WARNING!: I going technical in my explanation. (this is long...)
(very little science investigation has been done on cast iron seasoning chemistry but I think I have an explanation after researching and thinking about the process for many years).
All fats and oil are triglycerides: like the capital letter E, the chemical structure of triglycerides is like 3 long ribbons tied to a central stick.
Triglycerides (fats and oils) are made using only 3 types of atoms Carbon (C), Hydrogen (H) and Oxygen (O).
On average, for every C there are 2 H. A triglyceride molecule has a minimum of 9 C to a maximum of 70. Oxygen atoms are only located were the 3 ribbons (called fatty acids) connect to the stick (glycerol backbone). There are 6 O in all.
This is the process:
Oil of fats can form really thin layers (molecular thin) particularly when hot. In addition, since cast iron contains a high amount of carbon, these layers are chemically attracted to the surface because fats and oils contain a high percentage of carbon.
When long chains of carbon are heated at high temperatures (without flashing and burning) they spontaneously break up in smaller pieces. That is the basis of converting heavy oil/tar to gasoline, the process is called cracking.
During the cracking process, the carbon chain lose some Hydrogen atoms and Oxygen and some Carbon. The smoke that emanates from the hot cast iron is water (H2O) vapour, small chain carbons and also CO2 plus Hydrogen gas (can't see).
Most carbons on a carbon chain are linked to 2 carbons and 2 hydrogen. At high heat, the carbons lose their hydrogen atoms and relink themselves with other hydrogenless carbons. Since the oil starts off as a very thin layer, these carbon rearrangements create a layer of carbonaceous material similar to graphite. Graphite is very heat stable and a natural solid lubricant (non-stick, nonadhesive)) (ever heard that you can lubricate a door lock with pencil powder?)
Any fat or oil will season a cast iron pan. Highly unsaturated oils unfortunately can become rancid while heating by making a gummy substance instead. The gummy substance is a carbon-oxygen epoxy polymer that can be very stable and will not convert to a graphite-like substance. Saturated fats (like lard) do not turn rancid as easily so they work better then oils to avoid the gummy substance.
To season correctly:
Long chain saturated hard fats work better (like lard, suet)
Oil can be used but keep the oil coating very thin.
Keep the heat high enough to make the reaction happen Above 250F
Keep the heat low enough to prevent flash burning the fats (instant combustion) below 500F
Several applications are needed: usage add more layers (patina build up).
I season my pan on the stovetop by applying a thin coat of peanut oil in my hot cast iron pan by rubbing the surface with a oil soaked paper towel. I let it carbonize, take the pan off the heat, let it cool slightly, rub more oil and repeat the process (5 to 10 times). I don't use the oven method because it only creates one layer per cycle.
I think that several coating on the stovetop followed by one oven cook could be a good process to apply coats then curing in the oven (i should try that).
I eat science everyday, do you?
I eat science everyday, do you?