The scientific chef

Peter Barham on tea scum

When making teabag tea, an iridescent film forms on the top, and then clings to the side in the form of a brown scum while the cup is drained. What's causing it - the tea, or the London water?

Leanne White, the Big Smoke

It never fails to amaze me that such seemingly simple questions can be the cause of real scientific controversy. Up to the mid-1990s scientists believed the scum on a cup of tea came from a thin layer of a waxy substance that coated the leaves and helped to waterproof them. When the leaves were put in hot water, the film melted to form a thin oily layer that floated on the surface of the tea. However, in 1994, chemists from Imperial College did some very careful research. They sampled the scum from cups of tea made in different ways and with different types of water, and did detailed chemical analyses to find that a key component of the scum layer is calcium. The scum, or at least 15% of it, is calcium carbonate - the rest being a lot of complex organic chemicals. In other words, it is not oil.



· email questions to Peter Barham is a physicist at Bristol University.