Bob Kiss's answer is what I also remember from my reading.
He had the advantage of being taught directly by T.H.James, who
succeeded Mees as head of Kodak Labs. My source is about the same
but from James's and Mees's books plus a better memory and
probably more basic chemistry than I have.
Gelatin is, of course, the magic key to all this. It is
possible to make light sensitive coatings without it, as
illustrated by collodion (wet plate) and many alternative
printing methods, but none have the sensitivity of gelatin based
coatings. I don't know the latest research but at least until
recently a complete understanding of the effect of gelatin was
not understood. The nature of the gelatin is extremely important,
something that caused Eastman to create his own gelatin making
business early on. Mees tells the story of how Eastman had a
failure of his dry plates early on which was traced to the source
of his gelatin. Mees says it depended on what the cows were
eating (gelatin comes from cow bones). Some were eating mustard,
I am not sure if this was the good stuff or the failed stuff but
the amount of sulfur and its compounds in the gelatin have a
profound effect on its photographic properties. AFAIK and I may
not have the right (Bob Shanebrook please) eventually the gelatin
was highly refined to remove trace materials and the desired ones
added. While various polymers have been tried and are added to
the gelatin nothing has ever been able to replace it.
The chemistry of gelatin is very complex, several books have
been written about it, and is mostly beyond me.
BTW, while the sensitive coating on film and paper is called
an _emulsion_ it is not, its a suspension. Curiously, the
support, if cellulose, _is_ an emulsion. I remember many years
ago when Technicolor discontinued its three color printing method
attending a special meeting of the SMPTE where several Tech
executives talked about the process. They kept referring to the
_emulsion_ until someone in the audience got one of them to
clarify he meant the _support_ not the coatings. He was correct
from a formal standpoint but the term _emulsion_ for a gelatin
based sensitive layer (or even one without gelatin) has become so
well established there is no chance of changing it and it would
be confusing to try.
Kodak discontinued the use of cellulose nitrate for motion
picture film in 1951 as I believe was general throughout the
industry. AFAIK the last movie to be shot on nitrate was "Sunset
Blvd". It was thought at the time that nitrate was clearer than
acetate. Maybe it was but nitrate was so dangerous it had to be
eliminated.
I rather like chopped liver, especially on Ritz crackers.
On 2/9/2019 2:25 PM, ROBERT KISS wrote:
What am I, chopped livahhh!?!?!
On February 9, 2019 at 8:21 AM ROBERT KISS <bobkiss@xxxxxxxxxxxxx> wrote:
Just to add a few things to Richard's info: (DISCLAIMER! This is from memory of classes taught by Dr T. H. James 47 years ago!!! You can confirm it by reading his bible...oops...book, THEORY OF THE PHOTOGRAPHIC PROCESS.)
1) Gelatin is what is called a Zwitter ion; it has an alkaline end and an acid end at opposite ends of a long organic molecule. That is why it has the iso-electric point Richard mentioned where the solution balances them and makes the swelling minimum. In the usual alkaline developer environment, the gelatin swells.
2) Latent image sites, where photons have struck halide (Br, Cl, I) ions releasing electrons which reduce nearby ionic Ag+ to Ag atoms, can occur on the surface of the silver halide (AgX) crystals or in the interior. They also provide an auto-catalytic site for more interstitial Br- ions to deposit more electrons which, in turn, reduce more interstitial Ag+ ions to make a latent image site with more neutral Ag atoms. There is a minimum number (Was it 6 or 8?) Ag atoms that constitute a "developable" latent image site. The developing agent, carrying an extra electron in the alkaline solution deposits it on the latent image site which then attracts and reduces more Ag+ to silver atoms. Some "surface" developers can only do this slowly on the surface of the AgX crystal. Others, with higher concentrations of complexing agents like Sodium Sulfite, dissolve some of the unexposed AgX crystal providing more available Ag+ ions and, with sufficient dissolution, reach the internal latent image sites. I think these were called "internal" developers. Anyhow, this process of building up more and more Ag atoms to create a visible silver grain is called "amplification". The fact that most dev agents contribute their extra electrons to the latent image site as a catalytic action is how the dev recognizes the latent image site. Might have to do with the fact that a site of a few Ag atoms has a different surface oxidation potential than anything else in the lattice work of the AgX crystal. No surprise that, when the dev agent reduces Ag+ in non latent image sites you get...fog!
There is sooooooooooooooooooo much more but, rather than bore you and probably make more mistakes due my 68 year old memory, I think this is a reasonably accurate and accessible description.
CHEERS!
BOB
On February 8, 2019 at 5:36 PM `Richard Knoppow < dickburk@xxxxxxxxxxxxx <mailto:dickburk@xxxxxxxxxxxxx>> wrote:
I am not sure what allows the developer tell an exposed grain
from an unexposed one. There is a chemical difference caused by
being light struck. I think the charges but one of the books on
the process will explain it better than I can right now.
It does not have to do with the pH of the solution because
there are developers which work in acid solutions (Metol will
work in neutral or very slightly acid solutions and metallic
developers work in acid). The concentration of salts in
processing formulas, both developers and others, affects the
swelling of the gelatin. Gelatin is what is called amphoretic,
that is, it is neither acid or alkaline. It does, however, have a
characteristic pH, called its isoelectric point, where the
swelling is minimum. Photographic gelatin is very slightly on the
acid side of neutral. One important effect of this is the binding
of hypo and fixer reaction products to the gelatin. When the
gelatin has been in an acid solution, like hardening fixing
baths, it tends to bind the fixer and reaction products. This
binding is released when the gelatin is treated in an alkaline
bath. Sodium sulfite washing aids, like Kodak Hypo Clearing
Agent, are just on the alkaline side of neutral so can release
the binding without destroying the hardening action of the fixing
bath. A more alkaline bath, such as sodium carbonate, will also
accelerate washing but destroys the hardening action of metalic
hardening agents. Sulfite has the additional action of being an
ion exchanger for thiosulfate and its reaction products.
The action of the developer in selectively converting light
struck halide particles is covered by the standard texts in
photographic chemistry. I think at least one edition of "Theory
of the Photographic Process" C.E. Kenneth Meese is available on
line free. The later edition by Mees and James is still available
for sale.
Kodak was certainly the center of knowledge of photographic
chemistry for decades. At least one of the retired Kodak
researchers is on this list and I hope will respond to this.
Chemical photography is having a resurgence after a few years
of becoming very obscure. None of the old time companies has
survived although Ilford is doing OK. Tetnal recently announced
it was dissolving after failing a chapter 11 type bankruptcy. Not
good.
On 2/8/2019 12:58 PM, joe mcguckin wrote:
What is the magic bit about a developer that is able to tell a sensitized grain of silver halide from an unsensitized one?
Is it true that the alkalinity of a developer makes the gelatin more porous? I’ve seen where people mention that the developer ‘opens’ the gelatin, making development
happen more rapidly.
On a side note: It sounds like all the old guys at Kodak with years of knowledge about film emulsions and chemistry are either retiring or dying off. Whats happening with the
institutional knowledge necessary for companies like Kodak and Ilford to continue mamufacturing film. Are they going to have to start over or redo the last 30 or 40 years of r&d?
Joe
--
Richard Knoppow
dickburk@xxxxxxxxxxxxx <mailto:dickburk@xxxxxxxxxxxxx>
WB6KBL
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