Caring for Cast Iron Cookware

Properly tended, cast iron will not rust nor will it emit the retchsome reek of rancid oil. Why anyone would consider cooking food in a vessel coated with rancid oil, I cannot conceive. Another consequence of the old Troop 451 method is that the iron, predictably, rusts.

This time, while we had no rusty iron to remediate (a first, I think!) we did find most of the iron reeking of rancidity. Yuck!

The proper care is trivially simple. Clean the vessel in whichever way you wish. Detergent and steel wool will NOT harm a properly seasoned piece of cast iron. This because of the nature of the chemistry involved in seasoning. Like so many real-world processes, this too can be understood by the application of scientific understanding.

The root problem is that iron metal and oxygen gas (O2 in the air that we breathe) combine readily in the presence of water. This is an energetically-favorable reaction that occurs spontaneously and water acts as a catalyst in the process. The end product is iron oxide, rust. Anyone who has ever left their tools outside on a damp evening knows that rust forms easily and quickly on iron and regular steel surfaces. The trouble is, unlike the tarnish that forms on the surface of silver or the green patina on copper, for example, rust continues beyond the surface. Left unabated, rust will continue until the entire iron object is corroded. Tarnish creates a protective film that precludes further corrosion - not so rust.

Thus, to preserve the iron vessel, we need to protect it from rust.

For cast iron, this means keeping it completely dry. We cannot keep it from oxygen in any atmosphere in which we can operate, needing oxygen ourselves. Thus, we aim to eliminate the catalytic water molecules. Here, we encounter two issues: 1) moisture is almost everywhere we are since we need water just as we need oxygen; and 2) cooking our food almost always involves water. Moreover, clean-up afterwards means even more water.

Iron and water readily interact, in stark contrast to, say, oil and water which, famously, do not mix.In fact, once iron and water interact, we need to add energy to the system to drive off the water. The easy way to do this is to heat up the cast iron vessel once it has been cleaned. Heating it up to above 212 °F will do the trick quite nicely.

This alone is not sufficient, however, to preclude rust. This is because iron’s interaction with water, as noted above, is energetically favorable. Left to its own devices, the iron will attract water molecules from the air and the rusting process will begin.

We need to prevent water molecules from reaching the iron.

We can do this, as we might for storing a firearm, by coating the metal with a layer of oil. Oxygen, being a highly polar molecule, does not like to interact with the strongly non-polar (“oily”) chemical nature of oils. Thus a coating of most familiar oils can prevent or severely retard rusting. However, we do not want to use a coating of edible oil on our iron.

This is because the oils we eat, such as canola oil, are composed mainly of triglycerides (triacylglycerols) that contain unsaturated fatty acids. These unsaturated fatty acids have carbon-carbon double bonds that can react with oxygen. When this happens, a wide array of chemical reactions occur that, collectively, are known as ‘rancidification.’

Skipping the complex chemistry behind this, the point is that food oils will go rancid when exposed to air. This is why vegetable oils come in sealed containers. This is also why we should never, ever store opened bottles of oil in Big Red. Simply open one of these ‘nose-bombs’ for the entertaining surprise of a snootful of short-chain aldehydes and ketones. Yummy, the smell is not.

So, if we want to seal our iron from oxygen yet liquid oil is not a good idea, what can we do?

The answer is simplicity itself. Heat the cleaned iron well past the point of dryness, to a surface temperature of about 400-450 °F. If it has already been seasoned, the vessel will start to smoke, just a little. Then, wipe it down with a thin coating of your favorite oil.

When applied to the hot iron, some of the double bonds in the fatty acids of the triglycerides will break in such a way that they form a free radical. Again, the point is not to teach all the chemistry involved but merely to say that the process is understandable to those who care to know it. These radical are highly reactive. One will bump into another molecule, linking to it and transferring the radical character. This new entity with bump into another molecule and link to it, and so on and so forth until we form a huge network of inter-linked molecules in a process called free radical polymerization.

The end product is a plastic-like coating on the surface of the cast iron. This coating is insoluble - detergents cannot affect it. And it is also duecedly difficult to remove mechanically. Moreover, it retains the non-polar, oxygen-resistant character of its oily precursors. This is the protective ‘seasoning’ of a well cared-for cast iron vessel.

Have you ever baked cookies on a brand new cookie sheet, only to have it emerge from the oven with a golden varnish-like coating on it? Try as you like, it seems impossible to remove that coating. Well, that coating is exactly the same as the seasoning on cast iron and it takes heating fats and oils to high temperatures to form it. If you recall how nearly-impossible it is to remove the golden coating from the new cookie sheet, you can see why a properly seasoned piece of cast iron is so tough. Soap, detergent, steel wool, etc. will not hurt a well-seasoned cast iron vessel.

So, clean the iron pot as you will then dry it over a high heat until it stops steaming and heat it more. When it starts smoking, turn off the flame and wipe it down with a light coating of oil. Keep wiping until all the liquid oil is gone. And that is it. Do this every time you use your cast iron and it will provide years of trouble-free service. My Grandmother’s cast iron has always been treated this way and my brother continues to use it daily, 100 years later.

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