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Name:  Alison A.
Status: other
Age: 20s
Location: N/A
Country: N/A
Date: 8/13/2004


Question:
Is it true that putting a metal spoon into a glass before pouring a hot liquid into it will keep the glass from breaking because the metal conducts the heat instead of the glass?


Replies:
Alison,

A lot depends on the fragility of the glass and any stresses that my be present in its composition. Indeed, the spoon would absorb some of the heat. However, the thermal shock to the glass may be more that it could withstand. So, I'd answer your question with a, "sometimes, depending ..."

Regards,
ProfHoff 905


Alison-

I do not think that is quite true, as stated. The effect exists, but I think a different reason is more important. True, the spoon is cold, and thermally conductive, and will take some heat out of the first 1/10 second of liquid poured. But I think spreading and calming the "assault" of hot water on glass is more important.

Pouring hot liquid into a glass does not always break it. The liquid might not be hot enough, the glass might have low temperature coefficient of expansion, like Pyrex, or the glass might have a low defect density (hence, be relatively strong) in the places that get stretched by the temperature change. There is a fairly sharp threshold for breaking glass by means of thermal stress. And this threshold can vary a lot between different types of glass, and even between identical items.

Putting a spoon in a glass before pouring the hot liquid probably helps. It might reduce the frequency of glass-breaking occurrences, by some amount from 10% to 90%. It does not help much by conducting heat. A little bit of metal in the liquid does not save the glass from being covered by a flood of hot liquid.

What I think it does do, is: baffle, scatter, and distribute the narrow stream of the poured liquid as it falls into the glass. If you pour a fine spout of boiling water right onto one side of the glass, that glass surface faces an intense "wind" of hot liquid for a couple of seconds straight. The inside surface is forced to heat up instantly all the way to the liquid's hot temperature. The outside surface is still cold. This provides, at that spot, the absolute maximum "thermal stress" you can generate with a liquid of that temperature. If you want to break the glass as often as possible, that is how to do it.

What breaks the glass less often is: when some fresh hot liquid slowly rises to cover it, and then sits still for some fraction of a second. Liquid and glass have about the same thermal conductivity and thermal mass, so as the glass heats up, the liquid right next to it cools down, and the number of degrees of fast temperature change is about halved.

The spoon does this for you, provided you pour onto it and do not miss it and hit the glass instead . It breaks up the liquid stream, spreading it into a splatter or sheet in all directions. Then the smaller and less focused streams assault most sides of the glass almost equally, and less often at any one spot. Then it is less likely that any one spot will experience thermal shock above the threshold required to initiate crack growth. For some glasses this will not be enough to save them, but for some it will.

Pouring your hot water onto a ping-pong ball floating in the glass might work as well, (not that you would want them in the kitchen). Ping-pong balls are a light, thin plastic, and do not conduct any significant heat. They do make the flow spread out or deflect. Golf-balls too, and they would not float so high. (I guess they sink.) A smooth plastic toy golf-ball 2/3 full of water would be perfect.

Pouring 5%, swirling a second, repeating a few times with progressively larger quantities till done should work very well, for similar reasons. But it risks that if the glass does break, your hand will be on the broken glass and gushing hot water. Pouring 25%, merely waiting a second or two, and repeating helps too, a little less, but likely enough to prevent breaking. Steadily rotating the glass while pouring against the inside side-wall might be another experimental equivalent.

If you ever try these things as an experiment, make very sure that the hot water cannot spill against you when the glass breaks.

Jim Swenson



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