I just purchased a new cast iron skillet. What's the best way to season it?
And if I need to re-season an old pan, is the process any different?
First, there is no difference between seasoning and re-seasoning, unless you need to do some extra work to remove rust (see instructions below). In fact, for new cast iron, scouring is also usually a good idea since you need to get off whatever wax or protective oil the manufacturer or seller may have put onto the cookware. (They don't use cooking oil for that sort of thing, believe me.)
If you need to remove rust: use a mixture of salt and oil and scrub that hard into the rust, then rinse thoroughly in hot water. Use steel wool if necessary.
Next, scour the pan completely under hot water. Do this for several minutes or until the water runs clean. I've heard varying things about whether soap or an SOS pad is OK at this step. My personal opinion is that it's OK, but you must wash the soap completely off before continuing with the seasoning so that your seasoning doesn't taste like soap!
Coat the cookware with grease or oil. (I do this with the whole pan, not just the cooking surface, to reduce the possibility of external rust.) Crisco, vegetable oil, and lard all work well. Don't pick something with a low smoking point, or too strong a flavor. Make sure it's a light coating... you shouldn't have pools of oil anywhere on your cookware.
Bake your skillet in a 250 - 350 °F (121 - 178 °C) oven for an hour. If you used liquid oil, you may want to put the cookware in upside down so excess oil drips off. But it's good to put a cookie sheet or something underneath the cookware to catch the drips if you do!
Let the cookware cool, and wipe off any excess oil.
For best results, do this two or three times, though a skillet can be satisfactory after a single treatment.
To keep the seasoning happy:
Don't let the cast iron sit too long without using it (you may notice a rancid smell or flavor if the seasoning turns bad; I'm not sure at what point this happens, but it's happened to me before).
Don't cook anything acidic (e.g. tomatoes) during the first or second use of your pan.
Don't use dishwashing liquid or soap on the pan (hot water and scrubbing only).
After cleaning the pan after each use, wipe it lightly with another bit of oil, using simple vegetable oil.
Another trick sometimes used to season Chinese woks: rub Chinese chives over the surface of the cookware when the oil is being heated (this works best on a stovetop, not in the oven). The juice of the chive has sulfur compounds that help remove remaining flavor from the previous coating of the cookware. Be aware that this technique really kicks up a lot of steam and smoke. I've never tried it on cast iron skillets, but I'd be curious if anyone out there has.
See this excellent article about the chemistry of seasoning. You want flaxseed oil (which incidentally has a low smoke point) but a high iodine value, allowing it to polymerize readily.
I read so many blogs about oil "impregnating the cast iron," but this doesn't make any sense chemically. What happens is that the oil polymerizes, and you want an oil that does that really well.
To season a pan, preheat your oven to 300°F (150°C). Preheat the pan on the stove top. When warm, coat the inside surfaces of the pan with vegetable oil or lard. Continue to heat just until you see ripples appear on the surface of the oil. At this point, pour off any excess oil, give it a quick wipe with a folded paper towel held in a pair of kitchen tongs, and then put the pan into the oven for 45 to 60 minutes. Remove the pan from the oven and allow it to cool to room temperature.
The more important part may be how not to unseason the pan. You can never, ever, clean it with soap. Or scrape it with brillo. Just warm/hot water and cloth.
Seasoning is literally "greasing" the pan. And soap is the enemy of grease. Good when washing your hands. Bad when cleaning cast-iron.
This creates a bit of a catch-22 since a poorly seasoned pan will require heavy cleaning. But once you do it right, it will last for months as long as you don't undo the seasoning with soap or scraping.
From the excellent Mike Saxon over at Chef's Tales:
On receiving a new, straight from the supplier, cast iron frying pan or sautéing pan, we used to first place it on a solid top stove, empty with absolutely nothing inside and get it so hot that it would be smoking.
We would then place enough course sea salt in the smoking hot pan to cover its entire cooking surface until it was about 2cm thick. We would leave the hot pot on the stove with the salt and slowly burn and cook the pot with the salt inside. The salt will very slowly “cook” by smoking, burning and turning the salt very slowly to a dark brown colour.
We would then discard the salt in a bin, get a piece of cloth (a towel cloth like an old bath towel or face cloth) dip it in oil and wipe or rub the pot until the hot metal would absorb the oil making the pan shiny.
When the pan is smoking hot, we would them wipe the pan clean with another piece of kitchen towel and then proceed with the whole process all over again from the beginning.
After doing this 2 or 3 times the metal will have absorbed the oil making the metal very shiny, “seasoned” and have a homemade non stick effect. When you have done this you need to try out the pan by making an omelet, if the eggs stick, you have to start all over again until the pan cooks an omelet without the eggs sticking.
The act of “seasoning” the pan may take a few days, a week on more than a week, but it is definitely a loving care process.
This doesn't directly answer your question about a full re-seasoning, but is related to seasoning.
When cooking with cast iron, I find it easiest to clean the pan immediately, while it's still very hot, with very hot water (our tap gets hot enough to scald if you're not careful).
Then, I thoroughly dry it with a towel, then a paper towel (to get what the towel may have missed).
Then I immediately oil it (I use grapeseed because of its cost compared to its smoke point, but have used bacon grease, olive oil, or whatever else is around) and put it back on the hot burner.
Using this method, I haven't had to re-season my cast iron pans yet.
As long as you don't use soap or scouring pads on a (seasoned) cast-iron skillet, you should practically never need to re-season it. Just dry it thoroughly and rub on a little vegetable oil after you clean it and before you put it away.
Chris's answer is the right one for initial seasoning: Just "bake" the skillet coated in oil. Most cast iron cookware comes with directions regarding temperature and time, and there are minor differences between brands, but 350° F for 1 hour is a good rule of thumb.
One tip to use on a cast-iron that is severely encrusted or has burnt sugar residue is to burn it off. I have done this twice with great success. In one case we made a large hot charcoal fire in an outdoor BBQ grill and buried the pan in the coals. The other was done similarly in a fireplace hearth making sure that the inside and the outside were both in contact with hot coals as much as possible. After letting the fire burn out, and LETTING THE PAN COOL!!, it was necessary to remove a small amount of residue with sandpaper and/or wire brush. The pan is now in brand new condition except that it will look very rusty and will continue to rust unless you treat it immediately. First scour it with a stainless steel pad and some kind of oil (I actually prefer mineral oil for this step) to remove the rust. Wipe it thoroughly with old cloth towels and then proceed to season by one of the methods detailed above. I prefer to do this several times before cooking with the pan. I have always used a solid vegetable shortening for seasoning with good results.