I don't drink coffee, so I use my Bodum French coffee press for tea. The press is elegant, it’s a perfect size, and it can withstand the shock of boiling water I pour over loose leaves. It also usually stays in the cabinet because when I pour from it, it spills. Every time.
When we spill liquids, we do so for very specific reasons. We are drunk, for instance, or clumsy. I myself am stranger to neither state. But even the most steady and sober imbiber can end up with a spreading wetness when pouring from a vessel that has the wrong kind of lip. As much as I like the Bodum press, its lip — thick, rounded — is the wrong kind.
Any number of videos online may be found showing Indian chai wallahs “pulling” or “throwing” tea for their customers (see, for instance, this one). Bartenders may recognize the move as first cousin to the back-and-forth tossing of high-proof whiskey needed to create a Blue Blazer. Well, minus the flaming whiskey. Some think that thick mugs able to withstand high temperatures are de rigeur for bartenders and home enthusiasts wanting to recreate the 19th century Blue Blazer. But it turns out that they may be handicapping themselves by using clunky old pewter mugs.
In their book Mangoes & Curry Leaves, Jeffery Alford and Naomi Duguid describe the long arcs of hot liquid Indian tea sellers pour to froth their tea:
“Throwing tea” is a subcontinental tradition. A person making tea will often pour the milk and tea mixture from one container to another and then back again, over and over, in order to blend and froth the tea. You’ll see people do this all over the Subcontinent, but nowhere as dramatically as in South India where a tea maker will have an arc of tea that is three to four feet long flying through the air. An expert thrower never spills and can work with the smallest of containers, even while gazing in a completely different direction…They go on to say, almost in passing, that one of the tricks to learning the move is to use containers with thin lips. This is an important note. It turns out that fat-lipped containers are particularly prone to dribbles and spills. In fact, there’s a name for the phenomenon: the teapot effect.
It is simply that at the pouring lip the pressure in the liquid is lower than the pressure in the surrounding air, so that the air pushes the liquid against the lip and against the outside of the pouring container.In a pouring container with a thick, fat, or rounded lip, this actually can cause the liquid to flow backwards along the rim of the pouring container and along its outer surface. That’s where the dribble comes from and why I end up with tea on the counter. There’s more — much more — to be said about the teapot effect; streamlines, flow rates, atmospheric pressure, velocity vectors, etc. Jearl Walker offers a more detailed examination of the forces at work here.
The take-home points for bartenders, drinks enthusiasts, and those who would practice throwing tea with minimal spillage, though, are:
- Use containers with thin lips. Most two-part Boston shakers, for instance, are perfect. But pouring from the metal canister rather than the glass is less likely to cause spills.
- Pour from containers that are only partly full. Once it hits the lip, the liquid from a partially full glass is moving at a greater velocity and is less likely to spill along the outer container. Also, in order to spill, the liquid would have to turn a large angle — which is unlikely.
- Increase the angle of the pour as much as possible. Poured at a right angle (90°), a liquid has far more opportunity to travel back along the outer surface of the pouring vessel. Increase that angle, and you’ll end up with a cleaner pour.
- Pour quickly. Liquids traveling at greater speed is more apt to go where you want it.
I still use the Bodum press — after all: perfect size, can withstand boiling water, and all that. But after reading Keller, I now know why it's better not to fill it quite so much and to pour quickly. There's nothing I can do about that lip, though.
Goes well with:
- Jeffery Alford and Naomi Duguid (2005) Mangoes & Curry Leaves: Culinary Travels Through the Great Subcontinent. Artisan Books, New York.
- Joseph B. Keller (1957) Teapot Effect. Journal of Applied Physics, Vol. 28, No. 8, pages 859-864.
- -- (1988) Spilling. In Kurti, Nicholas and Giana (ed) But the Crackling Is Superb: An Anthology on Food and Drink by Fellows and Foreign Members of the Royal Society. Adam Hilger, Philadelphia. I mentioned this book a few weeks ago in a confession for my love of port wine.