Rick Aylsworth wrote:

Tiny nit, Tony and Jim. Ferrous oxide is black (Fe+2). Ferric oxide
(Fe+3) is the red-brown pigment, and also rust, with Tony's caveats.
Different oxidation state = different crystalline structure =
different color.

True, I mistyped, reversing ferrous vs. ferric. But black
ferrous oxide isn't a paint pigment. What a paint guy might call
ferric oxide probably is a rather more complex material, not the
chemically and structurally pure laboratory compound, nor a pure
mineral.
Natural rust is a mixture of iron oxides, starting with the
first film which is yellowish, followed by the familiar orange-red
color (lots of ferric oxide), then darkening more and more into a
really dark brown. The various iron compounds responsible for all this
(relevant to freight car weathering, especially unpainted interiors of
gondolas and hoppers) are complex and far beyond the scope of this
list. But the color sequence is entirely relevant.

Tony Thompson Editor, Signature Press, Berkeley, CA
2906 Forest Ave., Berkeley, CA 94705 www.signaturepress.com
(510) 540-6538; fax, (510) 540-1937; e-mail, thompson@...
Publishers of books on railroad history

Hi Tony, notice that I did not state that ferrous oxide is a paint pigment, simply that it is black, and therefore is not the pigment used to make reddish-brown paint. It IS used as a pigment for other purposes. I spent a great deal of time in college attempting to identify various oxides and their hydrates through a petrographic microscope, and it is a complex and fascinating subject.

Every morning I walk my dog through an area that was once a railyard serving an iron mine, and I never cease to be amazed by the variety of colors in the ore that fell off the FREIGHT CARS ;)

I appreciate all the experts that inhabit this forum.

Peace, Rick Aylsworth

--- In STMFC@..., Anthony Thompson <thompson@> wrote:

Rick Aylsworth wrote:

Tiny nit, Tony and Jim. Ferrous oxide is black (Fe+2). Ferric oxide
(Fe+3) is the red-brown pigment, and also rust, with Tony's caveats.
Different oxidation state = different crystalline structure =
different color.

True, I mistyped, reversing ferrous vs. ferric. But black
ferrous oxide isn't a paint pigment. What a paint guy might call
ferric oxide probably is a rather more complex material, not the
chemically and structurally pure laboratory compound, nor a pure
mineral.
Natural rust is a mixture of iron oxides, starting with the
first film which is yellowish, followed by the familiar orange-red
color (lots of ferric oxide), then darkening more and more into a
really dark brown. The various iron compounds responsible for all this
(relevant to freight car weathering, especially unpainted interiors of
gondolas and hoppers) are complex and far beyond the scope of this
list. But the color sequence is entirely relevant.

Tony Thompson Editor, Signature Press, Berkeley, CA
2906 Forest Ave., Berkeley, CA 94705 www.signaturepress.com
(510) 540-6538; fax, (510) 540-1937; e-mail, thompson@
Publishers of books on railroad history