On Thu, 08 Sep 2005 18:44:37 +0100, Stuart Noble
wrote:

The only stains I've come across are from old water leaks. I guess the
reason they're always brown is either that they carry rust from fixings
in the masonry or oxides from the bricks. Maybe John Schmitt can
enlighten us on that.

It is normally hydrous iron oxides from either source carried by water
ingress. In short they both count as rust stains. Chemically there is
little to differentiate them, although it is a matter of some puzzlement
to me as to why these stains can penetrate an oil-based medium. I can only
conjecture that the ferric ion complexes with the part of the solvent
matrix and migrates in this manner. Iron does some bizarre things, like
form ferrocene. My best guess is that the reason for this is that there
are only 2 electrons in the outer shell, leaving the 14 in the next shell
down accessible thus resulting in this anomalous behaviour.

John Schmitt

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