I had success in cleaning cast iron by using electrolysis. An internet search can find you more info, but the basic idea is submersing the piece of cast iron in a conductive solution (I use washing soda and water) with another piece of metal, and applying a current, with the cast iron as the cathode (negative) and the other piece of metal as the anode (positive). This will strip any carbon, rust, and any other accumulations until you're back down to raw cast iron. This method minimizes any scrubbing needed, and you don't have to deal with harsh toxic chemicals, nor risk damaging the piece by superheating it in an attempt to burn everything off.