Oven safe dessiccant?
I like the suggested treatment suggested here which is to basically clean/scrub the pan, dry and coat with vegetable oil, invert the pan, place over aluminum foil on an oven rack and bake. To quote:

Now that you’ve returned your skillet to its base layer, you’ll need to season it for use. Drop a tablespoon of vegetable oil on the pan and spread it around with your fingers or a paper towel. (Take care not to burn yourself on a warm skillet!). Oil the outside and the handle, too. Next, take a folded napkin and wipe off the excess oil. You’ll want the skillet to be lightly greased. Set your oven to 350 degrees and put some aluminum foil on the bottom rack. Then pop the skillet in, upside down, on the upper rack for about 1 hour. Turn the oven off and leave the pan inside until it’s cool. The oil will bake into the pores of the pan, providing a non-stick finish.
Repeat this step a second time and you’ll have an almost glassy surface of seasoning.
Ta-da! Your rusted cast-iron skillet is good as new.

The created carbonized layer is safe, chemically inert and durable.