Re: Dry Plate Speed & Shelflife

From: MARTINM <martinm@SoftHome.net>
Subject: Re: Dry Plate Speed & Shelflife
Date: Thu, 03 Mar 2005 09:08:40 +0100

> Besides the ascorbate sensitization, one of its advantages stems
> from the high amount of silver halide present in the
> emulsion. Regarding ripening (and possibly gold sensitization), I
> guess this could be easily integrated into the schedule of this
> method. As you know, it's always much easier to inrease grain size
> than to achieve the opposite.

I disagree with pretty much all that's said above.

Reduction sensitization is effective but unless it is combined with an
effective way to control its reaction kinetics as well as
after-reaction stabilization, this technique is bound to fog. This is
a very tricky issue you can find in literature from 50s and 60s when
people at Kodak and Fuji were trying hard to make good use of
gold-sulfur(or selenium)-reduction sensitization with acceptable level
of fog.

In my experience, fog due to overdigestion with sulfur comes rather
gradually, but fog from excessive reduction sensitization comes more
rapidly. (These days people fight for speed with tellurium
sensitization, and of course fog is the problem.)

I don't know how you can incorporate ripening into that process
without undermining other photographic properties of the sensitized
plate. You may be able to increase the crystal size by adding a
thioether, thiocyanate or ammonium salts, but then unless you remove
them soon after crystalization, problems will most likely arise.

In conventional emulsion making system, it's easy to reduce grain
size. But it's also easy to increase grain size. I never aim smaller
than about 50nm (not that I measure the grain size-frequency
distribution or anything) or greater than just below a micron, smaller
grains are easier to make with high contrast and also take less time.
Amino acid composition of the gelatin (methionine and cysteine for
example) has influence on this, as well as dopants and addenda that
adsorb on AgX crystals to retard ripening. You can add a very small
amount of cysteine for this purpose. PVA is also known to retard
ripening greatly. You can also precipitate at a low temperature. After
all, in a simple single jet system, it's often that all you need to
decrease avg grain size is to dilute the salt in the kettle (keep the
salt constant but put more water and adjust gelatin). In order to make
crystals bigger, you'll need a ripener, or maybe combination of
multiple ripeners, like a thioether compound (which is always on) and
ammonium sulfate (which can be turned on or off). You'll have to spend
longer time to grow crystals, because if you try to grow them too
fast, renucleation will occur and you'll have too many dead grains.

> Well, you might be surprised to learn that the power levels involved
> with those lasers - most of the time - are extremely low. Whereas
> youre likely to handle several kilowatts of light power, we're
> usually dealing with a few milliwatts only.

High intensity is difficult to use because of high intensity
reciprocity failure. This is the major problem with chemically
developed color papers used for high quality digital output, where
paper is exposed by LED or other light source. In order to expedite
the exposure, the light output must be increased, but HIRF becomes the
problem. This is particularly problematic because color papers are
mostly silver chloride to make the development and fixing process very
fast. AgCl has very poor HIRF characteristics. So manufacturers use
all sorts of techniques such as core-shell profiling, multiple doping,
etc. to improve HIRF characteristics.

You say a few milliwatts but that's still a lot compared to exposure
from an ordinary enlarger or especially in camera exposure.

> I guess it depends on the scale of emulsions to be produced. For
> small experimental batches the bathing methods might remain
> attractive: no need for double-jets - though I have to admit
> reproducibility might be an issue.

I don't think so. Conventional emulsion is not that hard to make from
50ml up to a liter. If you are coating on glass, not paper, half of
the problem is solved as well.

Double jet system is certainly useful for greater control, but it is
not really a strict requirement for useful emulsions in the sense of
classical photography. Emulsions like Panatomic-X, Agfa Isopan, Ansco
Superpan Supreme, etc. are presumably all made by single jet
process. So are almost all AGFA emulsions from WW2 era. I can even use
a disposable plastic pipette as a jet to make an emulsion like
those. For that matter, even some rudimentary tabular grain emulsions
can be made with single jet setup if the precipitation conditions are
set just right.

pAg controlled, computer profiled precipitation system is something I
don't have. But I've made quite a few double jet emulsions using giant
disposable medical syringes, or simply two burettes hung over the
kettle. It's not that hard.

> Anyway, you might at least give the ascorbate reduction
> sensitization a try...

I've done it. Stannous chloride, ascorbate, sulfite, and a couple
other organic agents. However, I'm pretty happy with S+Au
sensitization without deliberate reduction sensitization.

--
Ryuji Suzuki
"Well, believing is all right, just don't let the wrong people know
what it's all about." (Bob Dylan, Need a Woman, 1982)