How to prevent broken glass when pouring hot tea?
A number of people, at some point in their life, have the unfortunate experience of pouring a hot liquid, such as boiling water or hot tea, into a glass, to find the glass shatter or crack. Why does this happen? And how can we prevent it from happening?
Even the most heat-resistant glass, like this borosilicate glassware, is subject to shattering under thermal shock.
Why does some glass shatter when heated?
The phenomenon which causes glass to shatter when we pour boiling water into it is called thermal shock. Wikipedia has a very technical article on Thermal shock that is probably more than most of you would need to know about it.
Unfortunately, not all cups or vessels are suitable for handling hot liquids with ease. Typical glass, in general, is not able to handle such heat very well. The reason is that as the glass heats, its density changes; it expands. Pouring boiling water into a glass is highly likely to shatter it, because the hot water contacts part of the glass first, whereas other parts of the glass (such as the outside of the cup) remain cooler. The glass thus does not expand as a whole, but is pulled in different directions as part of it expands and part does not; this difference produces the shattering.
Glass is less likely to break or shatter if we warm it up gradually. Pouring boiling water into an ice-cold glass is much more likely to shatter it, as is putting a hot piece of glass into a cold bath of water.
This effect of thermal shock is strong enough that it even can cause special types of glass like Pyrex or borosilicate glass, used in a lot of commercial labware, to shatter. I remember this from a chemistry lab in high school: we produced a solid in a test tube and needed to extract it, and to do so easily, we heated the tube to a high temperature, and then put it in a bath of cold water. This procedure caused even the heat-resistant labware to shatter.
Preventing breaking:
We can prevent teaware, cups, and mugs from breaking by following two guidelines:
- Avoid using generic glass for hot liquids; stick to ceramics or glass that we know to be heat-resistant.
- Even when using ceramics or heat-resistant glass, avoid very large, sudden changes in temperature.
Properly fired ceramics can handle the shift from room temperature to boiling water just fine, but a big enough shock will shatter just about anything. Have you ever had any of your cups, mugs, glasses, or teaware shatter due to thermal shock?
No, but I once used a glass plate to cover something I was microwaving, and then I immediately rinsed it off with cold water, and it broke. I guess that wasn't the smartest move.
ReplyDeleteOh yes, I learned this the hard with with a favorite, hand-blown glass pitcher!
ReplyDeletehttp://sirwilliamoftheleaf.blogspot.com/2010/04/fallen-comrade.html
ReplyDeleteIt was the unfortunate fate of this piece of teaware...
A couple months ago I went to add a small amount of cold water to my ceramic kettle, and as I was swirling it around the inside as I usually do to cool down the clay to avoid thermal shock from adding more cold water, I heard a loud pop. The bottom of that kettle is now an optional extra.
ReplyDeleteYes I had this experience with la opals tea mugs. 4 cups from my previous set and today I lost one from my new set. I generally wash the cups again just before serving, so washing with warm water may avoid this cracking problem.
ReplyDeleteI make coffe from ground coffe every day in a glass, I pour boiling water into the glass over the coffee grounds. In 10 ysars I've never had a glass shatter...
ReplyDeleteYou sound lucky--especially after some of the other posters here. It might have something to do with how you're pouring the water, with the type of glass you're using (including both the material and shape of the glass), or perhaps the glass being warmer. But hey, if it works for you, by all means keep doing it!
DeleteI just would caution others, because it seems to cause problems in many cases.
I would add a piece of advice, if you have a hot water tap, use it to warm up your glass vessel as hot as is comfortable for you to handle, both inside and outside, then empty the vessel and pour in your boiling tea/coffee/etc. With this pre-heating even the cheapest generic glass (probably) won't break.
ReplyDeleteUnfortunately I can't follow my own advice, since I live in Thailand, where most kitchens don't have hot water (since it's not really necessary). So I just had a glass pitcher crack by pouring hot tea into it. Fortunately it didn't break into many pieces, just developed a big crack in the bottom and started leaking. My glass tumblers can handle the same tea. Room temperature here can be around 25-35C.
It helps to thoroughly pre-heat the glass with tap hot water. Then it will most probably not break when pouring boiling water inside.
ReplyDeleteBut I can't follow my own advice since I live in Thailand and here most kitchens don't have hot water (since it's not really necessary).
I poured hot water from the tap all over the outside of a tall 0.5 litre drinking glass and then put cold water from a drinking cooler into it - the glass still cracked.
DeletePs. I took two of these from a holiday in Sydney back home to Perth on the plane (since the same thing happened to the other one I had) and one of them cracked within a week. I hope the second doesn't brake. I feel like a fool each time this happens.
Yeah...some glass vessels can be particularly fragile and sensitive to thermal shock. I've also noticed this with ceramics. People I know who work with ceramics have told me that this is often a result of improper firing.
DeleteWith glass, I wonder if there is something about the material, the method of manufacture, or the shape, which makes certain glasses prone to breaking in this way.
I will say, if you have a set of glasses and one breaks, I'd be cautious with the others. I have a set of mugs which have cracked in this manner, and I have had two crack...so I tend to just not use them with hot liquids any more.
MD, is this correct: "I poured hot water from the tap all over the outside . . . and then put cold water from a drinking cooler into it"? What you're aiming for is gradual temperature change, so the hot tap water is an intermediate step for boiling liquids; it would be the wrong way to go for cold liquids.
DeleteAhh, yes, pouring hot water from the tap is an intermediate step if you're readying the vessel for boiling water, but it would make things worse if you're getting ready to pour ice cold water. That's where you'd want cool/cold water that's an intermediate step.
DeleteI missed that because I didn't read MD's post closely enough. Thanks for pointing that out!
Hey there well explained blog. . I just got my glass broen due to hot water pouring. . Now I know the reason y. . . Thankx pal
ReplyDeletethx for the answer it helped me out in science class :)
ReplyDeleteStick a metal tea spoon into the glass before pouring in the boiling water. This will prevent shattering and cracking most of the time...but not always. The bigger the metal spoon, the better.
ReplyDeleteWhy would glass test tubes break when thawing at room temperature? I had a few blood specimens in glass test tubes and they broke after I thawed them and tried to centrifuge them.
ReplyDeleteI'm not sure of this one. Some substances expand upon thawing, and, while water has the opposite pattern, I could imagine that if these samples were in a confined space, there could be something going on with the other substances other than water, that caused them to expand.
DeleteBut really, I don't know...I haven't ever worked with these sorts of materials.
I heard the same thing about putting a spoon in the mug. On airplanes, they say to do this when there's turbulence. Supposedly, it keeps the cup from falling over and spilling all of its contents. I hope this helps.
ReplyDelete