A couple of ways that I clean up crusty old cooking iron are:
One is with an over night spaghetti sauce and hot water soak. Pour a jar or can of cheap spaghetti sauce into the pot or skillet, bring it to a low boil. Turn off heat, let cool for a bit, then put the whole affair in a sink with enough hot water to cover pot or skillet entirely. Let sit over night. Should look foundry new in the morning.
Next, is to use black strap molasses. Best to have a plastic tub to do this one.
Mix about 5 parts water to 1 part molasses and soak your pot for about two weeks. You can get a better deal on molasses from a livestock feed store than the super market.
CP