From: Ryuji Suzuki (rs@silvergrain.org)
Date: 10/31/03-12:48:43 AM Z
From: Richard Urmonas <rurmonas@tpg.com.au>
Subject: Re: Test for Silver Metal in Print?
Date: Fri, 31 Oct 2003 23:48:27 +1030
> 1) Density is a logarithmic scale, so the density drop is actually 13%.
> This may not relate directly to silver content as the optical density
> of silver and palladium may differ. Further if we have the case
> where a silver "grain' has been plated with palladium the silver could be
> removed from inside this shell with no change in optical density.
> So I would exect the 13% density shift to indicate a minimum of 13%
> silver. I guess to be more exact, would require weighing the
> "coating" before and after bleaching, as someone else on this list
> has done.
I didn't want to get into that but since you brought up. I'm not
familiar with the silver grain morphology of kallitype prints. But the
relation between amount of silver and density is already complicated
by various factors. In a macroscopic level, this factor is called
covering power. In the case of silver gelatin process, this is
affected by many factors such as grain size, grain morphology,
gelatin-to-silver ratio, gelatin composition, and substrate
surface. This is one of several reasons why modern films can attain
high Dmax with less silver compared to old films. So I wouldn't argue
anything based on per cent change in log density, unless something
specific about kallitype is well studied and established. It's just a
poor basis to argue anything about chemical level, in my opinion.
> 2) My experience with ferri - bromide bleaches is that the density
> of the bleached image can vary significantly. When bleaching
> a silver-gelatine print I can end up with anything from a very pale yellow
> image, through to a dark brown image. I would suggest redoing the
> test with a process which dissolves and washes away the silver, rather
> than simply bleaching it. Perhaps the nitric acid others have mentioned.
For the same reason, even if you remove any and all silver compounds, I
can't see much you can do to relate density change to change in amount
of silver.
Also, the amount or ratio of amount of residual silver may even be
density-dependent. By measuring one value, one cannot know if the
degree of conversion is greater or otherwise in low density areas.
Anyway, simplest and easiest way I can think of to arrive at a cleanly
interpretable result is x-ray fluorescence spectroscopy, as I said
before. Both toned and untoned prints of identical test pattern should
be measured, with the material and processing held fixed for all
samples. Maybe next simplest way is to dissolve all image forming
metals in a mixture of nitric acid and hydrochloric acid, and subject
the solution to a silver-specific assay.
-- Ryuji Suzuki "Reality has always had too many heads." (Bob Dylan, Cold Irons Bound, 1997)
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