Re: Some points of ponder

There are reasons why silver gelatin emulsions are made in the way
they are, not the way you mentioned. AgX crystals are made from
soluble halide and soluble silver salts. This allows to form crystals
of high quality with little impurity and crystal defects. Oxidizing
metallic silver particles won't result in the crystals of the same
quality. (There may be a way to do it but it won't be simple.)

Also, in order to make sufficiently light sensitive AgX crystals of
good developability, you need real tiny amount of impurities, defects,
etc. The method you mentioned didn't factor in the importance of these.

One virtue of silver gelatin process is the development
process. Although the concept of developing existed in Calotype
process, this is most successfully used in silver gelatin process.
Say we have films consisting of AgX crystals of 1 micron diameter.
Each individual crystal has about 20 billion silver atoms. In
non-developing processes like POP, much of these silver atoms must be
reduced to silver by light exposure. This requires a lot of lot of
light. In the case of silver gelatin, all that's needed is that as few
as about 3 silver atoms be reduced by light exposure. Developer
solution will reduce the rest of 20 billion atoms as long as the
particular crystal has more than a couple of metallic silver atoms
present. (that number, 3, is for film materials; typical paper
materials need a few more silver atoms.) This makes chemical
"amplification" factor of a few billions, and the material can
register exposure by as few as several photons.

You have to say bye bye to development and silver gelatin process if
you try to incorporate more than 2 or so of silver atoms in individual
crystals.

--
Ryuji Suzuki
"You have to realize that junk is not the problem in and of itself.
Junk is the symptom, not the problem."
(Bob Dylan 1971; source: No Direction Home by Robert Shelton)
From: MARTINM <martinm@SoftHome.net>
Subject: Re: Some points of ponder
Date: Fri, 16 Jul 2004 10:48:06 +0200
> There are two things I would like to add to your very thorough explanations:
> 
> 1) Adding another dye to an already dye sensitized emulsion might turn out
> in practice to be difficult.
> What about fully exposing and developing the film, applying a rehalogenating
> bleach and then re-sensitize in a IR dye bath?
> 
> 2) Since colloidal silver is said to shift the sensitizing wavelength to the
> red and NIR (I think up to 800 nm or so), wouldn't it make sense to slightly
> pre-expose the emulsion and subsequently carry out colloidal development?
> The colloidal developer might be formed of a very diluted (say 1 : 100)
> standard
> developer.
> 
> Martin