the science of cast iron

It didn’t take much Googling to see that just about every food blogger out there has a post on cast iron, how to clean it, how to season it, etc. It seems borderline obligatory, so for fear of being left behind, I’m joining the swaddled safety of the herd. Today at DFK we talk about cast iron, how to clean it, how to season it, etc.!

Joking aside, there really is a hell of a lot of information out there on cast iron, so if it’s a topic you’re keen on, a little searching will get you a lot of material. That’s what I did, and over the last days and weeks I’ve read damn near all of the how tos on cast iron, cross-referenced with my own notes and experience on the subject, and fallen down a deep rabbit hole of chemistry in the process. Some of the info I found is good – ironically a lot of it literally copied and pasted from one source to another – but a lot of it is highly questionable or outright wrong. My post today is a sifting of all of the good stuff and none of the garbage.

the pros and cons of cast iron

pros

Cast iron is extremely durable. It’s not unusual to have one (properly cared for) that lasts generations. Even if you find a beat up pan, there’s plenty of life under it if you know the tricks to save it.

In general, cast iron is very affordable. I have always trusted in Lodge brand pans (Lodge if you’re out there, feel free to now give me cast iron goodies for life. I won’t mind.) and you can find all sorts of shapes and sizes at a good price.

As a metal, cast iron has exceptional heat retention and heat capacity. This means it gets really hot and stays hot. The ability to achieve high, sustained heat makes it the best metal to use for searing proteins.

Cast iron can be seasoned to create a natural, durable, non-stick surface. Much more on that in a min.

It’s dope af. While I don’t think this is a technical point, there’s just something cool about a well-worn, well cared for cast iron skillet. Call me sentimental.

cons

Cast iron is a poor conductor of heat. This means it takes a long time to heat up (thanks in part to its density), and doesn’t heat up evenly. When using cast iron, make sure the flame or heat source is large enough to cover as much of the pan as possible, so heat is evenly applied. A large skillet on a small burner will absolutely create a hot spot where that flame is in direct contact with the pan.

They take more than average care and maintenance. To be sure, nothing about caring for a cast iron pan is difficult, but it’s admittedly more work than simply rinsing and drying. Personally I think the simple care is more than worth it for the value of a nicely seasoned pan, but the lazier cooks of the world may think of this as a deal breaker.

why you should season your pan (and a few cast iron myths)

Food sticking to a pan during cooking is really simple; molecules of the food bond to the surface of the cooking vessel (protein sticking to metal being the most prone-to-sticking combination). That’s it. Although your metal cooking pan or glass dish may look and feel smooth, at a microscopic level they are at best porous like a sponge or at worst, a craggy landscape with lots of opportunities for molecular bonding.

Seasoning is the process of treating your iron pan so that it develops non-stick properties (which also helps preserve the pan itself). Oil is added to the cast iron in very thin layers, which fill the cracks and crevices of the pan’s surface. The oil is exposed to high heat, which breaks the oil molecules down causing it to polymerize, firmly bonding to the cast iron and effectively becoming part of the metal.

The new surface of the pan is smoothed, since the oil filled in the rough surface, and just as importantly becomes hydrophobic or water-repelling (oil not liking water and all). Additionally, the high heat of the process causes the oil to carbonize, adding single-bonded carbon to the matrix of bonded oil. These properties create the non-stick surface of a good cast iron piece.

Once you have a seasoned pan, it is perfectly ok to:

1. Use soap and water on your pan to clean it. This will not remove the seasoning. Just make sure to use regular dish soap, and dry the pan thoroughly immediately after washing.
2. Use metal utensils in the pan. Mild use of metal utensils (like a whisk or spatula) will not remove the seasoning of your pan and will in fact gently abrade the pan’s surface making it less prone to food sticking.

You should avoid:

1. Using abrasive cleaners or cleaning products on your pan. Stick to regular dish soap and a sponge. Avoid using steel wool or similarly rough cleaning pads.
2. Highly acidic foods. The acid in these foods can break down the molecular bonds of your seasoned pan.

how to season your pan

what oil to use

First of all, when you purchase a cast iron pan that claims to be pre-seasoned, this almost always means it’s been treated to avoid rusting, not to be non-stick. Seasoning your brand new pan is the best time to do so since it’s as clean as it’s gonna get. If you have an old pan that happens to be badly damaged and/or rusted, there is a way to save it, and I’ve outlined that after this section.

