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(This question is about non-tempered glass.)

I broke my favorite glass (tumbler) today, dropping it in my (ceramic) sink while trying to refill it. :(

I'm kind of a klutz - that's far from the first time I've dropped that glass in that sink. However, there was nothing different this time from all the other times. It's never gotten chipped, either (AFAIK).

It did get me curious, though. When some particular glass object is dropped a lot of times, which of the following is true?

  • Something invisible (at least to the naked eye) in the structure "degrades" each time, gradually weakening it, and eventually it becomes so weak that it breaks
  • It doesn't change at all as long as it doesn't break - there's just a random chance for it to break each time (e.g. based on the exact angle it collides at), so you could get unlucky and break a brand new glass

I suspect it's the first, since whenever I break something in a situation like this, it always seems to be something I've had for years and grown rather attached to, rather than something brand new that I could just replace like-for-like right away.

What actually happens on the inside when glass hits something hard?

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    $\begingroup$ Was the tumbler made of tempered glass, eg Duralex? Tempered glass is pre-stressed, and shatters into small chunks, rather than shards. $\endgroup$
    – PM 2Ring
    Mar 1 at 17:25
  • $\begingroup$ @PM2Ring No. But thank you, I should have mentioned this! I've clarified in the question now. A quick image search suggests I'm talking about "annealed" glass but I'm not sure if there are other types of non-tempered glass. $\endgroup$
    – Keiji
    Mar 1 at 17:43
  • $\begingroup$ So did the tumbler break into a lot of uniform-sized chunks, or did it break into jagged shards of very random shapes & sizes? $\endgroup$
    – PM 2Ring
    Mar 1 at 18:12
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    $\begingroup$ The fact that you only tend to break things after dropping them many times may just indicate that the random IID probability of breakage each time is low. That would be perfectly consistent with it being very unlikely to break the first time you drop it. You'll notice that you rarely roll a 6 on the first roll of a fair die, but that it inevitably happens the more you roll, even though the chance of getting a 6 never changes. $\endgroup$ Mar 1 at 18:31
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    $\begingroup$ Possibility 3: It's not the drops over time, but everyday wear & tear that create small scratches, which have a greater chance of being opened by a future drop. $\endgroup$
    – BowlOfRed
    Mar 2 at 4:47

3 Answers 3

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The MIT 'Ask an engineer' website: How does glass change over time? has some information about that:

Quote from Michael Cima, Professor of Materials Science and Engineering and Faculty Director of the MIT Glass Lab

If tiny flaws or weak points in the glass are exposed to water over time the water attacks the tips of the cracks and dramatically decreases the strength of the glass.

We know of course that to obtain a clean break of a pane of glass a very superficial scratch is sufficient. But that is a long straight crack. The comment by Michael Cima appears to describe that a very short crack can also compromise the strength of the class. Repeated exposure to water having an effect on microcracks is also a surprise to me.




After further looking down the list of search results for:
glass degradation over time

quote from an article in Nature magazine, titled: Glass alteration in atmospheric conditions: crossing perspectives from cultural heritage, glass industry, and nuclear waste management

Microcracks penetrate in the hydrated layer, intersect, and many of them pass into the pristine glass, often redirecting parallel to the surface. Microcracks have a very detrimental effect on glass durability, especially in cyclic climates. They may allow capillary condensation of water at the interface with the pristine glass during humid periods, extending glass hydration. Furthermore, they are the seat of salts crystallization during dry periods, which may induce stress and cause in-depth propagation of the cracks

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    $\begingroup$ Brittle fracture usually occurs when a pre-existing crack is overstressed so it will propagate uncontrollably. Prior dropping way we’ll have caused a small crack, or grown one a bit bigger, ready to go more easily. Further, even with out those, the relative direction of the shock forces and the pre-existing flaw matter. $\endgroup$
    – Jon Custer
    Mar 1 at 18:05
  • $\begingroup$ To better understand Cima's quote, refer to "The Fracturing of Glass," "Environmental Effects on the Static Fatigue of Silica Optical Fiber,"... $\endgroup$ Mar 1 at 22:24
  • $\begingroup$ ...and "Degradation of Stressed Optical Fibers in Water," and the references within. $\endgroup$ Mar 1 at 22:24
  • $\begingroup$ After quoting all the articles which say "glass degrades over time" you should really change your first sentence which is clearly and completely wrong. $\endgroup$ Mar 2 at 16:30
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    $\begingroup$ @Peter-ReinstateMonica You are correct. I should have searched first. Here I answered first, then started searching, and what I found was (indeed) the opposite of my expectation. I have edited my answer extensively. $\endgroup$
    – Cleonis
    Mar 2 at 17:14
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Glass is brittle, which means once a crack has a chance to get started in it, it tends to let go all at once. with time, a used piece of glass accumulates very tiny cracks in its surface and if subjected to a sudden shock, one of those cracks will grow- and kaboom.

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As @Cleonis states in his answer, glass has an amorphous molecular structure. It is not a solid, but is a super cooled liquid that has undergone a secondary transition. Some references say that it can deform over time due to creep, usually over centuries.Rheology - flow of matter (although others disagree link ). It may however very slowly crystallize. Unless you are extremely old, I doubt that either of these happened to your tumbler.

The surface of glass is very important for it to maintain its integrity. To cut a sheet of plate glass, first a line is scored across the surface. Slight pressure and a small tap are enough to cause a crack to propagate along the score line. If you have ever had a crack in your windshield you will have no doubt noticed how the crack will grow under thermal and other pressures.

When glass is fabricated there are always internal stresses. One most interesting example is Prince Rupert’s drops. wikipedia Prince Rupert's drops These are made by dropping molten glass into water and are tear drop shaped. The bulb of the drop is extremely strong - you can hit it with a hammer and it won’t shatter, but even a small deformation of the tail will cause explosive disintegration into powder! youtube video

Most probably what happened to your tumbler is a combination of growing internal fractures, surface scoring and chance (i.e. bad luck).

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