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Film can be damaged by physical wear and tear such as scratching, biological damage because the gelatin is a protein and makes a good nutrient source for various biological vectors such as moulds, bacteria and insects, and because of shrinkage.
Care in handling film and ensuring that any mechanical equipment is correctly adjusted will prevent damage.
Tearing and scratching of the film base and emulsion layer are the most common forms of physical damage.
Cellulose based films can be easily torn by poor handling. Common places for tearing damage to occur are outwards from perforations and around splices.
Polyester based films are very resistant to tearing. Even so, polyester can tear if the edge of the film is damaged in some other way — for example, cut on a sharp sprocket or broken piece of equipment.
Perforations are particularly prone to wear and damage as this is where the most tension is applied to film while it is being transported through sprocket driven equipment. Small tears leading out from the edges of the perforation are known as 'crowsfooting’ and may eventually tear further.
Damage by crowsfooting will cause a film to be unstable during transport through equipment.
Tearing around splices is also common. After many years the cement splices may suffer from loss of plasticiser, causing the area surrounding the splice to become more brittle or the adhesion between the film sections to weaken. This is often described as a 'dry splice’. The brittleness may not be consistent across the whole of the films width.
If the splice begins to break apart, part of the splice may hold sufficiently well so that the film will tear rather than the rest of the splice breaking.
Scratching is the most common form of damage that occurs to film materials. All film handling operations involve a degree of risk of scratching. Light scratching is often called rain as it can appear as if rain is falling in the projected image.
Cinching is caused by the action of film moving against itself in the roll. This movement can be caused by loosely wound film, where it has been rewound without sufficient care. As the reel tightens up on itself, any dirt or other irregularities will cause scratching. The result can be seen as fine scratches angled in the direction of the movement (Fig 6.3).
Machine transport mechanisms are another source of scratching. Any dirt that comes in contact with moving film will almost certainly cause a scratch. Machine scratches are usually very long straight lines. It is sometimes observed as a waving line oscillating with a regular frequency (Fig 6.4).
Any scratching on the base of the film will have an effect on the quality of the image by causing the transmitted light to diffract (Fig 6.5). This causes annoying lines to appear on the screen during projection and will be transferred during duplication to either film or video (Fig 6.5).
The emulsion holds the image and as such any scratch will cause a loss of information. Black and white films will show a scratch as loss in density, as well as diffraction on projection. Because colour films are comprised of layers of dyes, a scratch will show as a different colour depending on how far through the dye layers the scratch extends. Any scratch on the base or emulsion will attract dirt that will show very distinctly on projection.
Water droplets can cause localised damage to the film emulsion. It is possible that the damaged could be caused either by biological action (e.g. bacteria) or by dissolving the gelatin. Also the emulsion may so strongly adhere to the adjacent layer of film that it can be torn away by careless winding.
Small foreign objects, such as particles of rust, can cause the image or base to become stained. Often 'normal’ cleaning will not remove these marks. The foreign material may have altered the image forming material by chemical action or become permanently attached to the base polymer.
It is well known that gelatin is a good food source for a variety of micro and macroscopic life forms. It is also entirely possible that cellulose ester base polymers can also be a target.
Moulds are microscopic, plant-like organisms, composed of long filaments called 'hyphae’ with cell walls made from 'chitin’, the same material as the hard outer shells of insects and other arthropods. However, because of the filamentous construction, lack of chlorophyll and the fact that plants do not make chitin, moulds are considered by most biologists to be separate from the Plant Kingdom and members of the Kingdom of Fungi. They are related to mushrooms and toadstools, differing only in not having their filaments united into large fruiting structures.
When mould hyphae are numerous enough to be seen with the unaided eye they form a cottony mass called a 'mycelium’.
Moulds feed by absorbing nutrients from the organic material in which they live. Since moulds do not have stomachs they must digest their food before it can pass through the cell wall into the hyphae. To accomplish this the hyphae secrete acids and enzymes that break the surrounding organic material down into simple molecules they can easily absorb.
The hyphae grow in the search for food, apparently randomly spreading across the surface of the gelatin. The furrowing or channelling marks are left where the gelatin has been digested.
Mould damage is apparent on the emulsion surface as a dendritic pattern that looks a bit like a fern leaf, Fig 6.6. It is more usual for the mould to start at the edge of the film and work inwards, but it is not unheard of for most of the damage to appear in the centre of the film. This may have something to do with the tension under which the film has been stored.
