I've been following this thread carefully in the hope of finding a cure for my blackened aluminium pot (c. 1950 or so) which (like Richard's)most definately is not any kind of deposit that can be scraped off. It is a stain that resulted from boiling water with a few drops of olive oil in it a couple of nights ago with a plan to make pasta. The water was turned off and left for the night and by morning the pot was black up to the water line. Despite doing first year introductory chemistry last year, I don't understand the carbon/aluminium reaction (oxidisation?) so if anyone is up to explaining it (along with what de-staining method to use), I'd be very interested. We are new to this area (Northern Territory, Australia) and I'm wondering if the high chlorine or calcium levels play a part as well. Speaking of which, I'm used to dealing with kettles going black, but here they go white!!!! Any ideas?