Re: Dry Plate Speed & Shelflife

Though Ryuji didn't summarize my position entirely accurately, I don't
intend to play the "yousaid-Isaid-yousaid" game. The relevant posts can be
found in the archives.

No doubt, the bathing method is unlikely to replace those professional
emulsions (specially designed, full of tons of any kind of dopants &
epitaxies). Having said that, it might still be fun to try.

To make it very short now, the passage I quoted from Glafkidès (3rd
edition), clearly indicates sulfur sensitization. Adding the possible
effects through reduction sensitization (ascorbate/Quinaldine Red) as
previously described, may well provide acceptable levels of speed.

Martin

----- Original Message -----
From: "Ryuji Suzuki" <rs@AgX.st>
To: <alt-photo-process-l@sask.usask.ca>
Sent: Monday, March 07, 2005 4:36 PM
Subject: 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)
Received on Tue Mar 8 04:40:44 2005