Rust, Steel, Heat, and Magnetism
We rusted some steel wool with bleach and vinegar as
catalyst. We removed the wool and drained the liquid through a coffee filter. We
tested the dried residue, which was no longer magnetic. We then heated it in a
spoon over a candle flame, which turned it black. It was magnetic
again. What was the process that "remagnetized" it?
Probably what you did is drive off some of the oxygen from the rust
(hematite or goethite) turning it into magnetite, which is black and
magnetic. The balanced chemical reaction starting with goethite would
12 FeO(OH) --> 4 Fe3O4 + 6 H2O + O2
That is the most realistic, since rust is generally hydrated. The
reaction is a little easier to balance from hematite:
6 Fe2O3 --> 4 Fe3O4 + O2
Richard Barrans, Ph.D., M.Ed.
Department of Physics and Astronomy
University of Wyoming
Some oxides of iron are non-magnetic, others are magnetic.
Fe2O3 . nH2O (red rust) is definitely non-magnetic.
"Magnetite" mineral is an iron oxide about like hematite,
something like Fe2O3 . FeO. (aka Fe3O4)
FeO is blackish not red, and the mixes tend to be gray-black too.
Your residue lost all water and a little oxygen
and maybe also recrystallized while in the solid state,
to a form more appropriate to its new composition. .
Check out whether your estimated peak temperature, in degrees K,
was more than 1/3 to 1/2 of the melting point of Fe3O4, also in degK.
If so some recrystallization should be able to occur on the molecular
level, though perhaps not to crystals large enough to be visible.
It is possible the gasses in the candle flame helped chemically,
not just thermally, with some of the oxygen removal.
If the flame wrapped around the spoon on all sides,
it might create an oxygen-depleted or reducing atmosphere in the spoon.
Carbon monoxide, unburned wax fumes, and soot are all reducing agents
for the conversion of Fe2O3 to Fe3O4.
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Update: June 2012