Why would glass shatter spontaneously?

[jkettle]

A few years ago, I purchased a lump of glass from an estate sale for no other reason that it was beautiful and I wanted it. I set it on a shelf with the rest of my collectibles to collect dust. This morning at 8AM on the dot, the lump of glass shattered into a million pieces with a pretty loud explosion. There were no visible forces acting on it. No sunshine or anything else I could detect. I would love some theories about why this might have happened. Very curious.

 

[marcusl]

Glass is amorphous, generally has a large coefficient of thermal expansion, and is a poor heat conductor, so as it cools from the molten state it can freeze in a lot of stress. This can happen even with heat-treated tempered glass like car windshields--which is why a rock that pits a windshield can cause cracks to spread across the entire windshield a day or two later. I'd expect a large mass of glass to be more susceptible than a thin sheet since it will cool unevenly (rapidly on the outside compared to the inside). To avoid this requires special treatment. For instance, the Mt. Palomar telescope mirror was cooled only one or two degrees per day for a year to reduce internal stresses, even though it was made of low expansion pyrex. Sounds like your chunk was a stress bomb waiting to go off.

 

[jkettle]

Thank you for the response Marcusi.

I have little training in physics other than 101 at college. I do remember glass is a special material that is almost like a liquid semi frozen into a solid state. Assuming my 'lump' of glass was very old would gravity have been the 'trigger' to ultimately cause the shattering? Did gravity exert influence on the glass over a long period of time (it had a flat surface on one side on which is was probably displayed for years) to essentially build up internal pressure? IS it normal for glass to explode? I would have expected the 'lump' to fissure, not explode.

 

[Dr_Morbius]

http://www.youtube.com/watch?v=Pdy2_vi0FfM&feature=related

 

[marcusl]

I don't know if it's normal. Maybe someone else knows...

 

[elterrible]

Glass is amorphous, generally has a large coefficient of thermal expansion, and is a poor heat conductor, so as it cools from the molten state it can freeze in a lot of stress. This can happen even with heat-treated tempered glass like car windshields--which is why a rock that pits a windshield can cause cracks to spread across the entire windshield a day or two later. I'd expect a large mass of glass to be more susceptible than a thin sheet since it will cool unevenly (rapidly on the outside compared to the inside). To avoid this requires special treatment. For instance, the Mt. Palomar telescope mirror was cooled only one or two degrees per day for a year to reduce internal stresses, even though it was made of low expansion pyrex. Sounds like your chunk was a stress bomb waiting to go off.

hmm, this seems a really good explanation. Plus its tensile strength will be a lot lower than its compressive strength. So even though its just setting on the counter, it still can have a lot of built up tensile stresses and just a few more thermal cycles to send it over the edge.

 

[NascentOxygen]

Glass shatters when stresses in it cause cracks to propagate. Unlike metals (which typically get stronger as they stretch in the region of a developing fracture), glass has no such mechanism, so the cracks propagate without limit. This is one reason why glass is so dangerous. With metals we can see it bending and usually have time to get out of the way of danger, but with a glass object there is no warning, and we can suffer frightfully deep gashes because we are still exerting full force on the sharp shards, it breaks so quickly, unexpectedly, and without warning.

A few examples that come to mind: toughened glass dinner plates are known to shatter for no apparent reason, so are the glass windows in oven doors, and glass coffee table tops. Vehicle windscreens do shatter seemingly for no reason, but generally this can be traced back to a recent chip by a flying stone.

 

[jkettle]

Thanks all for the valuable input. I've learned a lot about glass. I've told my story to several people who were stumped so I will pass on your explanations.

Side note: most novices that I spoke with went immediately to molecular resonance. I knew that couldn't be the answer.

Thanks again and this is a great forum!