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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