Thermal shock

[Moriel]

We all know that if you heat up glass, then run it under cold water, it can shatter. Or, a hot engine block can crack if you drive through too much water.

But what about putting hot water on something frozen? My room mate went out to his car yesterday and there was frost on his windshield... he went out there with a cup of hot water and just splashed it on... instantly melted the ice, but I told him that wasn't the smartest idea... but then again, I don't think I've seen thermal shock from going from cold to hot... it's usually only from hot to cold.

So, is there a danger of thermal shock going from cold to hot? The room mate said he always did that, and it's never cracked yet... maybe just lucky?

 

[Tinker Grey]

Try dropping an ice cube in a pot of boiling water.

Even in room temperature water, I've heard ice cubes pop & snap. I should think that there is some risk.

 

[Wedjat]

Windshield glass is pretty durable.

 

[laconicstudent]

Windshield glass is pretty durable.

Yeah, but what Moriel's roommate did still seems fairly careless. That's a good way to blast a hairline crack wide open.

 

[Moriel]

Indeed... any small cracks or chips you already have could be opened up that way (or just by a frozen windshield without instant heat)... I'm not sure if ice cubes are a proper illustration though. I don't know if I've ever dropped one into boiling water... but when one or two escape and fall on the ground, I toss 'em in the sink and usually melt 'em with hot water... Maybe mine are just magic, but they seem to melt away pretty silently. The cracking may have something to do with the standing water, because even when I drop it into cold liquid, they'll crack all over the place (like putting ice cubes in an already cold drink).

... other than ice cubes in water, anyone know definitively about this?

 

[bigbadwilf]

Thermal shock is where you rapidy raise or lower the temperature of something that has a high coefficient of thermal expansion and a low thermal conductivity. This results in the exterior of the material expanding or contracting more rapidly than interior.
The key to avoiding the (fairly remote) chance of doing any damage to the windscreen then I'd recommend using larger volumes of warm water rather than smaller volumes of hot water.

Moriel, your hot water probably doesn't result in the cracking effect as it is too hot. Rather than making it contract but leaving it solid (temporarily) it's melting the exterior rather than making it contract I think.

 

[Cabal]

It's the same thing, afaik.

 

[pgp_protector]

Yeah, but what Moriel's roommate did still seems fairly careless. That's a good way to blast a hairline crack wide open.

Bingo.

If there's any defect in that windshield, they could be looking at having to replace the whole thing.

 

[Wedjat]

Yeah, but what Moriel's roommate did still seems fairly careless. That's a good way to blast a hairline crack wide open.

True dat

 

[ArnautDaniel]

We all know that if you heat up glass, then run it under cold water, it can shatter. Or, a hot engine block can crack if you drive through too much water.

But what about putting hot water on something frozen? My room mate went out to his car yesterday and there was frost on his windshield... he went out there with a cup of hot water and just splashed it on... instantly melted the ice, but I told him that wasn't the smartest idea... but then again, I don't think I've seen thermal shock from going from cold to hot... it's usually only from hot to cold.

So, is there a danger of thermal shock going from cold to hot? The room mate said he always did that, and it's never cracked yet... maybe just lucky?

The local news here ran a piece last week which was advising against doing this sort of thing...

...I assume there had to be something behind it to justify the time it took away from reporting on crime....

 

[kangitanka]

... other than ice cubes in water, anyone know definitively about this?

Well I remember taking a glass pitcher out of the refrigerator (not even the freezer) circa 20 years ago....it was empty because my roomate drank the last of the lemonade and just put it back in the fridge. When I took it out of the fridge, the glass frosted almost immediately.
Not thinking anything of it, I ran warm (not hot) water in it in preparation for cleaning it............and SNAP! A crack formed immediately along one side of the glass pitcher.

Had to throw it away, obviously.

So yes, cold to hot (or even warm) does work to stress glass to a breaking point.
However, windshield glass in a car is usually laminated glass, which has much different properties from "normal glass" (although I would still not pour hot or even "warm to the touch" water over such glass, due to explanations above).

Hehe I'm going to try an experiment tonight (not with our car windshield, obviously). I'll let you know the results.

 

[Split Rock]

We all know that if you heat up glass, then run it under cold water, it can shatter. Or, a hot engine block can crack if you drive through too much water.

But what about putting hot water on something frozen? My room mate went out to his car yesterday and there was frost on his windshield... he went out there with a cup of hot water and just splashed it on... instantly melted the ice, but I told him that wasn't the smartest idea... but then again, I don't think I've seen thermal shock from going from cold to hot... it's usually only from hot to cold.

So, is there a danger of thermal shock going from cold to hot? The room mate said he always did that, and it's never cracked yet... maybe just lucky?

True Story

Last January I moved from Madison WI to Fargo, ND (don't ask me why). The moving truck driver was from Florida. He had to keep his engine running constantly because of the very low temperatures we were experiencing at the time. One morning he turned up the defroster full blast to deice his windshield. Because the engine was warm, the defroster was immediately hot and he cracked his windshield badly. I do not know if he already has a small crack or dent to begin with or not.

 

[kangitanka]

Because the engine was warm, the defroster was immediately hot and he cracked his windshield badly. I do not know if he already has a small crack or dent to begin with or not.

So yes, apparently, cold to hot does work regarding stress on glass.

Worked for me (badly), worked for him (similarly).

Thermal stress works both ways.

 

[Moriel]

makes sense. Guess I'd always just seen it done going one way and not the other.

 

[MorkandMindy]

We all know that if you heat up glass, then run it under cold water, it can shatter. Or, a hot engine block can crack if you drive through too much water.

But what about putting hot water on something frozen? My room mate went out to his car yesterday and there was frost on his windshield... he went out there with a cup of hot water and just splashed it on... instantly melted the ice, but I told him that wasn't the smartest idea... but then again, I don't think I've seen thermal shock from going from cold to hot... it's usually only from hot to cold.

So, is there a danger of thermal shock going from cold to hot? The room mate said he always did that, and it's never cracked yet... maybe just lucky?

A solid object is more likely to be destroyed going from hot to cold

because cracks result from tension, so if the scratches or other imperfections are only on the outside

when cooling the outside is shrinking relative to the inside and is therefore under tension and therefore able to crack

Try dropping an ice cube in a pot of boiling water.

Even in room temperature water, I've heard ice cubes pop & snap. I should think that there is some risk.

A nice experiment and relevant; there are imperfections inside the ice cube,

similarly if the hot water is not applied absolutely evenly to the windscreen, the same could happen in cold areas next to warm or hot areas

Yeah, but what Moriel's roommate did still seems fairly careless. That's a good way to blast a hairline crack wide open.

I've heard of that happening

Windshield glass is pretty durable.

Car windscreens are made of three layers, the middle layer protected by a double sided sticky plastic film doesn't get scratched and is therefore very strong.

It would be the outer layer most likely that would fracture. This could result in it damaging the wipers and counts as an obstruction to vision in this country and if it passes through the driver's viewing area would result in a vehicle test failure

 

[Steezie]

I know working with metal, it can deform or crack if the temperature change is too dramatic in a short space of time.

Im sure glass will react the same way