It's called scale and on modern steels is basically an iron oxide with a bit of the alloying elements thrown in. Scale may not rust at first but it will tend to flake off with temperature changes or impact and so doesn not make a good base for a protective coating. It can be removed mechanically, thermally or chemically---soaking overnight in vinegar will usually allow you to scrub it off---then rinse/neutralize the vinegar on the steel.
Or if you are using a coal fire with a dirty or green coal it can be a coating of coal tar and impurities.
If you are using real wrought iron it will be a glassy coating of the iron silicates and the iron oxide
If you have zinc *IN* your steel something is *WRONG*! (save as miniscule ammounts as a tramp element) If your steel is galvanized/zinc coated then please read the first entries on the i-forge link on anvilfire.com about the death of a friend due to putting zinc coated steel in his forge.
Thomas
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Thomas
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