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In addition to humidity and temperature under the environmental list, air contamination would be the third most significant type of environmental deterioration. One of the serious problems in preservation is the relatively large quantity of oxidizing gases in the atmosphere in certain areas. Large cities must contend with coal-burning industries, gasoline and diesel engines and oil and gas-burning systems. High pollution also exists in areas where paints, printing inks, lacquers, enamels, varnishes and cosmetics are being used. Near the seacoast, very small amounts of airborne salts may infiltrate into storage areas which not only accelerates chemical degradation, but also encourages the growth of microorganisms.
Chemical
Degradation
The second most common form of deterioration is chemical degradation. The
two kinds of chemical degradation that are most frequently observed are image
fading (discoloration) and stains. The black image which is most commonly a
silver product, will turn a yellowish-brown with inadequate processing
techniques. This occurs most often when fixer is left in the print or film
because of insufficient washing. The job of the fixer in the photographic
process is to reduce the unexposed silver to a salt product that can be washed
away. When the fixer is not removed, it acts like a bleach to the silver and, in
effect, reduces or removes the silver. It acts on the highlight (lightest) areas
first and thus reduces the contrast n the print. When fixer is not removed from
the print, it causes the silver to appear as if it is 'fading.' This can be
improved by the restoration process which will be explained in a later section.
The second type of chemical degradation is created by insufficient fixing.
During the later half of the 19th century, photographers were not aware of the
consequences of inadequate processing. Additionally, they did not have the
monitoring devices we have today to let us know when the chemicals were no
longer useful. Consequently, at times photographers were using fixers that were
exhausted (used up) and this created dark stains on the prints. When the
unexposed silver is not removed from the print by fixing it, the silver
gradually darkens the more it is exposed to light.
Physical
Deterioration
The third most common form of photographic deterioration is physical
deterioration. This form is most prevalent in photographs that have been
poorly stored. Some examples of physical deterioration are holes, scratches and
spots that are caused from the abrasion of one material against another. Brittle
matte board or photographs can snap and create losses in the image area. Glass
plates, when not handled properly, can chip or break. Water can be a terrorist
against a friendly photograph. I have seen collections totally destroyed from
flooded basements, burst water pipes, spillages and have seen many photographs
that have stuck to the glass from a frame due to excessive moisture.
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