Re: Color of Vandyke Browns, was First Kallitype

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From: Joe Portale (jportale@gci-net.com)
Date: 01/03/01-05:50:42 PM Z

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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