Liam Lawless (lawless@ic24.net)
Wed, 03 Nov 1999 14:52:43 +0000
I mentioned Janet's question to Dallas Simpson, a chemist/photographer not
on this list, and he suggested what we've all overlooked: consult the
literature for information on stabilisation processing. I don't have
anything to hand, but vaguely recall that thiourea (presumably in acid
solution?) is one way of stabilising an image without fixing it.
Stabilisation does not confer the same permanence as fixing & washing,
however, and stabilised prints must not be washed.
Experiment 1
I've been testing my own suggestion: that conversion of silver
bromide/chloride into silver iodide by means of a 4% potassium iodide bath
considerably reduces the light sensitivity of unexposed silver salts (or
bleached silver). My first experiment was to take two sheets of unexposed
silver-gelatin paper (glossy, variable contrast RC - horrible stuff! Jessop
VC, if you must know, but I don't think it's available outside the UK) and,
under safelighting, paint one sheet over with the aforementioned iodide
solution, after which both were washed and dried in the dark.
They were then put out under normal room lighting (tungsten), without any
daylight or UV present. A coin was placed on each sheet, to show up any
changes caused by light (small coins, as things are a bit tight round here
at the moment).
The untreated emulsion contains silver bromide and silver chloride,
while the treated emulsion is now of silver iodide; there are one or two
bromo-iodide papers, but this is not one of them. And the first thing to
report is that the iodide emulsion has become - without exposure - a pale
yellow-green, whereas it was white before.
After less than 5 minutes in ambient light, the untreated, white emulsion is
starting to show the first signs of greying, when compared with the
unexposed portion under the coin. At this stage, the colour and density of
the green, iodide emulsion remain unchanged.
Darkening of the untreated emulsion continues - very slowly, but after an
hour has become a light pinky-grey that is far from the original white. The
green sheet, however, is still unchanged.
After an hour and three-quarters, still no change in the iodide sheet, but
the untreated sheet is showing a pinkish tone that I would put at about zone
VII. Growing impatient, I gave both sheets a short blast under my UV
lights.
After 2 minutes under UV: no change in the iodide sheet, though the
untreated sheet has become more neutral in colour and considerably darker
(almost a zone V tone). [Is everyone aware that we can use a number of
factory papers for print-outs from enlarged negs? Sensitivity, colour and
D-max vary with different papers; process as for salt prints, preferably
using an alkaline fix, and gold tone for max. permanence.]
At 5 minutes, still no change in the iodide sheet.
After 10 minutes, ditto. But the untreated sheet does not seem to have
darkened any more and is still about a zone V tone. This, I suppose, is the
D-max of this paper when exposed in this way.
Still no change after 25 minutes, and I decide to call it a day. I do not
know how much longer the iodide sheet will continue to resist darkening by
exposure, but it has survived an unhealthy dose of UV radiation, suggesting
that a pot. iodide bath might be of some value as a desensitiser or
preserver of bleached tones.
Experiment 2
The silver bromide + silver chloride of an unexposed sheet is not the same
as the pure silver bromide image that is the result of using an ordinary
sepia toner bleach, so the next test was to repeat the above experiment with
a bleached and washed print. This was cut in two, half treated with
4% pot. iodide, as before, washed again and both halves dried in the dark.
The bleached image is a weak, red-brown colour on this paper, and the first
thing to report this time is that no "greening" occurrred with the
application of iodide - no visible change at all, in fact. In view of the
result of the first test, I went straight to the UV lights with these
sheets.
The "normal" half darkens quite rapidly, though not so quickly as the
unexposed sheet in the first experiment, and the final D-max is somewhat
lower. The final image colour is a not unattractive greyish-brown, but not
enough density to be very useful.
The iodide half still shows no change, after an hour this time. My lights
are considerably less powerful than a Nu-Arc, but will print platinum in
about 40 minutes, so I am fairly confident in saying that iodide treatment
should be satisfactory for prints that are to be kept in albums. How much
protection it gives bleached prints against daylight remains to be seen, but
iodide looks quite promising on the basis of these tests.
Experiment 3
Just out of interest, I did similar tests with silver iodide, bromide and
chloride coated onto plain paper, and got a rather different result.
Paper was Altantic Silversafe, starch-sized and fairly inert. Separate
pieces were painted with 10% pot. iodide, 10% pot bromide, and 10% sod.
chloride, and then dried. Sensitising was with 10% silver nitrate which,
being applied after the salt, I would expect to produce an excess of silver
over salt (as in salt printing). The three pieces were each bisected, one
half then being exposed to ambient room lighting (tungsten), and the other
to my UV lights.
Freshly coated, the silver chloride is white, the silver bromide
cream-coloured, and the silver iodide yellow. Under tungsten light, silver
bromide is definitely the most rapid, starting to grey in under 5 minutes,
but the silver iodide starts to darken soon after, and then the silver
chloride. After several hours, all three have darkened considerably, the
silver bromide most, and the silver chloride least. The chloride has
produced a pale blue-grey tone, the iodide yellow-green, and the bromide a
fairly neutral grey.
Under UV, the story is much the same, except that all three have produced
greater density; the colours obtained are green (iodide), neutral (bromide),
and purple (chloride). The order of densities is again iodide, chloride,
bromide, from lowest to highest.
>From my salt printing experience I'd have expected the highest density from
silver chloride, and I guess that the difference is down to the starch
sizing since, as far as I know, I've always used gelatin-sized for salt.
Hope someone finds this interesting,
Liam
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