From: Joe Portale (jportale@gci-net.com)
Date: 01/06/01-10:51:24 PM Z
Sarah,
> Joe, I believe you are describing how fixer removes residual unexposed
> silver salts in silver gelatin film or paper or with salt prints,
something
> that I already did have a rudimentary concept of.
I may have misunderstood your question. The chemistry of the way fixer acts
is basically the same no matter if you are using VDB or conventional film or
paper.
>What I still wonder is why in vandyke printing, it is very dilute fixer
that darkens what is basically a rusty reddish iron print by >reducing
silver salts to metallic silver.
The molecules of silver halides (chloride, oxide, iodide, nitrides) are
being stripped of the halide salts and leaving behind the metallic silver.
This changes the structure of the crystals and the way they reflect the
light.
>Does the iron print remain beneath the silver or are they bound together?
No, the iron is removed (supposedly) from the paper during the development
or clearing stage. The ferric molecules attach to the excess halides from
the silver and form ferric and ferrous compounds, both should be removed in
the first wash. By the time the print is placed in the fixer, most of the
iron has been washed out. any residual iron should be gone by the time the
final wash has been completed.
>What is the role of tartaric acid - to remove highlight stains or to
facilitate the mingling of silver and iron? Why doesn't EDTA work >with
vandyke to remove residual iron? I will be home until tomorrow afternoon if
you can get back to me.
Theoretically, you could make an image with only the FAO and silver nitrate.
I've tried it , the image is very weak. The tartaric acid serves two
purposes, one is as a preservative. The second is like you said, it assists
in the formation of the ferric-silver compunds that create the image.
EDTA does remove iron, it just doesn't work as well with FAO. AFO clears
better with a neutral to slightly acid developer. That is why when faced
with some really stubborn VDB prints, a 2% bath of citric acid and water is
often recommended.
Good luck,
Joe
> Thanks, Sarah
>
> > Sarah,
> >
> > I CC'd you on this in the hopes you will see it before you leave.
Sodium
> > thiosulfate or ammonia thiosulphate are what we uses for fixer. For
most
> > alt applications we use the sodium because the ammonia is far too
strong.
> > The chemistry of what happens when we fix film or papers is that the
fixer
> > converts the residual silver salt to a form that is more water soluable.
An
> > over simplified chemical equaltion for this would look like this
> >
> > AgCl (silver chloride) + NaSO4(H20) (sodium hyposulfate the water is
part of
> > the hypo molecule) = AgSO4 + NaCl
> >
> > The actual process is much more complicated due to other factors, but
this
> > shows what happens. The silver salt is converted to silver sulfate
which is
> > more easily diffused in water. The common salt is simply a by product of
the
> > reaction and is highly water soluable also. The hypo effects the the
> > deposited metallic silver more slowly. It wants to grab onto the silver
> > salts first because they are easier to convert. Hope this helps.
> >
> > Joe
> >
> >
> > ----- Original Message -----
> > From: Sarah Van Keuren <svk@steuber.com>
> > To: <alt-photo-process-l@sask.usask.ca>
> > Sent: Tuesday, January 02, 2001 8:43 PM
> > Subject: Re: Color of Vandyke Browns, was First Kallitype
> >
> >
> >>One aspect of vandyke that I've never had a pop-science explanation for
is
> >>how the dilute fixer reduces the silver nitrate to metallic silver. Can
you
> >>help me with that? If I don't respond it is because I'm going to be away
> > and
> >>without a laptop Jan. 4-19.
> >>
> >>Sarah
> >
> >
> >
> >
>
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