Film Restoration Methods at Test: Scratches Cleaning - Text 3
Photochemical methods
Before digital technology was applied to film preservation, scratches were a big problem for film laboratories, as nothing could be done about those made on prints or negatives of previous generations (commonly referred to as ‘printed in’).
When a film emulsion is scratched, small portions of the visual information carried by the print or the negative have been scraped away and cannot be recovered by photomechanical means, unless the scratch is extremely light (in which case, the emulsion can be softened so that it expands slightly, thus filling the affected area). Scratches on the base of the film are somewhat easier to treat, because no visual content was removed from the emulsion. Dirt and speckles of dust can be removed before printing and projection through various cleaning methods (at present, best practice calls for ultrasonic cleaning in a chemical bath). At the printing stage, there are two techniques for reducing or preventing the appearance of base scratches on the duplicate.
In the first method (now infrequently used), known as “light diffusion”, the light source used for printing is directed from a variety of angles, so that the greatest possible amount of light reaches the grooves at a 90-degree angle as it does with the remaining flat surface of the film stock. No chemicals are required in this process, but the resulting image exhibits a lower contrast, and sometimes looks foggier than the original.
The second method, called “wet gate printing”, is far more effective. It requires a printing device where the optical gate is enclosed in a box containing a chemical substance so transparent that it fills the grooves, making the base appear smooth and free from scratches, just long enough for the light of the printer to pass through in a perpendicular direction. This is the best way for transferring scratched film into duplicates of excellent quality by photochemical means (only the very deepest grooves may leave a residual trace of their presence).