Moulds reproduce by spores. Spores are like seeds; they germinate to produce a new mould colony when they land in a suitable place. Unlike seeds, spores are very simple in structure and never contain an embryo or any sort of preformed offspring. Spores are produced in a variety of ways and occur in a vast array of shapes and sizes. In spite of this diversity, spores are quite constant in shape, size, colour and form for any given species. Accordingly spores are very useful for mould identification.
Research carried out in Australia, Vietnam and Europe has identified two main genus of moulds that affect film, 'Aspergillus’ and 'Penecillium’.
Bacteria are among the simplest, smallest, and most abundant organisms on earth. All bacteria are unicellular, single celled organisms. Most bacteria are only 1×10-6m in diameter.
The most readily identifiable damage caused by bacterial action occurs if the film is soaked in water (e.g. a flood) and left for a length of time without drying. Due to the microscopic size of bacteria they cannot be seen, only the damage caused after an infestation has occurred is noticeable.
The damage caused by bacteria is more of an altering of the gelatin structure rather an obvious physical sign such as the channelling left after a mould has digested a section of the emulsion. Bacterial attack can sometimes be seen by small bubbles of a black tarry substance on the edges of the reels. In very bad examples the bacterial action can cause a film to block together, once this degree of bacterial attack occurs a film is past salvation.
Cockroaches, silverfish and beetles (both larvae and adult) all find gelatin a good food source. Being large these vermin can be easily seen, as can the damage they do as they feed. The marks left by insects look like irregular holes or channels, but these tend to be wider than mould marks and have a more irregular edge.
The biological action of moulds and bacteria is rate dependant upon three factors, food supply, water and energy (as heat). By controlling any of these factors the rate of activity can be reduced. In reality we can only control two factors, heat and moisture. By storage under controlled low temperature and low relative humidity the rate at which biological factors attack the film can be minimised.
Moulds are particularly sensitive to relative humidity. Research performed in Vietnam has shown that moulds will cease to be viable below 60% RH.
Larger vermin can be controlled by an integrated pest management approach.
Changes in the physical dimensions of the film have a major impact on the ability of the film to be accessed. As the degree of shrinkage increases it becomes more risky to transport the film through equipment.
Over time a certain degree of shrinkage occurs due to the evaporation of residual solvents, casting and release agents left over from the manufacture of the film base. Early acetates also suffered a loss of plasticiser which led to a strong smell of naphthalene or 'mothballs’.
A much greater degree of shrinkage occurs due to decomposition reactions. The plasticiser commonly used in cellulose triacetate, triphenyl phosphate (TPP), readily migrates from within the base polymer as the acid concentration rises. Since TPP can make up as much as 12-15% of the film base the shrinkage can be very significant.
The stresses induced by shrinkage that build up in a film wound as a reel are potentially very great and will distort the shape of the reel, 'spoking’ (Fig 6.8). Other ways in which the film can relieve the stress induced by shrinkage is by 'curl’ and 'buckle and wave’.
Sometimes a decomposing film may be unwound and appear not to be suffering from curl or buckle and wave to any extent, however with more handling the film will start to shows signs of these problems.
Shrinkage prevents film from transporting through film equipment. The perforation pitch becomes narrower and the sprocket teeth will no longer match with the perforations. As little as 1.5% shrinkage can cause problems with some equipment.
It is possible to modify some equipment to handle slightly shrunken film. Changing the pitch on the sprocket teeth or modifying the film path can be successful. This compensates for the longitudinal shrinkage however lateral shrinkage, which can be greater, still remains a problem.
The emulsion will compensate for a small amount of shrinkage in the base by compressing. Eventually the stress between the emulsion and the base will become so great that the emulsion will lift away from the base, 'emulsion lift’ or 'channelling’.
It is also possible to redimension the film base, to return the base dimensions to as close as original as possible. This is done by softening the base and making it absorb some form of filler. There are several proprietry processes that have a great deal of success with this treatment.
Simpler methods using just elevated humidity or a solution of water, acetone and glycerol can be adequate. However the effect is short lived and the film needs to be duplicated almost immediately. The later process is also unsuitable if the film is severely decomposed as the gelatin emulsion will become very soft and may disintegrate. Using this technique it is also possible for severely decomposed film base to become opaque, very soft and easily torn.