What kind of oil to use in cast iron seasoning always seems to be of some controversy. I can tell you from personal experience and scientific research that hands down the best oil to use is 100% pure flaxseed oil. Flaxseed oil is very high in a specific fatty acid within the omega-3 family that breaks down (polymerizes) readily and effectively. Getting pure flaxseed oil will ensure no other chemical additives interfere with the process.

Yes, you can use other oils like canola, but they aren’t going to do nearly as good a job as flaxseed. Yes, flaxseed is expensive, but you’ll be using very small quantities and a single small bottle will last you ages. Any oil you choose to use should be as pure as possible and unsaturated, and preferably high in ALA/Omega-3 fatty acid.

seasoning method

Step 1 – Preheat your pan in an oven at 200F/93C to dry it of any surface moisture and open the pores of the iron.

Step 2 – With a paper towel, apply a very thin layer of oil to the entire cooking surface of the pan. A thick application of oil will cause it to flake off at the end of the process, so there are no shortcuts here.

Step 3 – Once you have applied the oil, remove as much as possible with a second paper towel. The pan will appear to have no oil in it at all, but it’s there.

Step 4 – Place the pan upside down in the cold oven. No drip pan is needed beneath it.

Step 5 – Turn your oven to between 400-500F/204-260C, allowing the pan to heat up with the oven. Once the oven reaches temperature, “bake” the cast iron for 1hr. The oven temp. must be above 375F/190C in order to break down the oil and allow polymerization. Yes, there will be some smoke created from the process, so prepare accordingly.

Step 6 – Turn off the oven and let the pan cool inside it, about 2 hours.

Step 7 – Repeat this process 6-8 times to develop a thorough, hard seasoning.

If the surface of your pan becomes sticky after any cycle of the seasoning process, it is because non-polymerized oil has been left on the pan and that occurs for one of only a few reasons:

1. Too much oil applied to the pan.
2. Oven set at too low a temp.
3. Not enough time spent “baking”.

working with a damaged pan

Much of this information here came from a great post on the subject by a lady named Sheryl Canter. She really got into the topic of cast iron, and my thanks to her for passing on her experience. I have used her method for stripping down a cast iron pan and can attest to the process.

Please note: This method calls for the use of oven cleaner. Oven cleaner is real sh*t, and extremely toxic/caustic. Follow all instructions carefully, make sure you follow all safety precautions thoroughly, and take the use of the product very seriously.

Step 1 – Using oven cleaner (and all necessary protective gear – non-corrosive gloves, goggles, ventilated mask, etc.) coat your pan and leave it to sit for 24hrs. Once again – make sure it is in a well ventilated area and appropriately covered/stored to avoid contact with anyone or anything that may be harmed by it.

Depending on the level of residue on the pan, it may need 1 or 2 treatments of the cleaner. At the end of this step there should be no visible residue still on the pan after a rinsing and thorough scrubbing. The pan will be rusted, which is normal.

Step 2 – Submerge the pan in a solution of equal parts of distilled white vinegar and water, which will help to remove the rust created by the oven cleaner. Let the pan sit in the solution for several hours – usually between 10-15. Change the solution if necessary (once it becomes cloudy with rust), until you see little to no rust rising to the surface of the solution. Do not let the pan sit in the solution for more than 24hrs. or it may be ruined permanently.

The vinegar will counteract the lye in the oven cleaner, and help to remove the rust that was created, but at the same time the stripped cast iron is still susceptible to rusting, which will happen in the solution itself. Therefore you must monitor the pan in the solution to keep it in long enough to remove the majority of the rust caused by the oven cleaner, but not so long that the solution itself damages the pan.

Step 3 – This step must be done in quick succession, so have everything ready prior to beginning. Preheat your oven to 200F/93C.

Remove the pan from the vinegar solution and immediately wash it with sodium carbonate, also known as washing soda. This is the base that will neutralize the acid of the vinegar to prevent further rusting from occurring on the now naked pan.

Dry the pan thoroughly with paper towels and place it in the oven to further dry, about an hour.

Step 4 – Remove the pan from the oven and cover it entirely in a thin coat of flaxseed oil, using paper towels to simultaneously apply the oil and remove any excess rust. The oil will in effect clean any residual rust off of the pan and provide a protective coating.

Your pan is now ready to be properly seasoned and live anew!

For those of you new to cast iron, I hope this encourages you to go out and get a pan of your own. It really is an excellent (and in my opinion, indispensable) piece of kitchen equipment. For anyone who already owns cast iron, I hope this clears up a bit of confusion and myth surrounding how to care for it. With just a little effort and know-how you can own a pan your entire life, to pass on to the next generation of cooks and bakers!

Cheers – Chef Scott