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I started boiling water in a small, stainless steel, copper-bottom pot. Old story: I forgot about it, the water boiled away, and the pot spent a couple hours on maximum heat from my electric stove.

I noticed it as I sat reading: a funny smell was coming from the pot, not unlike the smell of burning plastic. I opened the pot and saw the bottom covered with this dark grey matter that doesn't wash easily. What could it be? enter image description here

Then, while washing the pot, the nice dark copper color seemed to wash away. Now I have this. What happened, here? enter image description here

Lastly, now I have white spots on my cooktop where the pot sat. What could these be? enter image description here

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up vote 13 down vote accepted

The first photo is of mineral deposits left behind when you boiled your water away. If you water is really hard (full of minerals) you may get these even in normal use. Barkeeper's Friend cleaner and some hard work can remove it.

The second photo is showing that you actually managed to clean the dark oxidation off some of the bottom, but not all of it. When copper is really clean (free of oxidation) it's quite bright and pinkish or orange. The way your pot was before, it had a fairly even layer of oxidation, which makes the copper brown. You could either clean it all the way--again, barkeeper's friend is a good bet--or leave it and it will soon be brown again.

The last one I'm just guessing about. I think it is probably mineral deposits too, if there was water underneath the pot, or if if boiled over. I do not know if something like Barkeeper's Friend would be appropriate for your cooker, though. It could also be oxidation or staining from the pot--something on the pot that burned off and left a residue or discolored the surface. It's also possible that the hot pot actually damaged the surface, in which case the discoloration may be permanent.

In any case, your pot is not damaged if the bottom is still flat (sometimes overheated pots warp). Cleaning it will make it look nicer, but none of the things you're seeing are likely to have an impact on how it cooks.

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I mostly agree, though having a much darker spot on the bottom of the pan will tend to produce something of a hot spot, because it'll absorb more heat from the burner. –  Mark Bessey Mar 16 '11 at 18:23
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That spot on the inside may actually be from overheating the stainless, which does discolor at high enough temperatures. Also, vinegar will usually remove hard water deposits. Be glad it wasn't an aluminum bottom (or core), that may have melted. –  derobert Mar 16 '11 at 21:51

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