Photographs

A brief overview of photographic deterioration mechanisms

There are two main mechanisms of deterioration: sulphiding and oxidation-reduction deterioration.

Sulphiding

Silver has a strong tendency to react irreversibly with sulphur. The product of this reaction is silver sulphide—a black material that appears yellow when it is in very small particles. The main sources of damaging sulphur are:

• hydrogen sulphide from atmospheric pollution.

• inadequate washing after fixing. It is virtually impossible to remove all the fixer as it is retained tenaciously by paper fibres and baryta coatings. Fixer—sodium thiosulphate—breaks down in the presence of moisture and releases reactive sulphur.

Both of the above cause yellowing, fading and loss of detail in the highlight areas. Eventually the whole image is affected and will fade and change colour.

The highlight areas of this print show fading and loss of detail due to sulphiding. Photograph courtesy of Fred Francisco

• use of exhausted fixer baths. Thiosulphates combine with silver ions; many of the resulting molecules are soluble and so the fixer and excess silver can be washed away.

If the bath is exhausted, insoluble molecules can form; these are retained by the gelatine. So there are sulphur and silver ions still in the system. The silver reacts to become metallic silver, causing yellowing. Non-image areas change in colour: from white, to yellow then brown.

High relative humidity accelerates sulphiding.

Oxidative-reductive deterioration

This is the most important deterioration mechanism for silver-based images. The main mechanisms and their effects are:

• metallic silver particles are oxidised to produce silver ions;

• while metallic silver is coloured and therefore makes up the image, silver ions are colourless and do not absorb light;

• as ions have an electric charge, they can move around the system. They can migrate to the surface where they are reduced back to metallic silver, creating the characteristic mirroring or silvering out; and

• they may also migrate within the system and then be sulphided.

Silvering out around edges of a photograph. Photograph courtesy Artlab Australia, reproduced with permission of Ann V. Nicholas

Oxidative-reductive deterioration can also cause loss of highlight detail, overall fading and colour shifts to warmer hues.

Images developed using chemical developers are less vulnerable to this deterioration than those on printing-out papers, because the silver particles in developed images are bigger.

Only minute quantities of oxidising agents are required to cause deterioration and there are many oxidising agents in atmospheric gases and pollutants.

Controlling temperature and relative humidity helps slow down the process.