# Cracking glass with hot/cold fluids? **URL:** \ **Category:** Factual Questions\ **Created:** [March 20, 2010, 9:03pm UTC](https://boards.straightdope.com/t/cracking-glass-with-hot-cold-fluids/533170 "2010-03-20T21:03:35Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![constanze](https://avatars.discourse-cdn.com/v4/letter/c/f9ae1b/32.png) [@constanze](https://boards.straightdope.com/u/constanze)\ **Post date:** [March 20, 2010, 9:03pm UTC](https://boards.straightdope.com/t/cracking-glass-with-hot-cold-fluids/533170/1 "2010-03-20T21:03:35Z")
Okay, the background to this question is this: I like to drink fruit juice, which I keep in the fridge (so it doesn’t spoil), temp. roughly 8 C. But I don’t want to drink it cold, so I mix it with boiling water - not 100 C, I often stop when it’s bubbling. At home, I have a half-liter earthen mug, and pour the hot water first, then the cold juice. I do this because I believe that the hot water rises, the cold juice sinks, so they mingle better without needing to be stirred. My subjective experience (tasting, not thermometer) bears out that this mixes better than pouring cold juice first and then the hot water. However, when I was recently at a friend and did the same in a drinking glas, he was upset because he claimed that pouring the hot fluid first and then the cold will crack the glass, it has to be the cold fluid first. Now, I did it my usual method for several times without the glass cracking. I can’t convince him, but I don’t want to have been lucky and start cracking his glasses the next time. I also know that it’s possible to crack even oven-proof glass if, e.g., the glass is in a sink with cold water and then filled with hot fluid. But I don’t believe that filling fridge-cold and barely-boiling fluids can crack a glass since in my opinion the two fluids mingle too fast. I also have a hard time believing that the sequence would make a difference with regard to cracking. However, I want to hear from more experienced or more knowledgable people - either solid theory or anecdotes - so I know what’s best in the future. ---
**Author:** ![drachillix](https://avatars.discourse-cdn.com/v4/letter/d/48db29/32.png) [@drachillix](https://boards.straightdope.com/u/drachillix)\ **Post date:** [March 20, 2010, 9:07pm UTC](https://boards.straightdope.com/t/cracking-glass-with-hot-cold-fluids/533170/2 "2010-03-20T21:07:26Z")
> **[Cutting Glass](https://chestofbooks.com/crafts/mechanics/Workshop-Receipts-5/Cutting-Glass.html)** > > (3) To cut glass jars, fill the jar with lard-oil to where you want to cut the jar; then heat an iron rod or bar to red heat, immerse it in the oil; the unequal expansion will crack the jar all round ... this is kinda what he is worried about, sudden temperature changes are used specifically for cutting glass containers. ---
**Author:** ![constanze](https://avatars.discourse-cdn.com/v4/letter/c/f9ae1b/32.png) [@constanze](https://boards.straightdope.com/u/constanze)\ **Post date:** [March 20, 2010, 9:15pm UTC](https://boards.straightdope.com/t/cracking-glass-with-hot-cold-fluids/533170/3 "2010-03-20T21:15:58Z")
Yes, but: method 3 is a red-hot iron - surely that’s much hotter (several hundred C at least) than normal boiling water? Also, I fill the hot water first, not the cold. Method 4 and 5 work with existing cuts or lines - I understand that a glass will crack if there is a weaker point. But a normal drinking glas isn’t cut or etched. There might be miniature abrasions from the dishwasher, but those will be randomly distributed. Moreover, I don’t see support for his contention that the sequence makes a difference, that it is safe when the cold fluid is poured first. ---
**Author:** ![Cazzle](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cazzle/32/75_2.png) [@Cazzle](https://boards.straightdope.com/u/Cazzle)\ **Post date:** [March 20, 2010, 9:26pm UTC](https://boards.straightdope.com/t/cracking-glass-with-hot-cold-fluids/533170/4 "2010-03-20T21:26:02Z")
My ex was doing the washing up one night and rinsed a glass straight from the hot dishwater under the cold tap. The glass cracked. I don’t know how dramatic the temperature change has to be to cause glass to crack, but dishwater is no where near boiling and tapwater isn’t as cold as fruit juice straight from the fridge… though of course, the fact that the boiling water and the cold juice are mixing will give a different result than pouring one, then the other over the glass. Anyway, I would be too cautious to use my drinking glasses the way you’re describing, and would use a china mug or similar, just in case. I completely see where your friend was coming from. ---
