Go to the hardware and get a can of Lye crystals that are used for stopped toilets.
Get a container big enough to submerse the skillet/pot/griddle in.
Add enough warm water to cover the item.
Carefully add the lye to the water - DO NOT BREATHE THE FUMES! DO NOT GET THE MIXTURE ON YOU OR YOUR CLOTHES!
using a hook,heavy wire, or something similar,gently lower the item into the lye water
There will be a fairly strong reaction and many bubbles
After about 20-30 minutes,remove the item and hose it off.
Dig a hole and dump the lye water into it.
coat it with vegetable oil, lard or bacon grease... dont use corn oil.
put it in 250 degree oven upside down,on a cookie sheet for 30 minutes.
After it cools, scrub it down good and repeat 2 or three times
It's easy to crack cast iron utensils by putting them in a fire or dropping them on a fairly hard surface.
I have a large collection of Griswold items and the above is the easiest method that I know of to clean them.
Old Geezer