Thank you!
By "shortening" you mean like Crisco?
Not sure how to get that "red hot". Maybe put them inside the grill on high?
You might no be able to get it glowing bright red, but get it as hot as you can. Put it in a hot wood fire and cover it with coals, or an oven with a self-cleaning cycle, or like you said inside a grill turned up high. My mom used to put her gunky cast iron frying pans in the wood stove once every few years.
By shortening, I mean like Crisco.