Once you’ve cleared your cast iron cauldron of all the rust and accumulated gunk, you may find that you’ve actually stripped it bare of any “seasoning”. If this happens, the piece will look more “silver” and less black (though perhaps not as silver as a new piece, such as pictured above). Unfortunately, iron plus oxygen and water equals rust, and oxygen and water are both present in the air. Unless the iron is protected from contact with the air by some sort of coating, it will rust again.
Enter the concept of “seasoning”. All this really means is accumulating layers of polymerized fat molecules onto the surface of the iron. If those semesters of high school chemistry are escaping you, polymerization basically just means that smaller stable molecules (monomers), get a bond or so broken when exposed to heat. That forces them to link up with each other and with other molecules, and that results in a very large molecular chains (polymers), which will have a distinct three-dimensional network. In the case of oils, the liquid oils would become a solid slick. If the slicks get layered, they will eventually bond to each other and form an impenetrable coating around the iron. This is the beginning of seasoning. With time and use, the seasoning will further incorporate more carbon and magnetite into the matrix, making the seasoning coating a shiny, rich black color. It also further hardens the coating in addition to making the cooking surface more even (and therefore more slick).
Seasoning a cast iron pot is really simple: all you need are oil and heat. Simply cooking a lot of bacon batches in the pan or pot will eventually season the pan. However, starting the seasoning process in this way can result in uneven seasoning, which can be discoloring and can impede function. To really get a pan off on the right foot, a little technique is required.
According to the blogger Sheryl Canter, whose experiments were later held up by the test squad at Cooks Illustrated, one big key in the seasoning process is to be discriminating in the type of oil used. Canter uses flaxseed oil in her seasoning because it is the food grade version of linseed oil, the very oil that polymerizes when exposed to air alone and creates a hard coating. In fact, linseed or flaxseed oil is one of the most “drying” oils that nature produces, and that “drying” factor is simply how quickly and evenly that oil turns into a solid film…which is fat polymerization. Linseed oil excels at this: it is the base for oil paints and stains, which ‘cure’ to solid touch at room temperatures in 1-2 days and cure to their final, most solid state within a month. Heat only quickens the process.
Using flaxseed oil on a cast iron pan and applying heat, then, will create a hard, durable seasoning much more quickly than with any other oil. The only other trick is to apply thin coatings of the oil and apply even steady heat.
The first step in the process is to take the stripped cast iron pan and heat it up a bit. This accomplishes three things. In the first place, it ‘opens up the pores’ of the pan, or expands the microfissures in the surface. This will allow the oil to penetrate more deeply into the pan and eventually fill up the microfissures to create an even, slick cooking surface. In the second place, heat will prevent any water molecules from being present on the pan’s surface as the oil is being rubbed on, and that will prevent rust from occurring behind the seasoning later on. In the third place, that heat will help transform any rust molecules already present on the pan’s surface to magnetite, which will help prevent further corrosion (and will help contribute to the even, black color of a properly seasoned pan).
Therefore, begin the seasoning process by heating the cast iron piece in a 200°F oven. When it’s reached temperature (15-30 minutes), remove it from the oven and place it on a paper towel. Pour a little flaxseed oil onto the piece, and rub the oil all over the cast iron with your hands, making sure to cover every surface and crevice. Then, with a paper towel or rag, rub all the oil off. Continue until it looks as though there is no oil left on the iron’s surface. That is, the iron should look dry instead of shiny with oil. Fear not: there is still a fine coating of oil on the piece, and that is just the amount we’re looking for.
At this point, put the pan in a cold oven. Ideally, the pan should be placed upside down just in case you were not thorough enough in wiping off the bulk of the oil, and–for further safeguarding–you should line the next rack down with aluminum foil on the off chance you’d need to catch an oil drip. (If you wiped properly, all this will be overkill.) Finally, turn the oven to a baking temperature of 450°F-500°F, or essentially as high as your oven goes, and let the pan heat along with the oven. When the oven reaches temperature, let the pan bake for 1 hour, then turn the oven off and let the pan cool inside the hot oven for another two hours. It should be cool enough to handle by that point.
The pan should now be starting to look a little black, but it will still be matte. In order to get that shiny proper seasoning, the process will have to be repeated. You need, at minimum, SIX coatings of oil on the pan before it begins to look shiny. All in all, this will probably be about 15 hours of bake time and maybe 15 minutes of hands-on work.
The end result, however, is definitely worth the seasoning time. Cooks Illustrated seasoned a pan in this way and another pan using the same heating technique, but with regular vegetable oil. They then ran both pans through a dish washer–a major sin in cast iron maintenance. The vegetable oil pan’s seasoning was ruined, but the flax oil pan still held up strong.