Many is the time I have left a pan on too high and ended up trying to scrape off annoying black marks welded like tar to the bottom. If a pan is non-stick, this is a frequent problem, even if your pan was not on a very high heat, especially if you are trying to caramelise onions. Often I had given up after spending half an hour scrubbing at it and collapsing in a slightly sweaty heap on the floor. However, when it happened to my new, beloved Le Creuset casserole, I had to find a way to get it off. The answer was infuriatingly simple- cover the bottom of your pan with cold water, bring it to the boil, then add a large teaspoon of bicarbonate of soda. This will bubble and boil quite violently; let it continue to boil for a few minutes, then pour away. The marks should wipe off easily, if not, repeat. I am so pleased to say that this does work and I no longer have a huge bill for new pans.