Why Does Glass Become Brittle After Heating?

[bw800402]

After heating some glass bottles to a very high temperature with a bonfire the bottles changed shape as you would expect molten glass to do, but once the bottles returned to ambient temperature they were extremely brittle. What is happening to cause this? I read that annealing requires heating glass then cooling it very quickly and this is supposed to strengthen the glass. What are the changes in molecular structure that leads to the the weakening of the glass in this case?

 

[Bystander]

After heating some glass bottles to a very high temperature with a bonfire the bottles changed shape as you would expect molten glass to do, but once the bottles returned to ambient temperature they were extremely brittle. What is happening to cause this? I read that annealing requires heating glass then cooling it very quickly and this is supposed to strengthen the glass.

"WORNG." Cool slowly, and uniformly to allow the material to relax --- a quick quench locks in all sorts of stresses, resulting in easy breakage.

What are the changes in molecular structure that leads to the the weakening of the glass in this case?

The only change in chemistry from heating is that you can burn the sodium out of a glass, leaving you with a higher silica content, slightly higher temperature softening point.

 

[Gokul43201]

After heating some glass bottles to a very high temperature with a bonfire the bottles changed shape as you would expect molten glass to do, but once the bottles returned to ambient temperature they were extremely brittle. What is happening to cause this? I read that annealing requires heating glass then cooling it very quickly and this is supposed to strengthen the glass. What are the changes in molecular structure that leads to the the weakening of the glass in this case?

You're using very specific terms in very general ways. Strength and brittleness are not "opposite" properties. In fact, they usually go hand in hand. Any heat treatment that makes something stronger (gives it a higher breaking strength) typically makes it more brittle (lowers the strain at failure).

So how did you cool the glass?

 

[Gelsamel Epsilon]

Gokul is right, a diamond is really strong, but it's also really brittle. Which is why you can break diamonds with a hammer. On the other hand elastic bands are fairly weak and if you stretch them enough it breaks without much force, but they're not brittle at all, you can fold it and bend it and nothing will happen to it.