Journal of Film Preservation - FIAF

Restoration of Faded or Yellowed Blackand-White
Silver Images
Xu Jianhe and Ge Xiangbei
The chemical restoration of faded and yellowed black-and-white silver
image is discussed in this paper. The restoration of black and white
motion picture film can be enormously complicated. The damage may
include physical problems such as abrasions and tears, chemical
contamination, change of geometrical size, warping, wrinkling, sticking,
and separation of emulsion layer. Chemical changes in the film may
result in decomposition of the film base, chain scission, reduction of
mechanical properties, reduction, yellowing, “mirroring’ or
disappearance of the image. Changes in the silver halide black-and-white
image are generally the result of the residue of the development process
and environmental contaminants to which the film has been exposed.
Can this damage be restored? If the gelatin adhesive has not deteriorated,
the answer is “yes.”
1. Mechanism of Restoration
In the last 100 years, chemists and specialists in image restoration have
put forward many methods for recovery of chemically damaged silver
halide images. The method of redevelopment after halogenation has the
advantages of simplicity, low cost, and acceptable results without the use
of specialized equipment. This method is based on the process of
turning the metallic silver, silver sulphide (the result of the deterioration
process) and other silver salts in the image layer into silver halide
through bleaching, and then developing the silver halide to metallic
silver. The reaction is as follows:
Oxidant
Silver image + silver sulfide + other silver salt ➝ silver halide
Bleach
2. Types of Bleach Reagent and Character of Gelatin in Water
Solution
Many kinds of bleach reagents can be used to convert the silver image
and silver salts into silver halide, including potassium dichromate,
potassium permanganate, potassium ferrocyanide, mercury chloride,
copper chloride, copper bromide, etc. In the case of the three former
reagents, a halogen ion supplier is also needed, such as sodium chloride
or potassium bromide, in order to turn oxidized metallic silver into
silver halide.
From the mid-1970s, the author has studied the chemical restoration of
faded and yellowed black-and-white films and photographs.
Experiments have shown that acidic potassium permanganate is a strong
oxidizer which has a fast bleach rate and is less of an environmental
44 Journal of Film Preservation / 58/59 / 1999