There are 45,667,998,532 people out there who know chemistry better than
me, but here's what I know which might help. When Ferric Oxalate is exposed
to UV light it is reduced to a ferrous state; in a highlight area where
little UV hits, little iron is reduced from a ferric to a ferrous state. In
shadows, much iron is exposed to UV light, therefore, much of it is reduced
to the ferrous state. On development, Pt/Pd metals are reduced
proportionally to the ferrous oxalate present; little Pt/Pd appears in the
highlight areas since little ferrous oxalate exists there, while lots of
Pt/Pd is reduced in the shadows since there is much ferrous oxalate to
cause the reduction. Now, when Ferric Oxalate (keep remembering that Ferric
is ferric and Ferrous is ferrous and rarely the twain shall meet) sits
around in a bottle for 3 months or six months or NINE YEARS it oxidizes and
becomes Ferrous Oxalate. So, since it is already reduced, the exposure of
the print causea little or maybe no more reduction (depending on whether
there is any ferric oxalate left at all, most of it, if not all, having
already been reduced by plain old sitting around and oxidizing). Since the
reduction of Pt/Pd requires iron in the ferrous state, if the entire print
is already reduced, it just goes dark. As Pt/Pd printers see prints
becoming muddier and muddier with little dMax, they know their FeOx is
going bad, that is it is oxidizing from the ferric to the ferrous state.
When John added hydrogen peroxide to the sensitizer, wonder of wonder, he
was reconverting the ferrous oxalate into ferric oxalate (H2O2 does that)
and so revivifying the sensitizer. However, the hydrogen peroxide won't
make all the ferrous iron into ferric iron so my guess is the prints didn't
have the tonal scale or dMax they would have had were the FeOx new and
fresh. This chemistry should also explain why Hydrogen Peroxide is an
effective contrast agent: adding it to fresh ferric oxalate inhibits its
reduction to ferrous oxalate and therefore adds contrast. So, given all
that, I'd still go with my earlier claim: add EDTA and Oxalic acid to the
FeOx to give it a 6 month shelf life, but throw it out after that; don't
rely on H2)2 to reenergize it. I mean, jeez, a bottle of FeOx costs only
$4-5; why risk your nearly $100 worth of platinum on a $4 sip of iron. I
mean, who adds ginger ale to 20 year old single malt whiskey?
whew...