From: Sarah Van Keuren (svk@steuber.com)
Date: 01/03/01-09:30:49 PM Z
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. 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. Does
the iron print remain beneath the silver or are they bound together? 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.
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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