This procedure is an experimental home chemistry approach intended to do three things:
- Clean and de-rust the cast iron,
- Bias the surface toward a black oxide / magnetite-like layer more than plain vinegar would,
- Blacken and unify that layer with tea or tannin chemistry before applying traditional oil seasoning.
It is not a claim of phase-pure industrial black oxide, but it is a more chemically targeted approach than an acid-only restore-and-season process.