The Black And White Reversal Process

Special black and white film stocks can be processed in such a way as to produce a positive image directly in the course of one sequence of processing stages. Sometimes this type of process is called diapositive or direct positive.

Technically, the reversal process is only one of several methods of processing a diapositive film.

 

1. 1st Development - this produces a negative image as before.

2. Bleach - the silver negative image is made soluble and washed away leaving the unexposed silver salts

3. Reversal exposure - the unexposed silver salts are exposed to light and completely fogged

4. 2nd Development - the remaining exposed salts are developed to silver and these are a positive image.

In practice, although these are the only essential stages, several other solutions are usually added to control the process.

A typical black and white reversal process is as follows:

First Developer
Rinse
Bleach
Rinse
Clearing Bath
Rinse
Re exposure
Second Developer
Rinse
Fixer
Wash

The first developer is essentially a normal negative type with the addition of a silver halide solvent usually Sodium or Potassium thiocyanate.

If we again consider our piece of film with one half exposed, after first development the film will consist of half silver and half silver halide. We now bleach the film with a Potassium Dichromate and Sulphuric Acid Bleach. This bleach will dissolve the silver leaving the silver halide untouched.

The film now passes through a clearing bath to remove all traces of the bleach. We now have a piece of film with one half clear and the other half silver halide diagram b .We expose the film to light and pass it through a second developer. The second developer is a very active solution containing Sodium hydroxide to increase the alkalinity to make sure all the exposed halide is converted to silver, determined by the first developer.

Because we are re-developing all the halide, we are not concerned about contrast, as that has been determined by the first developer.

After second development, the film is put through a fixer to remove any small traces of silver halide left, washed and dried.