**Author:** ![vanotd21](https://avatars.discourse-cdn.com/v4/letter/v/82dd89/32.png) [@vanotd21](https://boards.straightdope.com/u/vanotd21)\ **Post date:** [March 20, 2010, 9:33pm UTC](https://boards.straightdope.com/t/cracking-glass-with-hot-cold-fluids/533170/5 "2010-03-20T21:33:52Z")
I would say don’t use glass when you do this. I got to see this first hand as a server of a restaurant. I ended up taking a glass right out of the dishwasher and poured soda in. The glass actually cracked and part of the lip of the cup broke off. thought there was a crack earlier which is why it happened, but two weeks later I did the same thing again with the same result. given the temperature of water in the dishwasher (which can be pretty dang hot) and the sudden drop in temperature (soda and ice in the cup), it happened. ---
**Author:** ![Kimstu](https://avatars.discourse-cdn.com/v4/letter/k/ecd19e/32.png) [@Kimstu](https://boards.straightdope.com/u/Kimstu)\ **Post date:** [March 20, 2010, 9:59pm UTC](https://boards.straightdope.com/t/cracking-glass-with-hot-cold-fluids/533170/6 "2010-03-20T21:59:02Z")
“Pour cold upon hot and you’ll wish you had not, But hot upon cold is all right, so I’m told.” This little mnemonic reflects the fact that glass is more likely to crack when it suddenly contracts (e.g., when hot glass is suddenly brought in contact with cold liquid) than when it suddenly expands (as when cold glass meets hot liquid). That’s why, for instance, glassblowers have to be very very careful to cool their products slowly and evenly in an annealing oven, since if the cooling is uneven or too sudden, then kapow, glass shards everywhere. So if you’re heating up a glass with near-boiling liquid and then suddenly chilling it with cold liquid, I’d say yes, that’s running a risk of thermal stress fracture. If you’re pouring the cold liquid into the hot water very slowly and carefully so that it never touches the hot glass directly but just gradually cools down the hot liquid inside the glass, I wouldn’t worry so much. Still, I think it would be safer on the whole to pour the cold liquid in first. And safest of all, as other posters have noted, to use a non-glass vessel. All this fluid thermodynamics analysis just to avoid having to stir the resulting mixture, by the way? What have you got against stirring? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [March 20, 2010, 11:39pm UTC](https://boards.straightdope.com/t/cracking-glass-with-hot-cold-fluids/533170/7 "2010-03-20T23:39:44Z")
> [@](#): > > But I don’t want to drink it cold, so I mix it with boiling water - not 100 C, I often stop when it’s bubbling. If you’re at sea level, and it’s bubbling, then it’s 100 C. If it were less, it wouldn’t be boiling yet. ---
**Author:** ![constanze](https://avatars.discourse-cdn.com/v4/letter/c/f9ae1b/32.png) [@constanze](https://boards.straightdope.com/u/constanze)\ **Post date:** [March 20, 2010, 11:54pm UTC](https://boards.straightdope.com/t/cracking-glass-with-hot-cold-fluids/533170/8 "2010-03-20T23:54:11Z")
> [@Kimstu](#): > > “Pour cold upon hot and you’ll wish you had not, > But hot upon cold is all right, so I’m told.” Never heard of this before, but I will try to remember it from now on. > [@](#): > > This little mnemonic reflects the fact that glass is more likely to crack when it suddenly contracts (e.g., when hot glass is suddenly brought in contact with cold liquid) than when it suddenly expands (as when cold glass meets hot liquid). So it is actually a property of glass in general. Interesting! > [@](#): > > So if you’re heating up a glass with near-boiling liquid and then suddenly chilling it with cold liquid, I’d say yes, that’s running a risk of thermal stress fracture. > > If you’re pouring the cold liquid into the hot water very slowly and carefully so that it never touches the hot glass directly but just gradually cools down the hot liquid inside the glass, I wouldn’t worry so much. Still, I think it would be safer on the whole to pour the cold liquid in first. And safest of all, as other posters have noted, to use a non-glass vessel. Since I don’t want to risk his glasses, I’ll switch to earthenware mug and go looking for a nice one again. > [@](#): > > All this fluid thermodynamics analysis just to avoid having to stir the resulting mixture, by the way? What have you got against stirring? Well, I wanted to know, I was curious. ---
