This morning, as with many, I let my skillet dry by leaving it on the burner. Unfortunately, my attention was drawn away and I forgot to take it off the burner! Of course, the moment I noticed, I moved it to another burner to cool, but this light-brown ring appeared.
After it cooled, touched the ring and discovered it was a powder, not merely a discoloration:
I'm not sure what this signifies. How do I recover the skillet?
Give it a good salt scrub and re-season. Cast iron is virtually indestructible. As long as it's not cracked, it can always be cleaned up and used again.
It is hard to tell from the picture, but this doesn't look like burned off seasoning, it is more like a burned on residue. If that's the case, you can try cleaning it some way. The problem is that physical cleaning methods probably won't be sufficient, chemical cleaning with alkali will damage the seasoning and chemical cleaning with acid, if the seasoning is compromised, can rust the pan a bit. So there is a high chance you will end up reseasoning.
If this really looks and feels like a buildup and not like exposed oxidised metal, my preference would be to soak in warm, not hot, acid. Vinegar or a citric acid solution should work well. Afterwards, try to scrub off with a stiff plastic brush.
If it doesn't go away, or the seasoning goes away too, or if this was missing seasoning from the beginning, you have to strip and reseason. We have several questions on the topic, and the Internet is also full of suggestions. I personally prefer doing it with lye (best results, strips both old seasoning and rust), owners of self-cleaning ovens like incinerating it, and there are a list of other methods to work with.
Don't worry, it takes far more than that to damage a cast iron pan. The worst that could happen is that you overheat the seasoning on it, in which case it may flake off. If it does start flaking then you need to remove what is loose and then re-season.
I did this same exact thing and I believe the reddish-brown powder is rust. It stuck on my fingers like rust would. When I wiped off the powder, the bottom was no longer slick so I think I burned the seasoning off.
There were no flakes per se, but around the rust area there was a light yellowish ring which I beleived to be the seasoning starting to peel away from the metal.
I used sandpaper on it, rinsed it, and reseasoned it.
That's the great thing about cast iron. Rub it with salt, steel wool, sandpaper, heck even sandblast it. Give it a good rinse, re-season it and carry on. Like @Jason Whipple says, unless you've cracked it, you can usually recover it.
This looks like rust. I believe what happened is you burned the seasoning clear off and quickly so that ash didn't get a chance to develop but went up with the smoke. While cooling the bare iron developed a surface layer of rust.
This happens also when you strip your cast iron in the clean cycle of your oven. As it cools it turns orange, which then needs to be scrubbed off again, and the cast iron put back in the oven to warm just enough to drive off the moisture before wiping on the first layer of oil.
The resolution here is to use hot water and a green scrubby to get rid of the rust. The hot water simply preheats the pan so it dries faster. Put the pan back on the burner, and wipe out the water with a paper towel. Check thenair about an inch over the surface of the pan starts. When is starts to feel like the pan is near cooking heat, pour in a teaspoon or two of your prefered seasoning oil. Spread it out and wipe it back up with a paper towel. Leave the pan on medium low heat, and don't go anywhere. As the oil starts to lump up (it will climb into drops on the bottom of the pan) use a fresh paper towel to wipe them up. Let the bottom of the pan get dry from the heat, then pour in another teaspoon of oil, and repeat 3 or 4 times to restore a few layers of seasoning.
If you wish you can continue this for even up to 9 times SO LONG AS YOU DRY THE OIL EVERY CYCLE, but you eventually lose return on investment. It just won't get any better. The only way to continue to improve the seasoning is to use the pan and care for it properly and the seasoning will build up nice smooth and slick.
