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Glass melting point

  1. Aug 22, 2006 #1
    Why Do Glasses Doesnt Have A Melting Point?
    And Are Glasses Liquid?thanks
     
  2. jcsd
  3. Aug 22, 2006 #2
    Glasses are amorphous solids with melting points around 1000°C-2000°C
     
  4. Aug 22, 2006 #3
    For The Geeks

    Scientists do insist that glass is a liquid (see addendum), because it has no melting point
    IVE READ THAT IN A ARTICLE IN THE INTERNET
    IS THAT TRUE? GLASSES DOESNT HAVE A MELTING POINT AND THEY ARE LIQUID?
     
  5. Aug 22, 2006 #4

    LURCH

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    The "glass is a liquid" story has been around the web a long time, but I've never heard that bit about glass having no melting point before.
    1) Glass is a solid. Not crystaline, but an amorphous solid.

    2) Glass melts.
     
  6. Aug 22, 2006 #5

    FredGarvin

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  7. Aug 22, 2006 #6
    Ive also heard of glass being referred to as a super cooled liquid, but that sounds wrong to me, because it implies that the substance is still in the liquid phase, just at a temp below its freezing temp.
     
  8. Aug 22, 2006 #7

    ZapperZ

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    Well, strangely enough, this is not that simple, as is the case usually on here when what appears to be a simple question can some time results in a rather complicated response.

    Glass, for all "practical" purposes, can be considered to be a "solid", if we consider that under a normal time span, it maintains its shape. However, the "glassy phase" is actually quite complex and is an active area of study in condensed matter/solid state physics. If you study it like that, you can consider ordinary glass as being a liquid but with an extremely high degree of viscosity. That is why old stain glass windows in medieval churches appear to sag right now.

    Zz.
     
  9. Aug 22, 2006 #8

    nazzard

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  10. Aug 22, 2006 #9

    Astronuc

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    I remember this discussion before on PF.

    But here is an interesting perspective - http://dwb.unl.edu/Teacher/NSF/C01/C01Links/www.ualberta.ca/~bderksen/florin.html

    At the bottom is the ASTM test for defining the distinction between solid and liquid - "A material that flows a total of 2 in (50 mm) or less within 3 min is considered a solid. Otherwise it is considered a liquid." So with this one can define a "melting point", or the temperature at transition between solid and liquid.

    However, one can see little physical difference between glass 1 degree above melting and one degree below. There is no clear distinction between solid and liquid.

    I wonder if a fusion (melting) temperature based on the temperature at which powdered glass fuses would make sense?
     
  11. Aug 22, 2006 #10
    Until about 4 years ago, I was living in Brisbane. Wish I had of known about this then; I could have gone to see it.

    Notice the glass beaker appears to be sagging to the right?
     
  12. Aug 22, 2006 #11
    glass is classified as an amorphous solid.

    to oversimplify:
    solid - has definite shape and definite volume
    liquid - no definite shape, but definite volume (ok, "slightly compressable")
    gas - no definite shape, no definite volume
    and yes there are more such as plasma, but these are the basic three for the sake of saving time

    obviously glass can't be deemed a liquid as much as can a solid. it's therefore a subcatagory of solid, an amorphous solid. amorphous solids have an organized crystal lattice but it isn't organized in the long range...so it's "mostly solid"...what is meant by that is...for example if you have ever viewed a window maybe 100 years or so old, you may notice that the bottom of the window is thicker than the top, that's because it's has had the long amount of time required for gravity to pull down the molecules, it took a long time because it's mostly solid.

    i did my best to explain it as simply as possible from my own basic knowledge, but i'm no scientist. you can look into it further for an exact explanation.
     
  13. Aug 22, 2006 #12

    Physics Monkey

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    Actually, I believe this myth that cathedral glasses show signs of flow is actually not true. There was a nice article in the American Journal of Physics some years ago (Am. J. Phys. 66, 392-396, 1998) that basically showed the time scales were wrong. Also, I think the lack of flow in many Roman glasses is taken as further evidence that medieval glasses could not possibly have flowed. As I understand it, the irregularities in thickness are most reasonably attributed to the glass making process used.
     
  14. Aug 22, 2006 #13

    ZapperZ

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    Actually, I think there was another AJP (or was it Eur. J. Phys.) paper that did a study on this and show that there is actually a "sagging" effect on very heavy glass from medieval churches. I'm not talking about "thickness" irregularities (this is another separate unresolved issue because it can't be just a matter of coincidence that ALL the irregular shaped glass somehow are mounted thick side down). I'll see if I can find it tomorrow.

    Zz.
     
  15. Aug 22, 2006 #14

    Physics Monkey

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    Very interesting, please let me know what you find. As for the thickness irregularities, I would agree that it would be somewhat unusual for all the windows to be thick side down, but I think some have been found with thick side up.
     
  16. Aug 23, 2006 #15

    LURCH

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    I would also be interested in that paper. Acording to this link;
    http://www.cmog.org/index.asp?pageId=745
    by Robert H. Brill, research scientist at the Corning Museum of glass,
    Unfortunately, the part I put in bold is only a second-hand quote, and not the result of the good doctors own research. Let's look and see if any of us can find a statistical study showing how many windows have been found with the thick side up and how many with it down.

    Some have speculated that window makers in the old days would check to see which side of a piece of glass was thickest, and put that side on the bottom (for reqular house windows, that is).

    ("Off on a tangent" begins here) This makes alot of sense, IMO. Whenever my familly was building a house, we would lay the hardwood flooring so that the "ugly", knotty side was down (as do most builders). I can see archeologists a thousand years from now looking at our "ancient dwellings" and declaring that wood is a liquid because, over a thousand years, the knots sink to the bootom!
     
  17. Aug 23, 2006 #16

    ZapperZ

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    I stand corrected.

    The paper that I had in mind was

    Cinzia Buratti "Analysis of the Thermal Stress and Strain on Arrigo Fiammingo's Artistic Window in the Cathedral of Perugia", Journal of Heat Transfer v.123, p.1173 (2001).

    Here, the bowing or sagging effect was attributed to thermal stresses, not the viscosity of the glass. However, interestingly enough, the assumption made by the AJP paper that you cited (i.e. glass panes at uniform ambient temperature) may not be valid per this paper, since they have shown that there could be a difference of as much as 20 C between different parts of the glass window. So a naive application of the Newton flow method may not be valid.

    Zz.
     
  18. Aug 23, 2006 #17
    If church glass did flow, you would expect to find obsidian in a sagged condition.
     
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