Once free of rust and previous build-up, the pan must be given an initial seasoning. This first layer of polymerized fat will help prevent the return of rust and provide a foundation upon which to build a good, slick new coat of long term seasoning.
Methods for basic seasoning vary. Most people just end up adopting a routine that has proven successful for them and sticking with it. For purposes of illustration, this is what I (and many others) do:
After the piece is cleaned and well-rinsed (some advise a final washing using Dawn™ dishwashing liquid at this point, and that's OK), paper towel-dry it, and set the oven to 200°F.
As you dry the piece, you may get a black or brown residue staining the paper towel. It's not unusual. The piece may also start to flash rust if ambient humidity is high. That's OK, too, it will wipe off as you apply the first coat of seasoning fat. Tip: Using cold water for the final rinse seems to help lessen flash rusting.
Place the piece in the oven upside down and let the 200° heat dry it completely, about 15 minutes. Leaving the oven door cracked open a bit during this initial heat drying phase also helps to minimize flash rusting. Next, close the oven door and raise the oven temp in 70 or 80° increments about 15 min. apart until it reaches 425°F.
Potholders, an oven mitt, or welder's gloves are useful at this point. Carefully remove the hot pan and place it on a piece of thick cardboard. Using 2 full sheets of paper towel folded over multiple times to make a thick pad, apply a somewhat liberal coat of Crisco™ (the solid shortening, not the oil) all over the piece.
For pieces with nooks and crannies such as cornstick pans, muffin pans, or waffle irons, small pieces of stiff or corrugated cardboard are handy to work the seasoning fat into hard to reach areas. Dip an edge into the Crisco, and poke it into the crevices. A cheap, natural bristle basting brush can also be helpful in "painting" seasoning fat into the details of intricately-molded pieces.
Next, using a cheap terry cotton towel, wipe the piece like you're trying to remove everything you just slathered all over it. All you want remaining is an extremely thin layer of oil. Then, back in the oven it goes-- upside down, again-- for 15 minutes, then remove and terry towel again. Don't use paper towel for this step, as it will tend to shred, and, once saturated, will not remove as much of the excess oil as you want.
The pan is placed in the oven upside down to prevent any excess Crisco from possibly forming pools which would remain as sticky areas after cooling. The reason for the second wiping is simply in case you didn't wipe enough off the first time. If not, even with the pan upside down, excess Crisco can congeal into droplets on the cooking surface, resulting in spotting and stickiness.
Return the piece to the oven, once again upside down, and crank the temp up to 500°F. After 15 more minutes, turn the oven off and let the piece cool slowly without opening the door. This final period of high temperature helps darken the seasoning. After cooling, I tend to wipe the item down with an extremely small amount of Crisco or canola oil, just to give it a cosmetic sheen.
Some collectors advocate immediately repeating this oven seasoning process up to a half-dozen times, but I have found that to be tedious, and that simply cooking in the pan gives both the best seasoning and best-looking patina over time.
Don't be tempted to try to accelerate the seasoning process by baking on a thicker coat of oil in a single pass. The result will be a sticky mess, and it will tend to slough off, requiring you to start the whole stripping and seasoning process all over again.
Once your pan has been cleaned and given an initial seasoning, it is ready to begin cooking. The non-stick properties cast iron is famous for will not yet be up to their full potential. Only time and usage will provide the additional layers of seasoning necessary. Since seasoning is primarily hardened layers of polymerized fats, cooking foods like bacon and sausage will accelerate the process, as will baking cornbread.
About Seasoning Fats
Traditionally, seasoning fats typically included lard and bacon grease, but they are not found in home kitchens as commonly as they once were. Some say animal fats are a must, or that the fat used must be a solid at room temperature, but these are not necessarily mandatory. Besides the very popular Crisco™ shortening, other good choices for seasoning fats include canola oil, PAM™ Cooking Spray (contains canola oil), and refined grapeseed oil (475°F smoke point).
