Bob,
The hydrogen that is liberated on the surface of the pan will lift a lot of the crud. Make sure you have good connections. Even though 6A will do the job, there may be much less current flowing. If your connections are good, increasing the strength of the electrolyte solution will increase the current flow, as will moving the pan closer to the positive electrode. Avoid exceeding the rating of your charger or it could be damaged.
You can scrub off a lot of loose crud with a stainless steel Chore Boy pad without scratching the cast iron.
Jeff