Water has been used for cleaning film since the early days of cinematography. The re-wash process described for emulsion scratches above is a simpler version of the same principle. Film that has been processed can be put back into the original process solutions or just the last washing stage using water alone to remove surface particles. In the 1930's with the advent of more mechanical processors the technique became quite common and several film manufacturers devised special solutions to promote cleaning without additional risk to the film emulsion which does soften considerably when re-wet. Today some laboratories have special short path length processing machines, sometimes with rotating buffer rollers to help the cleaning process. For very many years, these were the only cleaning techniques in use. Technicolor's laboratories did a great deal to devise safe, effective cleaners using water.
Aqueous cleaners have a secondary action to the cleaning of the surfaces that is not found in solvent cleaners. The emulsion is re-softened in a water-based solution and swells. This has two resulting effects. Firstly it washes out any water soluble chemicals that were left in the emulsion by the last process stage and in the case of silver, black and white images is not a problem as no chemistry of this sort was intended to be left. Indeed a second wash may well help to remove any last remnants of sodium or ammonium thiosulphate, the fixer, which might eventually cause fading of the silver image. This effect is a problem on modern tripack colour films where the last stage of the process sequence is to use a solution, sometimes called a "stabiliser", to leave some residual chemicals in the emulsion to help conserve the dyes, and thus problem is the same as that described under emulsion scratches [please see].
The solutions that have been proposed over the years for re-washing film effectively are legion and washing to remove dirt has frequently been confused with washing to anneal scratches; and undoubtedly the best solutions for each are different.
For washing the use of wetting agents and various surfactants [detergents] are more effective than plain water. Several have been tried for years and it is difficult to know which is more effective. It is probable that many different trade names actually refer to the same active ingredient. Kodak's Photoflo is the most widely used as it is sold specifically as a film wetting agent [used at a concentration of 500:1]; Renex, Teepol, and many others seem similar. In many film-processing laboratories in countries where these brands are not easily available, other liquid dish washing detergents are used and "Fairy Liquid" is said to be as effective as any!
Not only does gelatine, the primary matrix of the emulsion, swell during cleaning with water, but cellulose acetate and cellulose nitrate bases also absorbs some moisture. Polyester does take in water but at a significantly slower rate. It takes about three seconds of contact of film with water to ensure that the film is evenly wetted and it is generally felt that film should be exposed to water for as short a time as possible, if the purpose is to simply clean the film surfaces.
It is quite clear that aqueous washing is very effective and using the best machinery may be at least as effective as using any solvent in an ultrasonic cleaner. Surprisingly a great deal of investigation is still needed to establish how to carry out this form of cleaning on a large scale fast. There are two unresolved problems - whether the brief washing needed to clean the surface of colour tripack film will remove stabilisers and result in a reduction in dye life [that can only be countered by reusing the last process solution again] and how to dry film quickly, without using a large costly processing machine drying cabinet again. Several experiments to attempt to dry photographic film using microwave and radio wave energy are continuing, so far without great success. Meanwhile most film laboratories use their existing processors for film rewashing only when they feel no other method is effective, and are more likely to use rewashing as a means to anneal emulsion scratches rather than as a cleaning technique.
Washing with water may remove some types of dirt which organic cleaning solvents are unable to dissolve and remove [sugar solution marks, e.g. coffee, is a good example!].
Cascade wash tanks.
Water is introduced at the bottom of the last wash tank and the overflow from
this tank is directed to the bottom of the previous tank, and so on.
Spray washers
Spray jets are directed at the film and the used water is drained off.
Spray washes are effective and fast but use large quantities of water.
Water knives
A high velocity stream of water is flushed across the film surface.
Adjustment is critical to prevent splashing.
Calder Washing Machine
Using Kodak recommended formula
Air Drying
Drying is usually carried out by air knives positioned across the film. The
knives should produce an even drying across the film and produce no streaking.
Air knives are slotted tubes through which a stream of clean air is directed at
both sides of the film.
Vacuum Drying
Small vacuum chambers are placed against both sides of the film and
sufficient vacuum is supplied to suck off the surface moisture.
Rollers or wheels
Wringer - Sling Squeegee
Two rollers, one with a flange, the other without, are used on either side
of the film to squeeze the moisture off the film and on to the rollers. The
speed of the rollers throws off the moisture by centrifugal force.
Roller squeegee
Two rollers, with the film running between them displace the moisture,
which is taken away by gravity.
Rotary buffer Squeegee
Plush covered rollers are used to wipe away the moisture.
Wiper blades
Flexible rubber or plastic blades are used either singly or in pairs to
wipe away the moisture is the film passes in light contact with them.
Curl
Over drying or uneven drying may result in the film curling either around
the base or around the emulsion.
Blisters
May be caused by too much heat being used
Water spots
Water droplets which have dried on the film, is usually caused by poor
drying or condensation.
Shoreline effects
Wavy lines along the length of the film, usually along the outside edges
but sometimes encroaching into the picture area are probably caused by uneven
drying.