Richard Sullivan (richsul@earthlink.net)
Tue, 08 Jun 1999 08:36:37 -0600
>
>It is however interesting that this only occurs with the Sodium
>Tungstate. WHY? Have you repeated (without the Sodium Tungstate) as
>you said this originally gave a print which "came out fine"? Or does
>this still give some mottled appearance?
>
>--
>Jeffrey D. Mathias
>http://home.att.net/~jeffrey.d.mathias/
The sodium tungstate will under certain chemical conditions react and form
tungstic acid. (I learned this from Tony Mclean) The order in which it is
added to the emulsion is critical. FO + ST + LIPD is the order not FO +
LIPD + ST which will clump and take several minutes to redissolve. It is
quite possible that the sodium tungstate is reacting to something in the
paper and forming the clumpy tungstic acid.
I came upon the use of sodium tungstate thinking it might be a substitute
for mercury that was commonly used in the older platinum papers and
processes. It worked and turned the prints brown but with a decidedly
different look than cesium palladium produces. BTW, lead compounds appeared
to produce brown prints as well but curiously uranyl nitrate appeared to
have no effect. My thoughts were that the presence of large heavy molecules
were interfering with the deposition of the metal particles somehow and
thereby affecting the color.
Light and heavy issues play a part in the Ziatype system. Lithium is the
third lightest element and cesium is way down at the bottom of the chart
and is very heavy.
I've tinkered a bit with really loading up the print with lots of tungsten
with some interesting results. It produced a soft delicate print with good
dmax. Sodium tungstate can be mixed in an 88% solution btw which is pretty
astounding from such a heavy compound. Its main industrial use is in fire
proofing fabric.
--Dick
505-474-0890 FAX 505-474-2857
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