Switch to English Language Passer en langue française Omschakelen naar Nederlandse Taal Wechseln Sie zu deutschen Sprache Passa alla lingua italiana
Members: 70,549   Posts: 1,544,652   Online: 734
      
Page 3 of 6 FirstFirst 123456 LastLast
Results 21 to 30 of 52
  1. #21
    Marco B's Avatar
    Join Date
    Jan 2005
    Location
    The Netherlands
    Shooter
    Multi Format
    Posts
    2,983
    Images
    169
    Quote Originally Posted by clayne View Post
    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?
    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
    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?"

  2. #22

    Join Date
    Jul 2007
    Location
    Netherlands
    Shooter
    Medium Format
    Posts
    5,686
    Quote Originally Posted by Steve Smith View Post
    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.
    Assuming that only the glass ages, not the frame.

  3. #23
    Marek Warunkiewicz's Avatar
    Join Date
    Sep 2008
    Location
    Toronto, Canada
    Shooter
    Medium Format
    Posts
    142
    Images
    11
    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.
    Marek Warunkiewicz

  4. #24

    Join Date
    Jun 2003
    Location
    Portland, Oregon
    Posts
    1,338
    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

  5. #25
    Sirius Glass's Avatar
    Join Date
    Jan 2007
    Location
    Southern California
    Shooter
    Multi Format
    Posts
    13,120
    Quote Originally Posted by clayne View Post
    Seriously? You believe this?
    I have had it happen. Can you prove it did not?
    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.

  6. #26
    clayne's Avatar
    Join Date
    Sep 2008
    Location
    San Francisco, CA | Kuching, MY | Jakarta, ID
    Shooter
    Multi Format
    Posts
    2,838
    Images
    57
    Quote Originally Posted by Sirius Glass View Post
    I have had it happen. Can you prove it did not?
    Strawman.
    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

  7. #27
    greybeard's Avatar
    Join Date
    Jun 2005
    Location
    Northern California
    Shooter
    Large Format
    Posts
    377
    Images
    6
    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.

    This could quite possibly happen, but not exactly because the glass has "flowed". All glasses can absorb water, and even corrode. A thin object leaning and under the influence of gravity would be in compressive stress on the upper side, but tensile on the lower. In theory, this would make the underside more subject to water absorption (and hence, swelling) and thereby create a stress state which would maintain some of the curvature when the gravitational force was removed. As it happens, some of the glasses used in stained glass work barely qualify as members of the same class of materials from which we make window panes, barware and lenses; they are easily corroded and this is something of an issue for restoration work.

    As it happens, a grad-school colleague of mine once made it his business to find out just what viscosity separated "solids" from "liquids" (leaving out issues like glass transition temperature, viscoelasticity, and thixotropy). He found a publication referring to the value 10E14.6 poise (that's ten-to-the-14.6th-power, for those more used to superscripts). This was apparently the smallest value that would be plausibly measureable during the age of the universe, or something like that

  8. #28
    Jim Jones's Avatar
    Join Date
    Jan 2006
    Location
    Rural NW Missouri
    Shooter
    4x5 Format
    Posts
    1,816
    Large thin lenses can deform enough because of gravity to limit their performance. The 40 inch Yerkes refractor is one example. In well over a hundred years no larger refractor has been built. Mirror lenses can be supported by more than their rim, and they can also be corrected for the effect of gravity and temperature.

    Another example of the deformation of glass is evident in the barracks at Thule Air Force Base in Greenland. There can be a very high thermal gradient between the inside and outside of the barracks. As a result, the warm inside expands and the cold outside shrinks. The windows retain some of this inward curvature even when there is no temperature gradient.

  9. #29

    Join Date
    Jul 2010
    Location
    Southern USA
    Shooter
    Multi Format
    Posts
    3,840
    Glass is a supercooled liquid and over time it will crystallize and become very brittle. I have seen this in very old laboratory glass. The glass takes on a slightly milky cast. If you try heat it it will shatter where younger glass will not.

    Theoretically glass will undergo plastic deformation but the rate must measured in thousands of years.

    To see this phenomenon on a more practical time scale consider cesium bromide. It is used for optics in infra-red equipment. While it is a solid it will undergo deformation in a matter of days. If you should leave an optical blank say 1-1/4 inches in diameter by 1/6 inch thick extending over the edge of a table, it will begin to look like a Dali watch in a matter of days.
    A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.

    ~Antoine de Saint-Exupery

  10. #30

    Join Date
    Aug 2005
    Location
    Earth
    Shooter
    Multi Format
    Posts
    1,560
    Quote Originally Posted by Gerald C Koch View Post
    ...consider cesium bromide. It is used for optics in infra-red equipment. While it is a solid it will undergo deformation in a matter of days. If you should leave an optical blank say 1-1/4 inches in diameter by 1/6 inch thick extending over the edge of a table, it will begin to look like a Dali watch in a matter of days.
    Cool. I want one.

Page 3 of 6 FirstFirst 123456 LastLast


 

APUG PARTNERS EQUALLY FUNDING OUR COMMUNITY:



Contact Us  |  Support Us!  |  Advertise  |  Site Terms  |  Archive  —   Search  |  Mobile Device Access  |  RSS  |  Facebook  |  Linkedin