is many hundreds of degrees above room temperature.

    It's an unusual amorphous solid sometimes described as a 'supercooled liquid', and that's what causes this widespread misconception.

    Have a look at the Pitch Drop experiment, running (pun intended) at the University of Queensland since 1927. http://www.physics.uq.edu.au/physics_museum/pitchdrop.shtml
     
  9. pgomena

    pgomena Member

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    I rotate my lenses on my lensboards 90 degrees every 3 months just to be sure.:wink:

    Peter Gomena
     
  10. michael_r

    michael_r Subscriber

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    Rotating the lens - funny stuff.

    I'll have to do some more reading I guess, but I don't think Roman glass is a very good example anyway. I'm not talking about such extreme deformity, but rather something merely measurable as opposed to visible. So if such deformity did occur, it surely wouldn't be visible in Roman or more ancient glass. I was just curious as to whether it would affect an optical ie: precision lens enough over a reasonable amount of time.

    It's not that I'm worried about it in my own work. I was just trying to picture classified ads hundreds of years from now for circa-2000 Schneider lenses for example, and whether gravitational deformity could be another condition factor. I mean if anyone would be concerned with such a thing, it would surely be photographers.
     
  11. Q.G.

    Q.G. Inactive

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    They certainly do.
    So do the panes in your windows when it's blowing outside.
    Glass is flexible.
     
  12. Vaughn

    Vaughn Subscriber

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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!)
     
  13. Marco B

    Marco B Member

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    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.

    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.
     
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  15. Marco B

    Marco B Member

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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 a moderator: Oct 18, 2010
  16. Worker 11811

    Worker 11811 Member

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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.
     
  17. Sirius Glass

    Sirius Glass Subscriber

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    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.
     
  18. Vaughn

    Vaughn Subscriber

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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
     
  19. AgX

    AgX Member

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    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/physicsfaq/General/Glass/glass.html
     
    Last edited by a moderator: Oct 19, 2010
  20. clayne

    clayne Member

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    Seriously? You believe this?
     
  21. Steve Smith

    Steve Smith Member

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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.
     
  22. Marco B

    Marco B Member

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    It is not such a strange assumption, at least not from a theoretical view point. Again, read the following link, it answers almost all questions, and there is specific details about "viscosity", "plasticity" and "elasticity" and an explanation of the differences of the phenomena, even in what appear "solid" substances:

    LINK: Is glass liquid or solid?

    Also interesting in the linked article, the described differences between crystalline solids, fluids and glasses, and the not always so clear boundaries and transitions between these "states"...

    Marco
     
  23. Q.G.

    Q.G. Inactive

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    Assuming that only the glass ages, not the frame.
     
  24. Marek Warunkiewicz

    Marek Warunkiewicz Member

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    The comment below is what I am commenting on.

    I, funnily enough, worked as a quality control inspector at Pilkington Glass back in 1977-78. Learned a lot about glass and how it is made and how it ages. One could ONLY do that kind of a measurement with modern glass which is made in a way that ensures a very close thickness over the whole large piece. Glass today is made by floating molten silica on a huge bath of molten metal. The speed at which they feed the molten glass onto the molten metal determines the thickness. The tolerances we worked with back then were in the order of a thousands of an inch. Glass in the "olden" days was made by rolling molten glass through a device similar to old wringer washers and they were never very accurate. We had some glass in our windows at home from the 1870s and you could see this lovely dappled light coming through the windows. We had to replace them with modern double-glazed windows due to our climate and heating costs and now its just a solid tone. The variations in old glass were fairly large compared to today's glass, so it would be difficult to ensure a benchmark across a whole plate - the variations would be such that one could not say with any accuracy that the glass is X thick. It's funny that this "myth" still persists. I was at Corning a few years ago for a few days on a course and one of the people there asked the teacher this question. The teacher laughed and said this was one of the more common questions but that there is no truth to it.
     
  25. pgomena

    pgomena Member

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    My grandmother's old house had very old rolled glass in the windows. It was beautiful to see fall color on the leaves outside through them. I sometimes wish I could have had a couple of the panes just so I could see the world as she did.

    Peter Gomena
     
  26. Sirius Glass

    Sirius Glass Subscriber

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    I have had it happen. Can you prove it did not?