**Author:** ![constanze](https://avatars.discourse-cdn.com/v4/letter/c/f9ae1b/32.png) [@constanze](https://boards.straightdope.com/u/constanze)\ **Post date:** [March 20, 2010, 11:56pm UTC](https://boards.straightdope.com/t/cracking-glass-with-hot-cold-fluids/533170/9 "2010-03-20T23:56:41Z")
> [@Chronos](#): > > If you’re at sea level, and it’s bubbling, then it’s 100 C. If it were less, it wouldn’t be boiling yet. At home, I’m 500 m above sea level; at my friend, close to. But when I heat water (in the electric kettle), then there are always the same stages: steam coming out, small bubbles and little noise, bubbles everywhere and lots of noise. I thought only the last stage is 100 C, but I don’t have a thermometer for that temp. so I never tested it. ---
**Author:** ![Kimstu](https://avatars.discourse-cdn.com/v4/letter/k/ecd19e/32.png) [@Kimstu](https://boards.straightdope.com/u/Kimstu)\ **Post date:** [March 21, 2010, 12:13am UTC](https://boards.straightdope.com/t/cracking-glass-with-hot-cold-fluids/533170/10 "2010-03-21T00:13:19Z")
> [@constanze](#): > > Never heard of this before, but I will try to remember it from now on. My imaginary lawyers 😉 remind me to warn you that this isn’t a **guarantee** that you’ll never shatter cold glass by suddenly bringing it into contact with heat: it can be done, but it’s less likely than shattering hot glass with cold liquid. (Which AIUI is why oven-safe glass baking dishes can often be whisked out of the fridge right into a pre-heated oven, but it’s not a good idea to stick the heated glass dish into cold water when you take it out of the oven.) ---
**Author:** ![si\_blakely](https://avatars.discourse-cdn.com/v4/letter/s/d9b06d/32.png) [@si\_blakely](https://boards.straightdope.com/u/si_blakely)\ **Post date:** [March 21, 2010, 12:22am UTC](https://boards.straightdope.com/t/cracking-glass-with-hot-cold-fluids/533170/11 "2010-03-21T00:22:50Z")
The real issue is with temperature differentials, and the sequence does matter. When you pour near-boiling water into a room temperature glass, the delta-T is 70-80 degrees C - enough to cause expansion in the thin-walled sides at a faster rate than the unheated portion or the heavier base, and thus possibly causing cracking. If you add cold juice into a glass, the delta-T is much less - about 20 degrees C. Less contraction, not much risk of shattering. When adding boiling water to this, the juice and water mix, delta-T is about 45 degrees (assuming 50-50 water/juice) and the glass still does not have the same sort of thermal stress. As for rapid cooling, if the glass was at 70-80 degrees C (like just out of a commercial washing system in a pub) and had cold drink added, then the large delta-T and compression can cause breakage. Si ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://boards.straightdope.com/u/matt)\ **Post date:** [March 21, 2010, 2:13am UTC](https://boards.straightdope.com/t/cracking-glass-with-hot-cold-fluids/533170/12 "2010-03-21T02:13:17Z")
> [@constanze](#): > > At home, I’m 500 m above sea level; at my friend, close to. > > But when I heat water (in the electric kettle), then there are always the same stages: steam coming out, small bubbles and little noise, bubbles everywhere and lots of noise. I thought only the last stage is 100 C, but I don’t have a thermometer for that temp. so I never tested it. You are correct. I think **Chronos** is talking about the last stage, a rolling boil, rather than the earlier stages when dissolved gases are coming out as their solubility decreases with rising temperature. I have tried to crack the bottoms off tabaso bottles to use as candle holders. You can bring a part-filled bottle of tabaso to a boil in an oven and then crack the bottom off by submerging in cold water, but the results aren’t very clean. ---
**Author:** ![DCnDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dcndc/32/2842_2.png) [@DCnDC](https://boards.straightdope.com/u/DCnDC)\ **Post date:** [March 21, 2010, 3:49am UTC](https://boards.straightdope.com/t/cracking-glass-with-hot-cold-fluids/533170/13 "2010-03-21T03:49:40Z")
> [@matt](#): > > You can bring a part-filled bottle of tabaso to a boil in an oven and then crack the bottom off by submerging in cold water, but the results aren’t very clean. Once I fried a ton of bacon for a group of friends that were staying with me for the weekend, and rather than pour the grease down the drain(never a good idea) I poured the still-sizzling grease into an empty _Prego_ tomato sauce jar. The sink was partially filled with room-temperature water, and for some reason after pouring the sizzling-hot grease into the jar I put it in the sink. _Crack!_ Jar was cut clean, right at the level of the grease. ---
