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I am in the process of seasoning a cast iron pan. (Creating the black non-stick coating typical of such pans.)

This is done by causing thin layers of flaxseed oil to polymerize and bond to the iron.

In the literature there is the recommendation that the polymerized oil bonds better to the pan, if the pan is covered with magnetite rather than blank iron, in part due to the porous nature of the oxide layer.

My question is: is there a DIY method that can either create $\ce{Fe3O4}$ on a polished cast iron pan, or alternatively convert a layer of $\ce{Fe2O3}$ into $\ce{Fe3O4}$?

In the gun manufacturing trade a process called "rust bluing" is used. In it the gun barrel is encouraged to form a thin layer of $\ce{Fe2O3}$ before being immersed in boiling distilled water. This causes the $\ce{Fe2O3}$ layer to convert into $\ce{Fe3O4}$ within minutes. Apparently due to the fact that boiling water lacks dissolved oxygen.

When I attempt to apply this "rust bluing" process to a cast iron pan, the $\ce{Fe2O3}$ layer stays red rather then turning black: Clearly the conversion is not taking place.

I am not clear why this is the case. Is it due to difference in materials, i.e. cast iron vs. gun steel?

Is there any way I can promote creation of/conversion to a magnetite layer on cast iron with DIY-compatible means? The industrial solution of caustic soda solution at 150$^{\circ}$C does not sound like a good idea for home implementation.

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I think the two steps that are missing from you approach are "carding", where, after immersion in the boiling water, the steel is rubbed/brushed to knock off the larger rust structures and repetition. The process is repeated several times before the piece builds up a stable blued surface.

Check out this video of Larry Potterfield applying a rust blue surface to a rifle action and barrel: https://www.youtube.com/watch?v=VPD3GWqk-yE

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Heating the polished and cleaned pan on your kitchen stove will oxidize the polished surface. The oxide will be magnetite. If you bring the pan to a dull glow, the oxide layer can be grown in thickness to suit your needs.

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