- Film Preservation Handbook Contents
- Film Construction
- Base Polymers & Decomposition
- Gelatin
- Image Forming Materials
- Damage to Films
- Cold Storage of Film
- Preparation for Long Term Storage
- Occupational Health & Safety
- Condition Reporting
- Film Identification & Handling
- Film Repair
- Conservation Treatments
- Photographic Duplication
- Disaster Planning
Conservation Treatments
Rewashing
Rewashing is an aqueous based solution that is designed to swell the gelatin used in the emulsion and the backing layer. The degree of swelling of the gelatin is dependant upon pH, (Fig 12.9) which determines the net charge of the gelatin molecule.
Fig 12.9 The effect of pH on the swelling of gelatin
Formula
| Chemical | Weight (g/100 litre) |
|---|---|
| Sodium polymetaphosphate | 500 |
| Sodium sulfite | 840 |
| Sodium metabisulfite | 1,000 |
The result of swelling the gelatin is to increase the distance between the gelatin chains. This moves many of the chains surrounding a blemished area closer together. As the gelatin is washed and dried it returns to near its iso electric point and the chains draw in closer together again, recreating many of the intermolecular bonds, crosslinking.
After full drying the gelatin returns to its original unswollen state, however the crosslinking has drawn the sides of small blemishes together effectively reducing the dimensions of the blemish (Fig 12.10).
Fig 12.10 The effect of rewashing on a scratch
Considerations
There are several issues that arise with rewashing archival film. These are:
- weakening of the base as it absorbs moisture
- damage to perforations and splices
- brittleness of the base
- solubility of any surface treatments or image dyes
- decomposition leading to acid levels causing increased solubility of the emulsion
Fig 12.11 Reinforcing a splice
To reduce the risk of damage to the film during rewashing careful examination of the entire film is needed and any damage or splices need to be reinforced or supported (Fig 12.11) with a piece of tape attached to the base. A small amount of overhang will make it easier to remove the tape once the film has been rewashed.
If the film has any decomposition or unknown treatment then test a small less important section of the film.
The machine used for rewashing needs to be well maintained so that there is no dirt in the system, especially the drier, or unevenness in the transport. The transport tension on the film reduced to prevent undue stress on the film, the idea being to provide transport through the machine that is as gentle as possible.
The degree of the reduction in the blemishing in the emulsion is largely dependant upon the severity of the damage. Deep scratches where part of the image has been lost will still be apparent, however light scratches or other surface blemishes can be made almost unnoticeable.
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