Removing carbon from cast iron skillet

I recently came across a rusty, carbon encrusted dskillet in a garage sale and tried to restore it. I got out the rust, but the carbon continues to flake off and I can't cook with It. Have tried salt, grill brush, soaking in vinegar and water (worked for the rust). I do not have a torch, access to a fire, won't use oven cleaner, carbon remover, or lye. Want a natural method without chemicals. Thanks, Myra