Years ago - I think a lot of it had lead in it. Maybe that is naturally where the color red came from or maybe it is just to hide small amounts of rust.
Did some work on existing BUT old (1920's) New York bridges. The old blue prints were very specific on assuring there was enough "lead" in the paint.
Now we spend a fortune (part of my project) to remove it.
Minio, PbO4, that I remember. At bridge demolitions even textiles are hung under to prevent fall of the environment damaging thing. I am not sure of the status of lead roofs, maybe it is still allowed for restorations. Not in a hurry to know.
And how good is to have good friends. In Spain, the period to dismantle the industries using asbestos allowed them to reconvert to greener (gypsum plasteboard) pastures. It seems some things are less venomous if you have the proper friends.
red iron is a broad term used by welders and erectors when referring to a steel bldg. my brother-in-law is a welder on an erection crew and he says red iron for everything, even though I know that it's A992 steel he's talking about.
it's kind of like people in the South saying Coke as an all-encompassing word for soft drinks.
As others have said, "red iron" is a colloquial term for structural steel. It is not limited to weathering steel or any particular grade, but comes from the red primer used as MiketheEngineering noted. The original red color came from the addition of "red lead powder", but ultimately was replaced with a red pigment.
