How to look after an aluminium pot so that it does not turn brown
The secret is vegetable cooking water!
Pure aluminium conducts heat well, spreading it quickly and evenly, which is why it is so widely used for Japanese cooking pots such as the yukihira (hand pot), the yattoko pot and the dansuki pot. As soon as it is used, aluminium forms a film on itself called aluminium hydroxide, which coats the surface so that it resists corrosion, lasts well and poses no danger in cooking. But the film that forms usually leaves brown staining inside the pot, so the pot no longer looks as handsome as it did when you bought it.
The trick to forming a clear film on an aluminium pot rather than brown staining is to boil water with vegetable trimmings the very first time you use it. This is a piece of Japanese wisdom passed down from one person to the next. It is believed that the complex chemical reaction from the vegetable water helps form a colourless film on the surface of the pot. Boiling vegetable water on first use keeps the pot durable and good-looking for a long time. Even so, if the pot darkens after a while, that is perfectly normal.
How to fix it If your aluminium pot does darken and lose its looks, boil water with sliced lemon in it for about 15 minutes. The staining on the aluminium will gradually disappear and the pot will look as good as new again.