From: Ryuji Suzuki (rs@silvergrain.org)
Date: 10/26/03-06:24:42 PM Z
From: Sandy King <sanking@clemson.edu>
Subject: Test for Silver Metal in Print?
Date: Sun, 26 Oct 2003 18:14:19 -0400
> Some of the literature on toning indicates that during metal toning
> the more noble metal plates the less noble one. Other literature
> indicates that the more noble metal actually replaces, either
> completely or in part, the less noble one.
The feasibility of the desired reaction has to do with a lot of
factors. One major factor is redox potentials. You can break down the
desired reaction into an anodic process and cathodic process, and for
each of them, look up the standard half cell potentials of reactions
involved and adjust the values for particular conditions
(concentration, temperature, etc.) and see if the reaction can occur
spontaneously. If this fails, you need to consider different reaction
path. All these stuff are described in any decent undergraduate level
chemistry book -- look for electrochemistry. What you saw in your
darkroom literature is a gross oversimplification of this principle.
However, even if the redox condition is favorable, there is no
guarantee if the desired reaction indeed occurs, and at a reaction
rate that's fast enough to make practical use out of it. For example,
there may be other reactions occuring simultaneously, preventing or
interfering the desired reaction. Or the situation might be that
something plates on or converts metallic silver surfaces, and this
surface conversion prevents further reaction. So it's not as easy to
theoretically predict the result of a particular toning process.
Niranjan Patel suggested one possible method, but I think X-ray
fluorescence spectroscopy is an easier and faster test that has also
been used in measuring degrees of conversions of important toning
processes like selenium toning. With this method, sample preparation
is a lot easier.
-- Ryuji Suzuki "Reality has always had too many heads." (Bob Dylan, Cold Irons Bound, 1997)
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