Is it called slag?

[metalworker77]

I notice a black outer layer that forms on the objects when I heat them to a high temperature. Is it just steel? or can it be impurities in the steel such as zinc rising to the surface? like slag? It's a black outer scale that has a bit of a waxy look to it. I ask this because I kind of like it, and it doesn't seem to rust the same way as steel? it find it gives the pieces a more natural look.

[ThomasPowers]

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 link removed at the request of anvilfire about the death of a friend due to putting zinc coated steel in his forge.

Thomas