What is the thin film that forms on green tea when left out?

chlorPHYLL?

Any chance it’s not from the tea but from the water?—i.e., from the kettle you boiled the water in? If you used tap water and have hard water, as we do, the stuff in it can seem to leave a film. Tea is thin and pale enough to show it, where coffee or soup might not.

Easy enough to test by pouring a cup of hot water from the kettle and watching to see what happens.

Meanwhile I would not expect any problems from the tea.

This happens to tea in my mum’s house but not mine. It’s an effect of hard water. Safe to drink, but tastes better and won’t leave a film on top if you use filtered or softened water.

I’m thinking it is the essential oil in the tea leaves which separates from the infusion created by steeping the tea leaves. Essential oil is a volatile oil rather than a fixed oil like canola or olive oil, but like a fixed oil it doesn’t remain dispersed in the tea infusion. Neither essential oils nor fixed oils remain dispersed in water unless an emulsifier is used.

I have noticed that, too, just in my regular Lipton tea. It couldn’t be the hard water, because I make it with Olympus bottled water here in the office. And it has to sit for a good while before the oil forms on top. Must be the natural oils in the tea leaves, as @CardAngel said.

My son is a serious tea fancier who buys and delicately brews expensive teas and boils his water in a glass kettle. He has complained of this film when he made tea at our house in California using the same batch of tea leaves (from the same foil pouch) in the selfsame kettle that he had used in Vermont. The only variable was the water. He said it never happened there.

The film is apparently caused by calcium in hard water: http://www.theguardian.com/lifeandstyle/2005/nov/04/foodanddrink.features

http://discovermagazine.com/1994/sep/teadoff421

You will not get the film if you use reverse osmosis filtered water to brew your tea.