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What Made My Window Shatter?

I'm guessing a high angle of incidence air rifle pellet or sommat from a catapult ( I just LOVE that word...much nicer than the USA equivalent 'slingshot'!!)
EDIT: Oh, yes, and I meant to mention: the fella that mows our lawn went over a pebble and it got propelled against our kitchen window, last fall...glass all over the floor, both layers of a double pane insul-glass window shattered, but NO sign of the rock. [cue eerie music...]:boggled:
 
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I find this unlikely but just for completeness' sake ought to mention another possible mechanism.

You might not have noticed it, but your window might have had a miraculous apparition, an image of the BVM or JC, and the combined force of the eyebeams of all the gawkers out in the lane staring up at it caused the breakage. Did you notice any "oohs" and "ahhs" of amazement from outdoors prior to the shattering event?

[edit] @slabsides: A pellet or other small projectile would've left at least a little pit at the site of impact, possibly punched a hole right through.
 
Meffy said:
A pellet or other small projectile would've left at least a little pit at the site of impact, possibly punched a hole right through.
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Airgun pellets leave beautiful round holes in greenhouse glass. Luckily for me my dad never noticed this phenomenon.
 
Just so. :-D

Sitting in a booth at a nearby fast food place I noticed a near-perfect cone punched out of its half-inch thick window by some such impact. The tiny hole on the outside was circular, and a shallow, broadly conical, conchoidally-fractured "plug" had been knocked out, widening toward the inside. I wonder whether the plug was a solid chunk, and if so whether anyone (police, employee) found it.
 
The poor glass probably cracked from pressure. It was there everyday and you just looked right through it as though it wasn't there. After a while it became so despondent because you couldn't see it that it just cracked. :)
Seriously it looks like an impact.
 
It looks like a classic example of a tempered window getting a scratch.

Tempering introduces tension in the glass (on purpose) putting the middle of the glass in compression and the outsides in tension. Problem is, a break in the part under tension can lead to just what you see.
 
From Wiki
Although toughened glass is most susceptible to breakage via edge damage, breakage can also occur from impacts in the centre of the glass pane.

Shattering may not happen when the damage originally occurs and can be triggered by a minor stress like heat or small impact that would not normally affect the toughened glass.
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The evidence is the glass broke from a clear point of impact or defect. Any number of things could have hit it but I'd think a bird's head fits the pattern better than a rock which should have had a more localized impact spot. There is no evidence there suggesting any kind of bullet or pellet. A baseball is another possibility.

Both the ball and the bird could have left the scene before you got outside to look. We hit a bird with our car once. I picked it up and put it in a box and a little further down the road it woke up and flew off. Alone though a small bird wouldn't hit the window hard enough to break it but there are some larger birds that possibly could. I would think a defect in the glass also contributed if it was indeed a bird.

But you can't rule that baseball out either unless the witnesses can say there were no kids running away with a ball. OTOH, the ball could easily bounce and roll quite a distance away on its own.
 
As I understand it, the little cones are where the glassblower held the glass to form it. It's not damage.
 
Silly Green Monkey said:
As I understand it, the little cones are where the glassblower held the glass to form it. It's not damage.
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Oh, this is damage. Not blown glass, this is heavy, modern, perfectly smooth, floor-to-ceiling sheets of plate glass. :-) Unmistakable. (Assuming you were talking about my fast food joint anecdote.) But it's also a completely different kind of damage, nothing like the shattering the OP is about.
 
No, it's still an artifact of the production process. There's even a name for them, but I can't remember it. Your fast food windows are not the only ones to have them.
 
I think I know the kind of feature you mean. This is a different kind of thing. I've just described it poorly. It's not an artifact of the manufacturing process. The hole used not to be there, then it appeared. It's just on one window, not far from one edge. It's definitely damage, there was even a razor-edged, arc-shaped little flake that hadn't quite detached around the edge. No competent builder would accept delivery of a window with a hole like the one I saw. :-) It's also very like shot damage I've seen in other glass. Just a little neater.
 
A few months ago I was sitting in the mall, and all of a sudden the huge glass window of the store next to me cracked up like that, and fell to the ground in dime sized pieces. This nitwit girl was also close by, and started telling everyone who was stopping to gawk at the glass on the foor that it was the noise of the nail file thingies that the workers were using inside on customers' nails that caused the glass to break.

I looked up to see the frame had given away a bit at the top, putting all the stress on the glass to stand in the frame on its own, while heaving out a bit. Try telling the yapper to shut it though, sheeesh. My eyeballs nearlly rolled their way right out of my head. If those nail file thingies could break glass with that little barely audible buzzing noise, then my teeth grinding in response to her supposition ought to have at least made her eyes water, or something like that, garrrr. The mall was recently renovated, and the glass was new, so some little buzzing noise would hardly cause it to smash into little tiny pieces!

