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.
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?