A Denver Family's Adventure Through The Ups And Downs of Life

Wednesday, August 3, 2011

Common Myth Debunked

I bet that you were told, like I was, that old glass runs and is thicker at the bottom of a window than at the top.

What prompted me to look into this?? I was watching Pawn Stars on the History Channel last night and Rick said that this was a myth. Since Rick "the pawn guy" is not the person I go to for educational information, I had to check it out myself.

According to Corning Incorporated, the world leader in specialty glass and ceramics, there are several reasons why the myth doesn't make sense.
  1. Liquids flow because there are no strong forces holding their molecules together. Their molecules can move freely past one another, so that liquids can be poured, splashed around, and spilled. But, unlike the molecules in conventional liquids, the atoms in glasses are all held together tightly by strong chemical bonds. It is as if the glass were one giant molecule. This makes glasses rigid so they cannot flow at room temperatures. Thus, the analogy fails in the case of fluidity and flow.
  2. Although the individual pieces of glass in a window may be uneven in thickness, and noticeably wavy, these effects result simply from the way the glasses were made.
  3. One also wonders why this alleged thickening is confined to the glass in cathedral windows. Why don't we find that Egyptian cored vessels or Hellenistic and Roman bowls have sagged and become misshapen after lying for centuries in tombs or in the ground? Those glasses are 1,000–2,500 years older than the cathedral windows.
  4. Speaking of time, just how long should it take—theoretically—for windows to thicken to any observable extent? The calculation showed that if a plate of glass a meter tall and a centimeter thick was placed in an upright position at room temperature, the time required for the glass to flow down so as to thicken 10 angstrom units at the bottom (a change the size of only a few atoms) would theoretically be about the same as the age of the universe: close to ten billion years. Similar calculations, made more recently, lead to similar conclusions. But such computations are perhaps only fanciful. It is questionable that the equations used to calculate rates of flow are really applicable to the situation at hand.
When all is said and done, the story about stained glass windows flowing—just because glasses have certain liquid-like characteristics—is an appealing notion, but in reality it just isn't so.

- According to the The Corning Museum of Glass

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