Magazine article Journal of Film Preservation

Restoration of Faded or Yellowed Black-and-White Silver Images

Magazine article Journal of Film Preservation

Restoration of Faded or Yellowed Black-and-White Silver Images

Article excerpt

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

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 pollutant. But it has three disadvantages. (1) It can't harden the emulsion layer, which may be damaged because of over expansion in solution. (2) The stability of the potassium permanganate solution is poor and crystallizes easily, adhering to the surface of the photosensitive material and at the same time becoming a less effective oxidizer because of the weakened solution. (3) The process requires the use of hydrochloric acid, which is harmful to the human respiratory tract. Another fast oxidization bleach is potassium dichromate, sulphuric acid and a halogenate. This solution has good stability and reduced effect on the emulsion layer (does not produce the expansion characteristic of the potassium permanganate solution). The draw-backs of this solution are that it is difficult to wash out and tends to remain on the film or photograph; it is also a stronger pollution hazard. The expansion factor of gelatin in these solutions is determined by the pH value of the solution and the presence of a chemical salt. Gelatin expands dramatically in an alkaline solution, but is restrained in an acidic solution. When the gelatin layer expands, the emulsion layer is weakened.

3. The Restoration Process of Black-and-White Silver Images in Film

The first step of the restoration process is to pre-harden the emulsion. In order to prevent the emulsion layer from being damaged by over-expansion, in chemical restoration, especially by the encroachment of acidic gases, a powerful alkaline formaldehyde bath is used to pre-harden the emulsion layer. Regardless of what is being restored, the state of the gelatin emulsion layer should be examined. This method of restoration should not be used on obviously deteriorating or decomposing film. If improperly used, there is a strong possibility that the process could cause irremediable harm to the gelatin layer, defeating the objectives of the process.

The second stage is to bleach the silver image. …

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