I was told by a condensed matter physicist that glass is a liquid with a very high viscosity (it would be more precise to say that it is a supercooled liquid). The example given was that in cathedral windows the glass flows very, very slowly, so they are thicker at the bottom.
I recently read this article in Scientific American, which contradicts the previous statement.
Are there any serious studies about this aspect of glass? Is it only a problem of definition?
Answer
False.
To quote from the specific section in the list of common misconceptions:
Glass does not flow at room temperature as a high-viscosity liquid. Although glass shares some molecular properties found in liquids, glass at room temperature is an "amorphous solid" that only begins to flow above the glass transition temperature, though the exact nature of the glass transition is not considered settled among theorists and scientists. Panes of stained glass windows are often thicker at the bottom than at the top, and this has been cited as an example of the slow flow of glass over centuries. However, this unevenness is due to the window manufacturing processes used at the time. Normally the thick end of glass would be installed at the bottom of the frame, but it is also common to find old windows where the thicker end has been installed to the sides or the top. No such distortion is observed in other glass objects, such as sculptures or optical instruments, that are of similar or even greater age. One researcher estimated in 1998 that for glass to actually "flow" at room temperatures would take many times the age of the earth.
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