Is your frame sound, and was there any other stress on the glass? If not, then maybe it was some imperfection that suddenly gave way.
 
wollery said:
In the UK it's fairly standard in blocks of flats to have hardened glass in the upper storeys. It helps to prevent idiots falling through them!
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My understanding of the reason that window glass is tempered is so the when it breaks it falls in many small and not very dangerous pieces. That is the reason glass in doors and on coffee table tops should be tempered.

If the same window made of ordinary window glass broke it might fall in large sabre like shards which can slice through anyone standing below. If you want to stop people from going through a window you make the glass thicker.
 
Note: Glass is tempered because it has great compressive strength but little tensile strength.

There are at least two kinds of tempering.

Heat tempering cools it rather quickly. The outside hardens fast, and the inside slowly cools. This pulls on the outside, ensuring that the outside is in compression. Any bending of the glass just relieves compression. If memory serves, the depth of this compression is around 30 microns. A scratch or erosion that goes that deep will shatter the glass.

Chemical tempering replaces atoms with bigger ones that have the same valence. The one I'm familiar with involves sodium and lithium ions. The increased side of the replaced ions results in the compression. This is very good; glass treated like this can be bent almost like plastic. However, if memory serves, the compression is only to the depth of about 5 microns.

Note that chemical tempering is similar to case-hardening, in which carbon monoxide is allowed to seep into a metal. I once case-hardened some aluminum rabbit in a smoky flame. It worked great.
 
Meffy said:
I think I know the kind of feature you mean. This is a different kind of thing. I've just described it poorly. It's not an artifact of the manufacturing process. The hole used not to be there, then it appeared. It's just on one window, not far from one edge. It's definitely damage, there was even a razor-edged, arc-shaped little flake that hadn't quite detached around the edge. No competent builder would accept delivery of a window with a hole like the one I saw. :-) It's also very like shot damage I've seen in other glass. Just a little neater.
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I've seen those in hundreds of glass objects since I was old enough to observe (funny -- I can't remember the wall covering or decor, but I will always spot mechanical details and defects). These are always in standard (non treated) float-glass or pressed glass (headlamps lenses, vases, heavy bottles, etc.). Have any of you knapped flint or chert? : push with a soft copper nib near an edge and *snap*, a flake peels off. Conchoidal fracture. Any *point* impact on untreated glass is likely to make one of these "cones", as the shock wave and stress riser propagates. Low energy, small, hard, fast projectiles (gravel, BBs, etc.) almost always do this if the thickness vs. width is right. The key seems to be a *point* impact, pushing the pounds/in^2 (pressure) and pounds/in^2/sec (rate of pressure rise) sky high.

Heat treated or otherwise modified glass stores high internal stresses (as others have noted), and any tiny dislocation will make it relieve itself... spectacularly.

Cheers,
Dave
 
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Yah, sounds like it. I saw a television news story a little before I noticed the hole -- county police were asking for help finding those involved in a pellet-gun shooting spree in suburbs not far from this restaurant. They mostly shattered car windows, but I suspect they might've done this too. Guess they were disappointed when it didn't turn to crumbled bits too. (Far as I know they never did catch the wretched little vandals.)

I still can't remember the name of the mark made by the pontil when glass is hand-blown. Annoying, as I used to know a lot about old-style glassmaking. Memory is such a traitor.
 
A window in the space shuttle suffered very nearly the same damage when a paint chip hit it at 17,500 MPH.

Done any renovating lately? ;)
 
I favour the differential heat theory myself, with expansion of part of the glass which was in the hot sun occurring while an adjacent bit in the shade of the window frame remained at a lower temp.

Another factor that will put stress on the glass panes is the fact that the window was double glazed. Differences in temperature between the room, the entrapped gas in the double-glazed section (argon I think), and the outside air can lead to the panes flexing inwards in a concave manner, or alternatively outwards in a convex manner if the gas inside contracts or expands. The added stress of this on the glass may have been sufficient to trigger the process, starting the crack at a previously imperceptible defective area in the outer pane.
 
Meffy said:
I still can't remember the name of the mark made by the pontil when glass is hand-blown.
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I think SGM is thinking of Bullseye glass, which is (as you say) caused by the pontil used to hold the glass in the Crown Glass manufacturing process. I don't think I've heard of another special name for the bit where the pontil is cracked off - it's usually just "the pontil mark" or somesuch.

The Crown Glass process involves putting a blob of glass on a pontil (a metal rod), blowing it open and then spinning it to make it flatten out. This leaves a thicker piece of glass where the rod connects, which is the characteristic bullseye. You then cut up the flattened sheet to give you your panes.

If you're blowing glass to make a sheet by blowing a cylinder then slicing it open you don't get the same bullseye effect. That method appears to have been used by the Romans to make windows. Clever lads, the Romans.

Edited to explain a bit about crown glass, for them as don't know.
 
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