Always wear latex gloves when using this method. Step one is to spray a heavy layer of Easy Off oven cleaner on the cast iron. This can be used to clean off heavy carbon build up, but will not remove rust. This method is completely safe and washing the cast iron in soap and water will remove all the lye from the piece. Once stripped and washed, if there is no rust you can now move to seasoning your cast iron. If there is rust, you will need to remove the rust next.
The second step is to place your cast iron in a trash bag and sit it outside in the sunlight. This will work in the cold, but works best on warm days.
After 24 to 48 hours, you should be able to wipe it off or use a SOS pad to scrub the carbon off. If any is left, simply repeat the steps until clean.