Re: Dry Plate Speed & Shelflife

From: Richard Knoppow <dickburk@ix.netcom.com>
Subject: Re: Dry Plate Speed & Shelflife
Date: Mon, 07 Mar 2005 00:10:03 -0800 (GMT-08:00)

> Ryuji, can you give a very rough time line for the differrent
  methods of emulsion making, i.e., ripening vs other methods like
  reduction sensitizing etc. There must have been very rapid changes
  between say 1880 and say 1900 or even 1890.

The point is not at the level of ripening or sensitization. The method
Martin is advocating is along the same line as the preparation of
salted paper, except that the crystals are formed in the gelatin
coated on glass instead of sizing of paper. Although this approach is
very simple, it has fundamental limitations that precludes useful
manipulations to the silver halide crystals that are almost mandatory
to obtain useful photographic speed (say ASA speed of 1 or faster) for
pictorial applications.

One fundamental difference in Maddox's approach was to make silver
halide crystals in gelatin solution. This way the size of crystals can
be manipulated by a number of different ways. Generally early emulsion
makers were interested in making crystal size larger to boost
photographic speed. One way is to add silver halide solvent such as
ammonia to the reacting solution(s) to increase crystal size. Or the
emulsion could stand for some time to enlarge crystals. The
concentration of free silver ion in the reacting solution is a very
important parameter in general, but it is very critical during
chemical sensitization (digestion) process. In order to increase free
silver ion in the solution to a suitable range for digestion, excess
salts must be removed from the emulsion, except for silver chloride
emulsions. Removal of salts were also necessary to ensure long shelf
life of emulsion. All these are additional steps to Maddox's approach
of making emulsions made during 1870s.

Maddox's emulsion was silver bromide. A small amount of iodide added
was found to be useful and it was studied by Abney. This was
1880. Chloride emulsions were made for developed papers but a couple
of years later, also by Abney. A lot more detailed knowledge had to
wait for electron microscopy. People like Carroll, Smith and Trivelli
did considerable work in studying the effects of iodide in terms of
quantity, method of addition, timing of addition, and various other
parameters. This way the emulsion making became more of engineering
crystals that meet the photographic needs.

Even tighter control of crystals became possible by double jet method.
Berry made this big contribution, and a lot more was added by Wey,
Strong, and other people at Kodak Labs.

By manipulating the profile of crystal formation, one can make
crystals that have different composition in different parts of a
single crystal. This is essential in today's emulsions, to achive high
speed to grain size ratio, good reciprocity characteristics, rapid
processing time, etc. However, this sort of manipulation cannot be
made by the method similar to what Martin adovocates.

Manipulation of crystals did not end with core-shell structure. When
people found ways to modify crystallographic habits of crystals in a
reliable way, those crystals having twinned planes were investigated,
leading to tabular grain technology. This, also cannot be done in the
method similar to what Martin uses.

Digestion is a process to create structures within single grains that
enhances the utility of photoelectrons generated when a photon is
absorbed by the grain. With undigested crystals, most of the
photoelectrons are lost without doing anything useful. In order to
secure those electrons to make stable latent image centers, the
crystal has to have a way to trap photoelectrons and photoholes at
different sites and also a way to facilitate formation of photolytic
silver specks. There are many ways to do this and usually a
combination of two or more techniques are used. Reduction
sensitization is one of them. Reduction sensitization occurs in
virtually all emulsions that use gelatin as the peptizer because
gelatin has some reducing effect. This effect can be further enhanced
by increasing the free silver ion concentration, increasing pH or
both. Or this effect can be achieved by addition of a very small
quantity of reducing agent like stannous chloride. But the most
powerful sensitization is sulfur sensitization. By adding a very small
quantity of labile sulfur compounds to emulsion and hold the emulsion
at 40-80C and other favorable conditions (pAg of 8-9 and pH of 6-8)
for some time, the speed and contrast increase dramatically. Other
middle chalcogen elements like selenium and tellurium can do the same
but they are a lot more difficult. Sulfur sensitization is also
implicitly present to some degree in many emulsions without deliberate
sensitization, because gelatin contained some active sulfur compounds,
identification of which was work of Sheppard. Today's photographic
gelatins are made inert by removing much of active compounds but yet
gelatins from different batch numbers work in different ways, even in
a very simple emulsion formula, especially those using silver
chloride. Soluble gold salts were introduced by Koslowsky and Mueller
(then at AGFA), which further doubled or tripled already sulfur
sensitized emulsions. This was the basis of doubling of film speeds
after WW2 outside AGFA and Ansco. Neopan SS from Fuji was one of the
first Fujifilm products that used this technology and it is still
available. Combination of reduction sensitization and gold
sensitization is very difficult unless the reduction specks and gold
specks are well "insulated" because gold makes small reduction specks
(thought to be silver dimer) more readily accessible to the developer.

The improvement in developer formulation was also a big factor in
increased photographic speed during 1870s. Burgess proposed alkaline
pyrogallol solution without silver nitrate as a wet plate developer in
1873. This was quickly adapted for development of dry plates, which
was one big factor that multipled into the photographic speed in that
era. Prior to this, physical developers were used, like those
containing pyrogallic acid, pyrogallol, or ferrous sulfate or oxalate,
together with silver nitrate. Burgess's developer was faster in
processing time as well as necessary in-camera exposure time. Burgess
used ammonia as the alkaline agent but it was replaced with carbonate,
and sulfite was added by Newton to preserve the solution and to remove
developer stain. These are all 19th century development.

Besides, sensitizing dye and desensitizing dye are relevant but
overviews of these are more easily found elsewhere.

--
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)