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According to an answer to Is there (or was there ever) a process that directly produced a reflective positive?:
In theory, you could even process any regular 'negative' photo paper in a reversal process and obtain a direct positive print of some sorts, just by modifying the development process.
What sort of modification is required? And what sort of results are obtained?
The original negative was created by exposing a light-sensitive emulsion. The grains within that emulsion are brought out by the intensity of light that hit them along with the development process.
What you have is an image where the more intense light is captured as a darker area on the negative and the less intense as lighter, thus, the negative.
The key here is that the lighter parts of your negative have had less exposure. So, to reverse the negative into a positive, one needs to bleach away the developed grains (causing them to go from dark, to light), re-expose the negative (causing the light spots to gain further exposure), and re-develop (allowing the light areas to now darken).
At the end of this process, you'll have a positive. To recap:
I researched this quite a bit ago and found the following two sites to be quite useful in my experiments:
Illford does make a photo positive paper that is most commonly used in pinhole cameras.
The photographic emulsion is a mix of salts of silver. These are crystals with varying sensitivity to light. The developer is formulated to be selective in that it seeks out exposed crystals and reduces them (splits them). This action liberates metallic silver. It’s the amount of silver in any one unit area that determines if that area is black, dark-gray, light-gray or clear. The other half of the crystal is called a halogen (Swedish for salt maker). When the crystal is reduced, the halogen portion is dissolved by the waters of the developer.
Now every photo film or photo paper image exits the developer with many tufts of metallic silver in every unit area. In each of these unit areas are salts of sliver crystals that were not exposed, and therefore not reduced by the developer. Actually these unexposed and undeveloped crystals are the exact counterpart of the image. If we could somehow remove the tufts of silver without disturbing the unexposed crystal, we could then fog the film with light and develop the film again. The second developing deposits tufts of metallic silver, but this time, the blackening will be a positive image.
OK – how is it done? Let’s talk about the positive process of a color film like the E-6 (Ektachrome slide process). We develop the film in a black and white developer. The color film contains (simplified view) 3 emulsions: one sensitive to red, one to blue and one to green light, the three primary light colors.
The silver salts in each emulsion is exposed in proportion to the colors and intensities of the vista being imaged. This film is then developed in a black & white developer. Three black & white negative images form. Inside the red emulsion is a colorless cyan dye. Inside the green emulsion is a colorless magenta dye. Inside the blue emulsion is a colorless yellow dye. These dyes are colorless because they are missing one ingredient. All three are missing the same ingredient. The black & white developer also contains this missing ingredient (called CD-4).
As the film is developing and the tufts of silver are forming, the waters of the developer contain dissolved gas. One of these is oxygen. As the tufts form, oxygen attacks and tarnishes the silver (oxidizes). The missing ingredient (CD-4) needs a catalyst to allow it to unite with the dye in film. The dye in the film, without the CD-4 is called a Leuco Dye from the Greek for hidden.
The film is then placed in a color developer. This developer contains a chemical fogging agent and CD-4 plus a black & white developer. The developer goes to work on the now-fogged crystals. As silver tufts emerge, they oxidize, and this is the catalysts that causes the CD-4 to unite with the Leuco dye. The Cyan dye blossoms in the red emulsion. Likewise the yellow dye blossoms in the blue emulsion and the magenta dye blossoms in the green emulsion. The film exits the developer with three negative silver images, three positive silver images and three positive dye images.
The film is then bleached. The bleach attacks the silver and coverts it back to a silver salt. Next comes the fixer. It is a solvent for silver salts but not a solvent for dye. The film exits the fix as a color positive (slide or transparency). Next is the stabilizer -- a wetting agent that prevents water from beading and a biocide to prevent beasties from eating the gelatin binder of the emulsion, which is made from bone and connective tissue of bovines.