You may have read elsewhere about the virtues of flaxseed oil as a seasoning fat. The belief is that the combination of its low smoke point and classification as a drying oil make it somehow superior to other commonly used fats in terms of polymerization and resulting hardness. That which I have read of others trying it has provided mixed reviews, including complaints of poor adhesion. I have tried using it myself, and have not been compelled to embrace it as a regular methodology. Perhaps I and those others who tried it and found it lacking did not follow the given instructions precisely enough. It also has a fishy smell when heated that I do not care for. My preference remains good old Crisco for oven seasoning (although recent trials with refined grapeseed oil have been positive), and canola oil for post-cook wipe-downs.
If you do decide to try it, be aware that caution must be exercised when using a drying oil. Flaxseed oil is the food-grade version of linseed oil, once commonly used in house paints. You may have heard of building fires being caused by the spontaneous combustion of oily rags. Drying oils auto-oxidize as they dry. The heat generated by this process can, under the right conditions, reach the temperature necessary to ignite the rags. You should therefore treat any drying oil-impregnated rags as potentially dangerous, and dispose of them safely.
Here are two pieces that were cleaned using a combination of lye and electrolysis, and seasoned using the Crisco method described above. Residual black stains on the inside of the small skillet were reduced using the vinegar/water solution described in Basic Rust Removal.
And here's a Lodge #7, after seasoning with grapeseed oil:
"Pre-Seasoned" Cast Iron
As more maintenance-free types of cookware emerged in the latter part of the 20th century, it became increasingly difficult for manufacturers to win new fans of cast iron. Lodge, the only remaining maker of cast iron cookware in the US, addressed this issue with the introduction of their Lodge Logic™ brand in 2002. Using a "proprietary soy-based vegetable oil", the iron is both protected from rust and given an initial seasoning layer at the factory. As with manual seasoning, however, additional layers must be built up in order to reach an optimum level of "non-stick" properties. This factory-applied seasoning is also subject to the same maintenance requirements as manual seasoning.
For Display Only
Ever see those photos online of vintage cast iron pieces, and they have a beautiful, even, satiny black color? They're not painted. But, are they really in such great condition? Maybe. Or is there a trick to getting them to look like that? The answer is yes.
If you are restoring cast iron for collectible purposes only and will not be cooking with your pieces, you may only want apply a rust preventative coating. For this, we can use mineral oil, which has several benefits.
With mineral oil, the oven time and temperature required are much shorter and lower. After cleaning to your satisfaction, dry and heat the piece in the oven as usual to 200°F. Then, crank the oven up to 300°F.
As an alternative step, next raise the oven temp slowly up to 500°F, in 75 or 80 degree increments. This actually helps darken the iron. Then, lower the oven temp back to 300°F, cracking the oven door open a bit to help the temperature drop.
Once the oven temp has reached, or, alternately, dropped to 300, remove and rub the hot iron all over with a thin application of food grade mineral oil, the stuff marked "USP" you can buy at the drug store. Then wipe off the excess, just as you would if you were seasoning with a cooking fat. Return the piece to the oven for a bit, then turn it off and allow to cool.
The additional advantages here are that you only need a single application, and you get a nice darkening effect, making the piece more cosmetically attractive. Further, the piece won't become sticky over time, as may be possible with cooking fats.
Don't be tempted to apply mineral oil to a piece while heated up to 400 or 500° to darken the seasoning as you would with cooking fats. You don't want to exceed the smoke point of the mineral oil. Mineral oil was formerly employed by theatrical fog generators, but its use was discontinued after widely-reported instances of respiratory irritation by those regularly exposed to it. Those who may have long ago owned toy train locomotives that came with a bottle of liquid drops to produce artificial "smoke" will likely recognize the odor of vaporized mineral oil.
If you ever decide you want to cook in your mineral oil-preserved piece, simply wash it with hot water and dishwashing liquid, and then apply your regular cooking fat-based seasoning regimen before use.