P3D Black and White Slides, part 3

["Brown, Fritz LABS" <brownf@xxxxxxxxxxxxxxxx>]

In my previous post, we saw that after developing a negative image in a
piece of black and white film a positive image also is present in the film,
we now need to explore the means of discarding the negative image and
developing the positive image.

To remove the negative image without disturbing the undeveloped silver
halide,  we use a strong oxidising reagent.  Both potasium permanganate and
potasium dichromate can be used successfully and are used in an acidic
solution commonly using sulfuric acid (note these are nasty solutions and
you need to use proper laboratory safety precautions when using them.  You
have been warned).  What these do is to oxidise the metalic silver making up
the negative image to silver ions ( Ag(0)->Ag(I) ) which are freely soluble
in aqueous solution. The silver halide chrystals of the unexposed emulsion
are not soluble and are undisturbed by this process.  After the bleaching
process you need to completely remove the oxidising agent and a clearing
solution is employed to fully reduce and help remove any remaining bleaching
agent.  At this point you have a piece of film that has unexposed silver
halide in the emulsion, but it is there in amounts that produce a positive
image.  In fact when you remove the film from the processing tank and look
at it, you will see a positive image on the film, but it is white (it is
really cool to see).

All that is left is to completely fog the remaining silver halide and
develop it to produce a positive image.  Kodak's kit uses a fogging develper
that fogs the film chemically.  The photographer's formulary kit requires
that you expose your bleached film to light for a minute on each side to fog
the emulsion.  Most home recipes also use light exposure to fog the film.
You can also chemically fog your film using a strong reducing agent such as
sodium borohydride (another highly reactive chemical that requires good lab
technique to use safely).  Whichever means you use to fog your film,  you
now just need to develop, fix and wash your film as normal to produce a
positive tranparency.

As I've said before, any silver based film can be reverse processed, but
different films will give different results.  Some films include a base fog
that will prevent you from getting clear highlights.  Some films (T-grain
films in my hands) dont produce enough total density to give decent blacks.
Contrast varies widely between different films.  In short, you need to
experiment with different films and with your own processing techniques to
get the results that are satisfactory for you.  But after all,  the
experimentation is what is most fun about all this anyway.  Right?

I didn't present specific recipes and protocols here because I have posted
them to this group in the recent past.  Search the archives and you will
find my protocol for reverse processing Technical Pan.  As a friendly
gesture,  I will offer the following:  contact me by email and I will
process a roll of Tech Pan for you so that you can see what kind of results
you can get.  Be sure and contact be before you shoot so that I can give you
information on exposure and metering and so I know what is coming.  This
offer is open until it starts to get out of hand.  What do I get out of
this?  I get to see your slides as I process them.

Later
 -Fritz M. Brown
brownf@xxxxxxxxxxxxxxxx