Forge scale, Fe3O4 is a "super rust". As opposed to regular rust, Fe3O3.
Forge scale is also pretty porus depending on the forge and the condition of the steel when it goes into the forge, this will lead to easier red rusting.
The upside of forge scale is that it's incredibly tough stuff. Way harder than hardened steel. So when I leave it on blades I boil the knife in water for a few minutes (working from memory I forget the specifics look up "rust blueing") and then soak them in a protective oil, like linseed.