View Single Post
Old Dec 13 2007, 05:33 PM   #18
Brenda
Senior Member
 
Brenda's Avatar
 
Join Date: Mar 2005
Location: St. Louis
Gender: F
Default Re: Wansor's equations

This is interesting.

*edit*

Oooh, so's this.
Quote:
>In article <199310050310.AA25843@shark.mel.dit.csiro.au>, I write:
>>I will try to get hold of the full paper and relate its findings.

And indeed I have. Some interesting points arise in the article "Antique windowpanes and the flow of supercooled liquids" by Robert C. Plumb (Worcester Polytech. Inst.)in the Journal of Chemical Education, 66(12), 994-6, 1989.

1)Robert Brill of the Corning Museum has records of the stories of window panes 'flowing' over the years, dating back as far as high school chemistry class in 1947.

2)The glassy state resembles a liquid in having short-range [molecular] order without long-range order ,but differs in that the entire network is rigid, whereas in the liquid state enough energy is available to break and reform bonds continuously.

3)The temperature at which a rigid glass becomes a supercooled liquid is called the glass transition temperature, Tg. For window glass, Tg (measured) is 550 degrees C. For the limiting case of infinite time the thermodynamically calculated ideal glass transition state Tg(0) for window glass is 270 degrees C. For Pyrex the values are 550 and 350 deg., respectively. [This alone should be enough to put to rest any argument for the liquid properties of glass at room temperature (which rarely gets above 50 degrees C).]

4)If a rod of glass is twisted it will return to its original shape. If it is twisted and held for a period it will retain its twisted shape BUT will gradually return to its original zero-twist state over time. This 'delayed elastic recovery effect' is explained in the paper but is too technical to go into here. Suffice to say, this experiment debunks any 'deformation of glass under pressure = glass flows' type experiments.

5)Those who are convinced that glass flows like a viscous liquid under its own weight sometimes cite the Corning Glass Co. instructions printed on boxes of tubing, "Lay flat, do not stand on end". According to R. Lemker (Operations Manager, Fallbrook Plant, Corning Glass, Corning, NY) the instructions are to avoid damage to the ends of the tubing, not to keep it from sagging.

6)The paper then goes into a long discussion of how glass windows were manufactured in the 1800's (the Crown glass process) and essentially comes to the conclusion (already offered in AFU) that antique window panes are thicker at the bottom because of variations in the thickness of the glass (which at the time were less important than other defects such as blisters, dust, lines, curves and scratches) produced during manufacture. The author surmises that glaziers would tend to put the thick end of the glass at the bottom for stability.
Brenda is offline   Reply With Quote