When a piece of cast iron is repeatedly covered in fat and exposed to heat it develops a hard, black, non-stick coating referred to as "seasoning".

What exactly is this coating made of molecularly? How is it bonded to the cast iron substrate? What is the chemical process that transforms a liquid fat into this coating? And what if any characteristics of the fatty acids or triglycerides used have an effect on the final coating or how well the it bonds to the iron?

  • 1
    $\begingroup$ Mixture of various black oxides (approx. Fe3O4), phosphates(?), carbonised oil of any sort. Sticks like hell, and scratches repair themselves during further use. :-) $\endgroup$
    – Karl
    Aug 23, 2016 at 15:06

2 Answers 2


Some hints:

  • if you mean that "what exactly" in the original sense of the words, then the answer is: nobody knows and will ever know.

  • If you are content with some more vague answer, then look at the pyrolysis chemistry occuring from 150 °C up with fats, proteins and starch/sugars, and all that at once. This is a vast field of chemistry, and all is gives to a trained chemist is a idea of the structures of that coating.

  • Bonds to iron will not occur, first because on cast iron and on steel pans there is always a layer of iron oxide, on cast iron you find a glaze formed by fusion of the molds finish layer (often containing graphite and clay)

  • $\begingroup$ Cast iron pans that develop seasoning are not enameled. There is definitely iron bonding, because the same seasoning doesn't develop on aluminum or other cookware, and the seasoning is developed before you let a pan rust. $\endgroup$
    – yincrash
    Aug 8, 2015 at 15:54
  • $\begingroup$ @yincrash Why definitely? He explains that the not even is iron in the first place in the third item. But iron oxide, and molten remains of the mold in which the pan was cast into. Similar to molten stone or so. $\endgroup$ Aug 22, 2018 at 8:42

Mostly- all the oils coated are of DRYIG oil -meaning they are polymeric. eg- the Linseed oil etc -they polymerize on heating. Since the cast iron itself is slightly porous- these oils penetrate along with the iron or , could be reacting with the iron salts, once they break down to fatty acids, even partly. This also explains - why this is not happening in pure steel cooking vessels.


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