>All the processes to date have, without exception, used silver nitrate as the
>commonly available source of silver. But nitrate is an oxidising anion, and
>tends to dissolve the colloidal image silver during the wet processing
>development procedure, especially under acidic conditions. To minimise this
>problem, alkaline buffered developers of high pH (e.g. Borax) were
>recommended. Alas, these create a new problem, because they cause hydrolysis
>of the excess ferric ion in the sensitizer and the deposition of insoluble
>ferric hydroxide in the image, which will ultimately cause it to fade. The
>cure is simple in principle: to replace the silver nitrate with a soluble salt
>of silver having a non-oxidising anion. There are a few such salts known to
>modern chemistry, but most of them, e.g. silver fluoride, have unacceptable
>properties or a level of toxicity that debar them from 'home chemistry'.
>There is, however, a little-known and relatively innocuous silver salt that
>does fit the bill: silver sulphamate, NH2SO3Ag. This cannot be bought, but is
>easily made in situ. It results in an acidic sensitizer of pH 2 to 3, which
>can be washed out of the paper cleanly, without hydrolysis of the excess
>ferric iron, and without any tendency to dissolve the colloidal silver image.
This is the chemical rationale for my Argyrotype process, which has been in
use now for five years. The reference is:
Mike Ware, 'The Argyrotype Process', British Journal of Photography, vol.
138 (no. 6824), pp. 17-19,(13 June 1991).
I look forward to reading Peter's article: where is it to be published?
Mike