The Black And White Negative/Positive
Process
A photographic image is formed when light falls on a photographic film resulting in a change in the materials on the film invisible to the human eye. That material is called an emulsion.
Photographic emulsions consist of silver halide; usually silver bromide, crystals suspended in gelatine. When these crystals are exposed to light or other radiation, minute quantities of silver are formed. These minute quantities of silver are the image. This initial image is called the latent (or hidden) image and in order to amplify it to a level when it can be seen the film must be 'processed', always, in the case of motion picture films, by soaking in a series of aqueous chemical solutions. In all processes, there are a number of essential chemical steps followed by washing and then drying. In addition there are usually additional chemical steps that are inserted in the sequence that are not essential but are there to ensure the process is controllable and repeatable, to improve the quality of the image, or to ensure that the solutions have a reasonable life.
Essential stages of black and white negative/positive process:
1. Development
Let us consider a piece of film where half has been exposed to light.
To turn the exposed film to an image we have to convert the exposed area into silver. We do this with a developer. A developing agent in solution converts the entire halide crystal on which there is an invisible latent image formed during exposure into black metallic silver.
Silver bromide
(Exposed) + Developing Agent
=
Metallic Silver + Oxidised Developing Agent + Bromide ions in solution
Go to: Developing agents
2. Fixing
We have turned the exposed area into silver, we know have to remove the unused silver halide and make the film non-light sensitive. We do this by putting the film into a fixing bath, which turns the unwanted silver halides into water-soluble salts of silver. The fix solution converts the unexposed silver bromides to soluble complexes.
Sodium thiosulphate is commonly used, and the complex salts formed are often called Argentothiosulphates. The white crystals of undeveloped bromide are dissolved and the film becomes transparent where they have been dissolved. The film is said to "clear".
Silver bromide + Fix salt = Soluble Silver Thiosulphate Complex
Some processes use Ammonium thiosulphate which is much quicker at fixing but more expensive, and many fixer solutions also contain a hardener to reduce the swelling of the emulsion and are usually acid as the solution is more stable when made acid. Sodium thiosulphate is sometimes known as Hypo, because it was incorrectly known as Sodium hyposulphate in the early days of photography.
3. Washing
The soluble complexes and fixer must be removed
from the
emulsion by efficient washing otherwise staining and fading of the image
may occur; and as little as possible fixer should remain in the emulsion
after washing.
4. Drying
After washing, the emulsion is then dried - the emulsion is very soft at this stage and should never touch anything as it dries. During drying, the emulsion, which swelled with water in the developer almost returns to its original thickness.