About Heritage Images
Inevitably and inexorably, old images degrade with time. There are several processes at work, one of which is unavoidable. The other major causes of image degradation are avoidable. Prior to discussing these, it is useful to consider some basic principles.
Introduction to the Chemistry and Physics
As a consequence of physics, the contrast - sharpness of photochemical images declines with time, when those images were originally made relying upon reduction of a salt of a metal to the metal itself. The most common photochemistry involves silver, but other metals have been used as well, including palladium, gold and platinum.
The chemical processing of the image results in the reduction of a salt of the metal to the actual metal within the carrier. The most common modern carrier is found in photographic film, but other carriers have been used, especially in the early days of photography. Some photographers even experimented with egg whites, with the metal salt suspended in it and coated onto a substrate such as glass.
When light hits the suspended light-sensitized salt, there are changes that occur in it which allow for chemical processes to create an image. Although the process is far more detailed than this outline suggests, this is enough to explain the fragility of images.
Unavoidable Change
Even when stored under perfect conditions, images produced as above decline in contrast and sharpness. This is a consequence of the physics ... atomic movement continues, inducing gradual changes. In effect, heavier concentrations of metal in some areas (the dark areas of a negative) are moving into the lower concentration areas (the light areas of the negative). What occurs is a gradual darkening of the light areas and a gradual lightening of the dark areas.
In practical terms, that means that the image is "smearing," losing the distinctions between those areas. We "see" this as a drop in contrast and apparent sharpness.
This process can theoretically be stopped only by storing at Absolute Zero. But even temperatures that would freeze water could damage the carrier, so this is not a solution to the problem.
The basic fact is that the Second Law of Thermodynamics rules this process and in time, the images will be beyond salvation. Countless motion pictures from the early days have been lost to this process, and to avoidable causes (see below).
To preserve the subject matter, the image must be copied in some manner, either onto another high-quality film or onto a digital medium. There are complications with both techniques, but they do work. Of course, storage on more film means more degradation in time, eventually requiring intervention. But some modern materials have been developed with archival storage in mind; whether they actually work as claimed remains to be seen.
Up to some variable point in the life of a film image, contrast can be restored by computer, sometimes very effectively. But it should be noted that even the best software, such as PhotoShop, relies upon approximations to make such restorations ... and with increasing damage, those approximations become less viable.
Avoidable Change
Many things can damage images, all of which relate to suboptimal handling and storage. High humidity is a major culprit but low humidity also takes it toll, dessicating the medium and causing it to crack and/or peel away from the substrate. The exceptional image below shows signs around its edges of such damage.
Storage of images in envelopes that react chemically with film and/or substrate is another important issue, especially where decades of physical contact are involved. There are modern envelopes that have been designed specifically for safe archival storage, some of which even allow for scanning through them without removing the image from its safe harbor. Again, whether such envelopes perform as claimed remains to be seen.
Images are extremely fragile, and they become more so with time. They must never be stacked on top of each other ... even modest vibrations cause them to abrade against their envelopes. Careful image storage is definitely not the "box in the closet" approach, nor for that matter, the "box in the museum" approach. Both of these suggest that the images are not protected sufficiently.
And of course there is the matter of physical handling. Sometimes this is necessary in the copying and/or printing process. Neophyte photographers know that one always handles only by the edges. But where the photographs are already deteriorating, if they are of value, expert help should be sought before doing any handling at all ... especially removing them from their envelopes. Experts can assist with such a delicate operation. If the images are too far gone for safe handling of any kind, it may well be too late.
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