Several formulations have been suggested by film manufacturers to promote the swelling of the emulsion, anneal the scratches and then to restabilise the emulsion to re-establish the best conditions for image permanence.
Kodak has a recommended process called RW1.
A solution of approximately 50 gms per litre of each of sodium sulphite and
borax produces a rapid and even softening of almost all emulsions [a high pH is
essential] and concentration and temperature is not very critical. Most
laboratories use a process temperature of 20C, followed by a wash for 10
minutes at 20C and a final rinse stage of 1 minute at 20C in a 500:1 Photoflo
solution to help uniform drying.
Commercial colour processing laboratories often use a process solution from the ECN2 process, the Prebath for rewashing black and white archive film, as it is already available in solution. This is a high pH sodium sulphate solution, and very effective as a rewash process. Any of these processes are good for black and white film of any period provided there is adequate washing following the softening solution. The wash must be as efficient as the usual final wash for a black and white process producing film for an archive. This aspect is covered on the chapter on Processing. A wetting agent rinse can be used after the wash.
Rewashing subtractive incorporated coupler colour films to anneal scratches
is a far more complex problem. The re-wash solution can be any of those listed
above although ECN2 Prebath is most commonly used, and the annealing process
seems to be just as effective as with black and white with most colour films.
However, in order to preserve the dyes for the future, most colour processes use
a final rinse or stabiliser solution to buffer the gelatine, leave a specific
internal pH, or carry out some final stabilising procedure - the dyes of most
film need this to ensure optimum dye stability. The answer is to use, as a
final stage, after the re-wash solution and water wash, the same process
solution as was used in the original process for this film stock. Clearly, this
requires identification of the film stock and knowledge of the original
process. Some last solutions were pH buffers and a wetting agent, some were formaldehyde,
which hardened the emulsion and restricted the breakdown of some dyes or
residual couplers, and some were solutions of specialised stabiliser chemicals.
One formulation used as a final rinse in the event of no information could be
as follows - it is similar to that used for many colour processes between 1960
and 1990:
Formaldehyde 37% 10 ml
Wetting Agent [e.g. Photoflo] 5 ml
pH adjusted to 4.0
Water to 1L
Washing is also used to remove some other defects on film. Washing is a
cleaning process and removes dust, stuck on dirt, oils and the general grime of
ages [see below], but is specifically used for the removal drying marks process
dirt and ferrotyping (see below).
Drying marks are the result of uneven drying of the emulsion during processing, and the result of droplets of hard water drying off the emulsion surface leaving a raised shoreline often with fine particles stuck to the emulsion. Soft water or distilled water leaves virtually no drying marks. Some film can be seen to have drying marks dating from their original process up to 80 years ago, usually in the form of a trail of droplet patterns or a continuous tide mark down the length of the film. Even after this time rewashing can remove these almost entirely.
Process dirt similarly is dust and sometimes fine gelatine particles from edge skiving that collects in process solutions and is picked up on the film emulsion surface. These too can be removed by most re-washing processes. Modern processors use continuous filtration systems to remove this material.
Ferrotyping is the term used for a local and often patchy change in the emulsion surface so that the normal matt or semi lustrous appearance alters to a smooth glossy finish.
Ferrotyping was the alternative term used to describe "glazing", the process of producing a shiny surface on photographic paper by drying the wet emulsion on a hot steel surface under pressure. In cinematography, "ferrotyping" is caused by tightly wound film becoming damp from humid storage. The emulsion swells and presses against the touching base layer and takes on the smoothness or glossiness of the back of the film. Thus, the process of glazing or ferrotyping on both paper and film is similar.
Ferrotyping is not serious in itself, as the image is usually unchanged, but it is rarely uniform and areas of ferrotyping have sharply edged boundaries, which show on the image as dark uneven wavy lines. Ferrotyping on a print is seen as a dark line; on a negative that is printed it is seen as a light line.
Ferrotyping is often removed by a water wash, and this can be hastened and made more effective by using a solution that encourages the gelatine to swell. The same solutions used to treat scratches seem to be effective for ferrotype lines.