2009-01-11
The Science Of Tea Cosies
A friend on Twitter (who has protected their updates, and thus gets to be anonymous) asked this morning:
Do tea cosies actually work? My physics brain say ‘not much’, but my British brain says ‘WHY WOULD YOU QUESTION THE WAY THINGS ARE?’
My response on Twitter was straightforward:
Why not do science? Get an oven thermometer, stick it down the spout, and let hot water cool a few times with + without tea cosy.
Of course, this is fine as far as it goes, but when asked “how fast do things fall”, physicists almost never go back to first principles, but instead use established theory. More to the point, I don’t have the right sort of thermometer.
So, one assumption, up front. As you’ve probably gathered from my reply, my definition of “work” is the perhaps narrow “does a tea cosy keep the tea warmer for longer”; there’s no consideration of how the tea actually tastes.
Now, there are two relevant mechanisms for heat transfer here: conduction and radiation. When you pour boiling water into a cold teapot, the water cools because some of the energy is conducted to the pot. This is why most instructions for tea say you need to “warm the pot”; it reduces the magnitude of this initial loss. (Since the initial brewing temperature is more responsible for the way the tea tastes than any subsequent cooling, this is probably more important if you want your tea to taste nice, rather than just stay warm.)
Once the pot is warm (technically, ‘approaching equilibrium with the water’, I suppose), radiation takes over: the pot loses heat to the air by radiating it away. This is where tea cosies come in, and they have two effects. Firstly, they replace radiation from the pot with conduction from pot to wool (or nylon, or whatever), and typically fabrics are not as good at warming up as porcelain is, meaning more heat stays in the pot.
Secondly, what heat does reach the cosy isn’t radiated as readily. This means that the wool won’t cool down, drawing more heat from the pot. (In contrast, the radiated heat from an uncovered pot means that the water inside has to lose yet more energy warming the porcelain up again).
In summary, then, even without experimental evidence, I’m convinced that a tea cosy will indeed keep tea in a pot warmer than tea in an uncovered pot.
