[pure-silver] Re: Chemical Fogging in Reversal Processing
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Wed, 16 Aug 2006 20:55:52 -0700
----- Original Message -----
From: "John Banister" <jbanister@xxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Wednesday, August 16, 2006 7:37 AM
Subject: [pure-silver] Re: Chemical Fogging in Reversal
Processing
> Thiourea is probably being used as a redeveloper in
> place of
sulfide. The
> result is a sepia image of sulfide. Because either
> Thiourea or
Sulfide works
> directly on the halide it does not need a fogging
> exposure or
chemical fogging.
> It also eliminates the need for a final fixing step
> because all of
the halide
> is converted to sulfide whereas there is always a small
> residual of halide
> which is not made developable by flashing. I am
> uncertain about
the effect of a
> chemical fogging agent but would guess its the same as
> the use of
Sulfide, that
> is, no final fixing step is necessary.
>
> --
> Richard Knoppow
> dickburk@xxxxxxxxxxxxx
> Los Angeles, CA, USA
Is this situation with Thiourea/Sulfide unique, or do you
think it's more
generally true that if a toned result is desired from a
B&W reversal image that
the fogging - second development steps can be bypassed or
would likely be
easy to incorporate in the toning process?
Normal two step sepia toning, for instance of a print, is
done by first bleaching the metallic silver image to some
form of Silver Halide, most usually Silver Bromide. The
halide is then treated in a solution of Sodium Sulfide or
Thiourea. When using sulfide or thiourea is used for
"redevelopment" in reversal processing the image is already
composed of a halide since the bleach used for reversing
_removes_ the metallic image silver and leaves the
undeveloped halide the film was composed of orinally. The
second development can be done with a normal, rather active,
developer, after treating the remaining halide with light or
a fogging agent, to make it developable, or, it can be
directly developed with the sulfide solution resulting in an
image composed of Silver Sulfide. The Sulfide process is
simpler since it eliminates both the flashing step and a
final fixing step, which regular development requires.
Provided the Sepia colored image is acceptable the sulfide
process works fine and the image is very permanent in the
same way as a Sepia toned silver image.
Either Sulfide or Thiourea can be used for redevelopment.
In toners the Sulfide solution is just Sulfide and water and
the same solution is used for reversal development. In
toners using Thiourea redevelopment the redevelopment
solution usually has some alkali in it to activate the
Thiourea. The color of the Sepia image will depend on the
nature of the original halide and, when using Thiourea the
formula for the redeveloper. I have never seen a Thiourea
formula for redeveloping in reversal development but
probably any toner formula would work. Defender Photo (later
owned by Dupont) published a series of formulae for toning
Varigam paper with Thiourea. The redevelopers contain either
Sodium Hydroxide or Sodium Cabonate, these formulas will
probably work for reversal redevelopment.
One key to reversal is the presence of a halide solvent
in the first developer. Some formulas used Hypo but the
solvent mostly used was Sodium Thiocyanate. The first
devloper must be a very active developer capable of
producing high contrast and also developing fast enough to
prevent too much solvent action. The solvent is needed
because some halide exists in the original emulsion which is
so insensitive to light that it is never exposed, no matter
how much exposure is given in the camera. This halide
remains to be developed in the reversal bath and causes
veiling of the highlights. The solvent is present in an
amount just sufficient to eliminate the veiling. The amount
of solvent varies with the emulsion being developed. When
using old formulas for modern film the solvent content, film
speed, devleopment time, etc., must be determined by
experimante
When the concentration of solvent is correct the film
speed for reversal purposes is close to the speed for use as
a negative. The original exposure and the first development
are the only really critical parts of reversal. Bleaching
and second development is carried out to completion. The
flashing exposure must be sufficient to make all the halide
remaining after the bleaching step redevelopable but not so
much as to drive the emulsion into reversal or to generate
photolytic silver. This is a pretty big window of exposure.
The second developer is, again, an active developer with low
solvent content. Any solvent in this developer may dissolve
some of the halide needed for the final image and cause low
density of the shadows. A good print developer or a high
contrast developer like Kodak D-19 will work for both steps.
When a flashing exposure and conventional development is
used for reversal there will always be some residue of
undeveloped halide that survives the entire the reversal
process. This halide must be removed by a final fixing step
or it will eventually attack the image in the same fashion
as in an incompletely fixed print or film. After fixing the
film should be washed as usual. Again, the fixing step is
not needed when sulfide is used to produce the final image
because no halide will remain.
I have some formulas for reversal of Agfa motion picture
films of the 1940's and Kodak publishes a set of formulas
for their current B&W reversal films. I will post these if
required.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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