my 'nuclear option' for cast iron skillets is to put them in a basin of lye solution, and let the pan sit for a week or a month (depending on how fresh and how concentrated the lye solution is.)
We keep a plastic basin out back for this.
lye is bad stuff, so if you have kids or pets, do please be careful.
The lye, however, will take off anything organic, and bring the skillet back to bare metal (well, some scrubbing may be necessary.
use a plastic scrubby).
in this condition, the skillet will start to rust instantly.
I work as follows: after the four weeks, take the skillet out of the bath, scrub it and hose it off, then immediately pour white vinegar on it: the acidity prevents rusting.
Then put it in a 450 F preheated oven for half an hour.
Then start the reseasoning process.
As I said, this is the method of last resort.
Here is a photo of a #3 cast iron pan, rolled back to 'year zero' after a few months in a lye solution, next to a seasoned #2 pan.
The iron actually looks like metal.
In this condition, you can check the surface for pitting, cracks, etc.
The vinegar is on hand to prevent rust.
cooking.stackexchange.com/users/1854/shabbychef
I thought that as it was cast iron you could just put the whole thing in a big fire and it would burn off whatever was burnt on and then you could scrub off the ashes, which should be fairly easy to do once the are realy
just char..
The second comment here seems to think you can do this as well.
You might also be able to do this in a couple of other ways:
Put the pot upside down over a high flame to burn off the food.
they used to do this at a Chinese restaurant I worked at to clean the woks.
The woks weren't cast iron though.
Put the pot in a self cleaning oven on the clean cycle.
Disclaimer:
I haven't done this, but I think it should work.
If it gets voted up, then you should be ok, if it gets voted down,
[peer pressure] for me.
You'll need to re-season afterwards.
cooking.stackexchange.com/users/210/Sam Holder
I had two pans that were in pretty bad shape from an antique store.
My "reset" button for them was an overnight camping trip.
I stayed in the tent.
They stayed in the campfire.
I built a large batch of coals and, before I went to bed, I buried the two pans in the fire circle under about 4 inches of coals.
In the morning, I pulled them out and scrubbed them down with sand and some steel wool
and they went straight to the seasoning process.
The fire carbonized the caked on food which then scrubbed off easily.
The #8 is now one of my favorites.
cooking.stackexchange.com/users/2291/yhw42
Scrub the heck out of it with Kosher salt and the tiniest amount of water, then start all over again seasoning it.
A fair amount of work, but well worth it to save a good skillet.
cooking.stackexchange.com/users/1393/Michael Natkin
Biological laundry detergent (just mix the powder into a paste) and leave somewhere warm for a day or so will remove anything organic -
this stuff is scary.
If anything is left, scrub with a paste of bicarb and water
cooking.stackexchange.com/users/4483/Martin Beckett
A sandblaster will quickly remove burnt-on stuff from a cast-iron pan.
I gave a pan to a friend with a sandblaster to treat it for me, and he never gave it back.
So, that's a risk with this method.
I have scraped with a table knife .
That worked well for burnt-on stuff in a small area, but it's a huge job to clean the whole pan.
Coarse steel wool or brass wool can work, too.
Many people don't realize how much stuff is burnt on to their cast iron.
They're not supposed to be black & rough, but dark grey and very smooth .
As others have said, be sure to season as soon as they're clean.
Or at least oil them to protect against rust until you're ready to season.
cooking.stackexchange.com/users/2054/Jay Bazuzi
Seeing as I am an hard core new Englander, having lived in new Hampshire all my life, and very savvy to the old way.
If your cast iron pan gets a lot of build up due to burnt food, throw it in the stove the next time you put fire wood in.
When that filling or wood is burnt down and ready for a refill, pull out the pan, using a set of tongs to grab it with.
The dam pan is hot and will burn you bad.
let it cool and then steel wool
it and your good to go
cooking.stackexchange.com/users/20622/user20622
I got most off with single edge razor scraper.
( a bit off work, heavily coated).
I then thought of oven cleaner.
Let it soak awhile in garage and was able to get it clean enough with scraper again and wipe off with rag and one off those scrub pads.
Much easier.
Rinse under hot water and dry off.
I think I'll try to find sandblaster though (local welder?)
for outside and touch up inside if cheap enough.
Will offer him 5 if he will do next time he uses blaster for his own work.
cooking.stackexchange.com/users/25664/Steven Donohue
I just thought I would add what to do with an ENAMELED cast iron pot, like a Le Creuset.
I had a doozy once, just black carbon scalded to the enamel.
After trying everything, I put it on a hot flame (gas range) and kept dripping in white wine and scraping the bottom with a wooden spoon.
Took about a half hour and a whole bottle of wine
but it worked...
or maybe I was just sipping on the wine as I went, and that's why it took a whole bottle :)
cooking.stackexchange.com/users/25572/user150153
I have this cast iron pot that my friend completely burnt. I was wondering what might be done to save this pot. I realize that this question seems similar to  this one  but in that one his skillet is not burnt and it is seasoned.   Thanks for your help.
How to clean a burnt cast iron pot?
cooking.stackexchange.com
<equipment><cleaning><cast-iron><pot>
cooking.stackexchange.com/questions/7127
cooking.stackexchange.com/users/27/Kyra
An extreme solution would be to put the pan in a lye solution for a week or month, depending on the freshness and concentration of the lye solution. You could also put the pan in a big fire or self cleaning oven cycle or scrub it thoroughly with kosher salt and a tiny bit of water or steel/brass wool before starting the seasoning process again. Covering the pan in a paste of biological laundry detergent and leaving in a warm place for a day or so can also remove anything organic. The easiest solution is to scrub with salt or steel wool. You can also try the self-cleaning cycle on an oven.
Leave to soak in a lye solution. Put it in a fire to heat and then scrub off. Try the self-cleaning cycle on a oven. Try scrubbing with salt or steel wool. Try putting laundry detergent on it and then leaving to soak.