Photographs

A brief overview of the chemistry of photography

Photography—drawing with light—includes any process in which pictures are produced by the action of light on light-sensitive materials.

Light-sensitive materials

Many materials are light-sensitive, but not all are suitable for the production of images:

• some materials fade in light;

• some materials darken in light;

• light causes molecules to decompose in some materials; and

• in other materials, light causes molecules to combine and to create larger molecules.

The most commonly used light-sensitive materials in photography are silver salts.

Light-sensitive silver salts

Silver salts decompose in light to produce metallic silver, which makes up the image. The silver salts which were found to be best for this process were the silver halides—silver chloride, silver bromide and silver iodide.

Initially, prolonged exposures were used to develop images. Negatives were placed in a printing-out frame in contact with sensitised positive papers and then exposed to light. Once the images had developed sufficiently, they were fixed chemically. This was the principle of the printing-out papers.

Developers

There are many advantages to be gained from the use of chemical developers. These include:

• reduced exposure times;

• allowing the photographer much more flexibility in manipulating his/her images; and

• making enlargements, reproductions and mass production of images possible.

With the introduction of developers, the sensitised paper was exposed to light for a shorter period. During this exposure time, an invisible latent image formed. This image was then intensified 109 times using the developer.

Developers are reducing agents, that is, they supply electrons to a system.

A number of reducing agents were tried, but not all were successful. If they are too strong they reduce all the silver halide to metallic silver—even the silver halide which has not been exposed to light. If too weak, they will not develop the image enough.

Much of the work done before a suitable process was found was experimentation—to find developers which had enough power to reduce the exposed silver halides, but not enough to reduce those that had not been exposed to light.

Fixers

If photographic images are not fixed, the chemical reactions will continue—leading to loss of image. Once the image has developed, the silver halides which have not been exposed to light must be removed from the system. Fixers are chemicals which dissolve silver halides and remove them from the system.

The fixer and dissolved silver halides must be washed away. Retention of these within the system will lead to discolouration.

Emulsions

Emulsions were produced from:

• albumen—egg white;

• cellulose nitrate—called collodion in this application; and

• gelatine.

Emulsions coated onto the photograph base-paper, glass or metal:

• allowed an even spread of the light-sensitive salts over the base material;

• provided a smoother surface than the surface of uncoated paper, giving more sharply focused images; and

• could be applied in liquid form and then dried in a thin film.

Gelatine

Gelatine survived as the most popular and widespread emulsion material. This is because gelatine has properties which make it particularly suitable for use in photography:

• gelatine is soluble in warm water, but remains as a gel in cold water. This allows water-based processing of the image without loss of emulsion. It is important to note that old degraded gelatine may be soluble in cold water;

• gelatine can be treated with formaldehyde to harden it, so that it is no longer soluble in hot water;

• gelatine contains minute amounts of materials which increase the sensitivity of the silver halides to light. This is due to the eating habits of cows, which are the source of the gelatine. Cows like hot-tasting food, which contains sulphur. Rabbits, on the other hand, do not like hot food and their gelatine is less suitable.