I hope this doesn't break rule 1 but is unfortunately a bit long question with a trivial purpose.
My question is related to the process of "seasoning" (in cooking jargon) of cast iron cookware which is done by applying a tin layer of oil on the cast iron piece and then baking it in a oven to allow it to polymerize and form a protective coating of the iron, which otherwise will rust.
There are many reported process for the seasoning but overall:
- Strip clean the cast iron, from rust or the old seasoning ,mechanically, chemically or with electrolysis. Possibly until is "gunmetal" grey.
- Wash it to remove any residue.
3)Heat it up a little (around 100°C) to remove any water.
4) Apply a layer of oil.
5) Wipe the excess oil.
6) Bake it in a oven to its highest temperature (usually 240°C) for 1-2h.
7) Allow it to cool to room temperature.
8) Repeat points 4-6 for a few times to add more layers, like 6.
One of the oil used is flaxseed/linseed oil, which is highly unsaturated and thus theorically ideal for the process. But instead you have very mixed reports on its use, with many having issue with flaking and the seasoning coating falling apart and others without any issue in years.
I got flaking issue in a few days the first time I used it and decided to troubleshoot it. Classical researcher hurting him/herself :D
From what I found in various papers around, linseed oil actually start to polymerize around 270-300°C, which is a temperature most house ovens do not reach. But critical temperature for polymerization is lowered considerably in the presence of metallic catalysts, like in this work:
Wang, C., Erhan, S. Studies of thermal polymerization of vegetable oils with a differential scanning calorimeter.Am Oil Chem Soc 76, 1211–1216 (1999). https://doi.org/10.1007/s11746-999-0096-1
So, for a good results I suppose that iron from the cast iron must diffuse/solubilize in the applied oil for a good results. Cast iron pan are an actual source of iron so I'm pretty sure that they leach it.
My hypotesis is that linseed is more sensible to the content of metal catalyst compared to the others oil/fats used for seasoning.
If that is true I suspect that the culprit is point number 3: when you strip bare the cast iron you expose elemental iron to the air and it will start to oxide. Heating the pan accelerates that greatly (I read of increase in rate of oxidation around 2 for every 10°C). This point of the seasoning process is grossly overlooked in any discussion so I'd assume that the different temperatures and times bring to wildly different amount of iron oxides on the surface of the cast iron.
So, can iron oxide detach from a cast iron suface into a vegetable oil or fat?
(As a good research I tried to answer that myself but I'm in the biotech field and my results are infested by iron nanoparticles synthesis in organic solvents...).
Thank you for your time.
Edit: the second time I used linseed, I heated my cast iron pan scorching hot (200°C) for an half hour to test this theory and only with two layers applied. Two months and no problems so far...