For dinner tonight I baked a sweet potato and spinach fritatta in one of my cast iron skillets. When it was done and I removed it from the pan I noticed a thick layer of baked-on egg all along the sides of the pan and realized it was time to re-season it.
I've had my cast iron for decades. I used to use it on top of the stove and in the oven, but several years ago we got a range with a glass cook top. I was afraid the heavy pans would scratch it up, so now I only used it for baking. I like how cast iron cooks food evenly, but I don't like that evenly-cooked food stuck on the pan!
The seasoning process is easy but time consuming. First I removed the cooked-on crud by scrubbing it with a mixture of salt and oil, rinsed the pan thoroughly in hot water, and dried it well. Next, I smeared it with a light layer of shortening (inside and out, while I was at it), then put it in a 300 degree oven and baked it for 2 hours. When it was done I took it out and wiped it down with a paper towel. I noticed the outside was nicely seasoned, but the inside still looked a little dull so I repeated the whole process again. This time it looked better, but I'll know for sure the next time I use it.
A good seasoning should last for months, as long as the person cleaning up after dinner remembers not to wash the pan with soap!