I first cleaned it with hot water to get the acidic stuff out. Then I scratched it with a spatula. After that I used this steelwool which contains soap (Ako Pad
http://upload.wikimedia....jpg/220px-Stahlwolle.jpg). I got it perfectly clean again but the part with scratching it off the bottom was a lot of effort. Maybe a more coarse steelwool could also help but with the finer one you shouldn't even try before you removed the burnt stuff mechanically.
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