**Author:** ![needscoffee](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/needscoffee/32/1076_2.png) [@needscoffee](https://boards.straightdope.com/u/needscoffee)\ **Post date:** [March 21, 2010, 4:11am UTC](https://boards.straightdope.com/t/cracking-glass-with-hot-cold-fluids/533170/14 "2010-03-21T04:11:11Z")
Why not microwave the juice for 5 or 10 seconds instead? ---
**Author:** ![constanze](https://avatars.discourse-cdn.com/v4/letter/c/f9ae1b/32.png) [@constanze](https://boards.straightdope.com/u/constanze)\ **Post date:** [March 21, 2010, 11:05pm UTC](https://boards.straightdope.com/t/cracking-glass-with-hot-cold-fluids/533170/15 "2010-03-21T23:05:51Z")
> [@needscoffee](#): > > Why not microwave the juice for 5 or 10 seconds instead? I don’t have a microwave, only an electric water kettle. Also, I want to dilute the 100% pure juice with water anyway. ---
**Author:** ![bizzwire](https://avatars.discourse-cdn.com/v4/letter/b/958977/32.png) [@bizzwire](https://boards.straightdope.com/u/bizzwire)\ **Post date:** [March 21, 2010, 11:27pm UTC](https://boards.straightdope.com/t/cracking-glass-with-hot-cold-fluids/533170/16 "2010-03-21T23:27:53Z")
Old Home Economics hint: If you want to play it safe, put a tablespoon in your glass before adding the hot water. This should prevent the glass from breaking. ---
**Author:** ![FatBaldGuy](https://avatars.discourse-cdn.com/v4/letter/f/ecd19e/32.png) [@FatBaldGuy](https://boards.straightdope.com/u/FatBaldGuy)\ **Post date:** [March 21, 2010, 11:51pm UTC](https://boards.straightdope.com/t/cracking-glass-with-hot-cold-fluids/533170/17 "2010-03-21T23:51:19Z")
One thing most people here seem to be missing is that the crucial factor is the temperature of the **glass** , not the liquid. If you take a glass directly out of the fridge and pour boiling water into it, it will probably crack. If you take a glass directly out of an oven or hot dishwasher and pour very cold water into it, it will probably crack. **However** , if you take a glass at room temperature and pour hot (or cold) liquid, then pour the opposite before the glass has stabilized at the new temperature, the shock will not be nearly as great. ---
**Author:** ![ducati](https://avatars.discourse-cdn.com/v4/letter/d/cc9497/32.png) [@ducati](https://boards.straightdope.com/u/ducati)\ **Post date:** [March 22, 2010, 4:20pm UTC](https://boards.straightdope.com/t/cracking-glass-with-hot-cold-fluids/533170/18 "2010-03-22T16:20:28Z")
Remember also, that a thinner-walled glass can take a bit more temperature abuse than a thick-walled glass. The thicker the glass, the greater temperature variation between the inside wall and outside wall. It’s the inner glass contracting more quickly than the outer wall that’s the crux of the matter. ---
**Author:** ![constanze](https://avatars.discourse-cdn.com/v4/letter/c/f9ae1b/32.png) [@constanze](https://boards.straightdope.com/u/constanze)\ **Post date:** [March 22, 2010, 4:47pm UTC](https://boards.straightdope.com/t/cracking-glass-with-hot-cold-fluids/533170/19 "2010-03-22T16:47:36Z")
> [@bizzwire](#): > > Old Home Economics hint: If you want to play it safe, put a tablespoon in your glass before adding the hot water. This should prevent the glass from breaking. I thought the spoon-in-water trick was only for the microwave to prevent supra-heated water? You mean the trick goes back before the microwave, to general hot fluids? ---
**Author:** ![constanze](https://avatars.discourse-cdn.com/v4/letter/c/f9ae1b/32.png) [@constanze](https://boards.straightdope.com/u/constanze)\ **Post date:** [March 22, 2010, 4:50pm UTC](https://boards.straightdope.com/t/cracking-glass-with-hot-cold-fluids/533170/20 "2010-03-22T16:50:09Z")
> [@ducati](#): > > Remember also, that a thinner-walled glass can take a bit more temperature abuse than a thick-walled glass. > > The thicker the glass, the greater temperature variation between the inside wall and outside wall. > > It’s the inner glass contracting more quickly than the outer wall that’s the crux of the matter. Ah, that explains why the Turkish tea glasses are so thin! (and small, but that’s another thing). (Turkish tea is kept in a small kettle on top of a bit kettle with hot water. You pour the very concentrated tea into the glass, and then add the hot water, both with a huge flourish by pouring from one foot above the glass…). Given si blakely’s explanation about Deltas, I wondered why those glasses didn’t crack more often simply from the hot tea. [Next page](https://boards.straightdope.com/t/cracking-glass-with-hot-cold-fluids/533170.md?page=2)