Glass in microwave

Clearly the glass wasn't microwave safe.

How long was the pint glass in the microwave? How cold was the milkshake when it was poured into the glass then placed in the microwave? Was there any metal/gilt in the paint or directly on the pint glass?

I'm fathoming a guess that the extreme temperature change of the glass from very cold (premicrowaving) to very hot (postmicrowaving) might have cracked the pint glass in places that were not noticeable to your son when he removed it from the microwave.

Then the repetitive strain of lifting the cup up and down onto the coffee table could aggravate the cracks until the glass becomes structurally unsound and shatters.

I am not prepared to believe we have all the relevant information.

After I stick my tongue out at you .... ((((pbbbbbbbbbbbbbbbbbbbbbtt!!!))))...

... I am prepared to agree with your cautious hesitation to assessment Roger.

I'm gonna go with a meteorite.

check for a small smoking hole in the roof...

Because he heated it in the microwave for 30 minutes before putting it on the coffee table?

Everything you'd ever want to know about superheated water in the microwave. Probably not the same situation, but I presume some of the same science is at work.

http://www.howeverythingworks.org/search.php?searchs=superheated+water

Glass is unstable. Glass can and will break without any apparent cause if there is an inherent flaw.

I learned that while talking to a glass blowing instructor when I was taking ceramics at an art school. That explained to me how one of my mother's wine glasses, hand washed then set on a towel on the plastic counter to dry suddenly and loudly cracked in a circle around the circumference of the bowl.

DID THE COFFEE TABLE HAVE A GLASS TOP?

you have a cold glass, its fulll of a substance that holds heat, you heat all up and place it on a cold surface. the cold surfaace absorbs some heat from the bottom of the glass. the heat in the milk and glass rises maintaining the (hot) temp of the glass in the upper area fo the glass whilst the lowere area of the glass becomes cold. the contraction of one part of the glass (lower) places stress on the uppper expanded (hot) portion.
Bang!
surprising that it took so long though.

Glass is extremely stable, resistant to chemical attack and very strong when used correctly, but glass is also brittle so it will fail dramatically compared to ductile failure modes. Unless you have special temperature resistant glass (Pyrex is the best known), you can induce very high stresses by inducing a temperature gradient across the glass (see dadpad's post) and then any flaw will set you off. The same thing in a ductile material will just cause it to expand or contract. In the right conditions, you can brittle fracture steel or iron as well. Navy lore says the Navy once brittle fractured a destroyer.

I would think that would take very, very low temperatures a/o very high pressure?

High stress really. That can come from high pressure but you usually plan for that. Where people usually get bit is rapid heat up of very thick materials, especially where there is a join causing a stress concentration point. (That's also what causes glass to fracture in most cases.) In my Navy days when we would heat the reactor up, we were limited to around a degree per minute until we reached a certain temperature due to brittle fracture concerns, making reactor heat-up to operating temperatures take several hours. The steel had a ductile to brittle transition temperature (DBTT) lower than our target, but we were inducing more stress than the 15ft-lbs the test is normally conducted at.

Yes, and did he hit it with a hammer ... I see where you're going with this.

Alright, but, the glass blower used the word unstable.

Was it African or European glass?

I never doubted you. I do disagree with him. I typed up a full explanation then started laughing at my geekiness so I spared you.

His description is obviously shorthand for describing resistance of glass to brittle fracture in the presence of a microscopic crack. Viewed in that sense, it's not wrong to say that such glass is unstable, as artistic glass-blowing likely creates microscopic defects - it's the nature of the manufacturing process.

But artistic glass blowing can induce extreme residual stress in glass as it cools if done rapidly, especially if using different glass compositions. (This is true for all glass forming processes.) Failing under high stress is not the same as failing under little stress. If you have an I-beam with a large weight on it and it fails, you wouldn't say the metal was unstable, you would say there was too much stress on it. The issue with glass is that you can freeze in stress so even though it is there, you can't see it like that weight on the I-beam. Annealed glass will sit in a room until the end of time and not crack. If you have ever tried to break window glass, it is actually pretty hard. People don't open their cabinet to find one of their glasses has cracked, but if you take a hot glass out of the dishwasher and put cold water in it, you can certainly expect it to crack. It didn't fail under minimal stress, it failed under high stress. I can design a glass that is under extreme stress so that if you add a little more, it will crack. (I can make one that not only will crack, but will explode.) That doesn't mean that glass is inherently unstable, it just means that I stressed the daylights out of it. I can also design a glass that has no stress in it and so you really have to work at it to break it. Pyrex is almost completely heat insensitive for example. The glass they make telescope mirrors out of does not expand or contract with temperature changes and is annealed so that there is no residual stress either. The overarching statement that glass is unstable is just not correct.

The point is that you can't compare the purity of glass used for, say, optical fibre, and the glass used for artistic glass-blowing, not least because the latter is usually colored, i.e. contains chemical impurities to begin with. Obviously the term "unstable" is used in artistic classes as shorthand for "delicate" - no sense explaining to artists thermal expansion coefficients of glass or the photonic-band-gap materials used in state-of-the-art technology for optical fibres.