Journal of Film Preservation - FIAF

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 overexpansion,
in chemical restoration, especially by the encroachment of
acidic gases, a powerful alkaline formaldehyde bath is used to preharden
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. The emulsion-hardened
film is immersed in a bleach bath composed of potassium dichromate,
sodium chloride and sulphuric acid for 1.5-2.0 minutes and terminated
at the point where the silver image has completely turned to pale yellow
silver halide. If the immersion is less than 1.5 minutes, halogenation will
not be thorough; if it is over 2 minutes, silver halide may be dissolved
into the bleach bath, resulting in the loss of silver image density. Sodium
chloride is the source of the chloride ion; without it, the silver image
would quickly be turned into white silver sulphate particulate and
precipitate out of the solution, in which case the restoration would be a
complete failure. The bleach phase is followed by a water wash.
After bleaching and water-washing, the film is immersed in a cleaning
bath composed of 10% sodium sulphite (or 5% sodium bisulphite) in
order to remove residual potassium dichromate. This process is
extremely important. If the film is not thoroughly cleaned, the residual
potassium dichromate contaminate the image, affecting the color and
clarity of the image. The more effective the cleaning process, the cooler
the tone of the image, and the higher its resolution will be. The above-
45 Journal of Film Preservation / 58/59 / 1999