Excellent, Griswald cast iron, pretty good stuff, if not the best.
For really bad grungy cast iron, I use the electrolysis method:
- A 6 amp manual battery charger, smaller will work, just take longer
- Arm and Hammer laundry detergent (not baking soda)
- plastic bucket
- sacrificial piece of steel (NOT stainless steel!); rebar works good and is cheap
I forget the amount of detergent for the amount of water, but it's not real critical. Attach the charger "black on black" i.e. the negative terminal to the cast iron, the positive to the sacrificial piece of steel. Rotate the pan to ensure complete coverage electrolysis works via "line of sight". A day or two later, the pan should be completely clear of crap and down to bare metal.
Here's a good link discussing electrolysis:
removing rust
Good luck.
Merlin