The correct preparation of film prior to long term storage is one of the most significant steps that can be made in film preservation. Incorrect preparation will accelerate the rate of base decomposition in cellulose nitrate and acetate films, and can lead to blocking in both cellulose and polyester based films.
Films should be stored either on a suitable core or reel, with an adequate length of leader attached to the head and tail. The films’ identification information should be permanently written on the leader.
Film cores make a convenient centre for the film that enables the film to be placed on winding equipment, and other film transport mechanisms, without damage to the film at the centre of the reel.
Film cores are specific to the format i.e. 35mm wide cores for 35mm film. Smaller gauges of film, e.g. 16mm or 8mm, may be stored on projection reels.
It is not good practice to use adhesive tape to secure the film to the core for any reason.
Not only are there concerns over the suitability of the adhesive but the sudden shock when the film is unwound and reaches the end can cause cinching.
The film core size plays a significant role in some forms of curl that develop during long term storage. A larger diameter core, either 75 or 100mm, will help reduce the effect of 'cold flow curling’. The smaller 50mm cores can be used although they are not recommended for this reason.
Cold flow curling occurs when the film base polymer adjusts to a new shape over a long period of time. In the instance of film the base will take on the curve of the film reel, the smaller the diameter the tighter the curl effect. Cellulose triacetate is faster to react to cold flow curl than polyester, however cellulose triacetate is also faster to reverse the curl. The cold flow curl in polyester film is referred to as core set. Core set can be reversed over time, however it does create handling issues.
If a film has developed cold flow curl then rewind the film in the opposite direction around the core, i.e. if the film was originally wound emulsion in, then rewind the film emulsion out.
The materials that the core is made from is very important. Whatever is used must not harm the film in the short or longer term.
In general a 'safe’ plastic is most suitable. Cardboard and wood should not be used.
A film leader is a short section of film that should be attached to each end of a film. Some types of unprocessed film, known as 'cream leader’ because of their light colour, are suitable.
The leader provides a convenient location to write identification information about the carrier. The leader also provides a degree of protection for the film during winding and other handling.
'Painted’ leader is also available for this purpose. However, this often becomes powdery over time and deposits the 'paint’ as a dust that may cause scratching. Therefore the dust needs to be cleaned away before the film can be used. Also, unless the exact composition of the pigments and carrier in the paint is known, there is a risk that these will harm the film, especially the image, in some way.
Along with storage temperature and relative humidity, wind tension plays a significant role in the rate of decomposition of cellulose based films. The correct wind tension allows greater diffusion of decomposition gases from the film during storage, which in turn has an effect in reducing the rate of decomposition. The ideal wind tension is referred to as a preservation wind.
For long term storage films should be evenly wound to a tension whereby they just hold their shape in a pack, Fig 8.3.
Too loose, the pack can distort and the core may drop out while handling.
However, the film should not be so loose that it can move easily against itself during normal return to storage handling. This tension is really a compromise between so loose that damage can occur if the film is moved and so tight that the decomposition gases cannot readily diffuse from the film.
To hold the film in the correct wind tension a small piece of 'photographically safe’ adhesive tape can be used to secure the end of the leader.
Films wound to a preservation wind must be rewound before transporting or being used on projection equipment. The lower tension will be taken up by machine transport mechanisms causing the film to pull against itself. This can potentially cause scratching. Similarly, during physical movement from location to location the lower tension permits the film to move against itself, again potentially causing scratching.
Other advantages of the looser preservation wind are that any dimensional change that may occur within the film, due to fluctuations in temperature or relative humidity (RH), will have less effect on the film. One of the major causes of blocking is the cross linking that occurs between the gelatin emulsion and backing layers under high RH. This is assisted by the pressure exerted by the gelatin swelling. A preservation wind reduces significantly the pressure that the swelling causes and reduces the degree of cross linking that can occur. Similarly, with changes in temperature the preservation wind can accommodate the dimensional changes that may occur.
During normal use, films are exposed to a wide variety of dirts and soils. Projector oils and greases from fingers build up on the surface over time. Many of these are acidic, or become acidic, and can accelerate dye fade and silver image oxidation. It is possible that they may even have an effect on base decomposition.
Over time these patches of oil may also harden and become difficult to remove and become visible on projection or during duplication. Small hard particles of dust and 'film dust’, the small amount of film that is worn off around the edges of the perforations by machine transport mechanisms, are sources of potential scratching when the film is handled. While cleaning can be expensive it does reduce the risk of the problems mentioned above.
Film should not be stored in a plastic or paper bag. Plastic bags will trap decomposition gases and this will accelerate decomposition. Unless Kodak’s 'acid scavenger – molecular sieve’ is being used, where it is recommended to enclose the film and the sieve together in a plastic bag inside the can. Paper bags are a source of acids and dust and all similar packaging, such as newspapers that may have been used to prevent the film from moving around during transport, must also be removed.
'Rubber’ or 'elastic’ bands must be removed from the films before storage. These can decompose and release sulfur compounds that will damage the silver or dye image. In extreme cases the band will decompose to a soft mass that will run down inside the film pack and requires a large effort to remove it completely.
Film splicing tape or other tape that has passed the 'photographic activity test’ can be used to secure the loose end of the leader. Any other tapes, including unknown tapes holding sections of the film together inside the film pack, should be removed and replaced with splicing tape known to be safe or the film should be cement spliced.
Any paper objects, such as grading or timing charts, loans information, invoices etc., must be removed before the film is sent for storage. Any such documentation should be noted and stored in a acid free envelope with the film I.D. information clearly written on it. Similar entries should be made in the cataloging system so that the two objects can be linked together in the future.
Some form of enclosure is useful for storing a film not only for a degree of protection from physical factors but also for affixing identifying labels. The selection of an enclosure is important as some enclosures may not provide adequate protection and damage the film over the longer terms of storage.
Traditionally, motion picture film has been stored in a film can made from either steel (coated or uncoated), aluminium or some type of plastic. The rigid barrier formed by the film can provides some protection from physical abrasion, dust and larger vermin.
Cans also provide some short term protection from water and may also slow down the rate of diffusion of pollutants from the environment. Cans buffer the effects of the outside environment to create a microclimate within. The buffering can slow down rapid changes in temperature and absolute moisture content of the air within the can.
Any identifying labels should only be placed on the outside of the can. No other materials should be placed inside the can. In particular paper and especially paper printed with inks. Paper may contain acids as well as being a source of moisture and dust, and is to be avoided. Adhesives and rubber bands should never be used inside a can as these are sources of harmful solvents, sulfur and oxidising agents.
The enclosure material should be chemically stable and free of acids and oxidising agents that may be released slowly over time. Plastics used for cans should be chemically inert and unplasticised. Some pigments may contain reactive metals, these need to be checked before use. Plastics identified as suitable in the ANSI Standard IT9.2-1991 Photographic Processed Films, Plates and Papers – Filing Enclosures and Storage Containers are:
- polyester (polyethylene terephthalate)
Although not specifically mentioned in the standard, polystyrene is generally considered acceptable due to its inert nature. Plastics to be avoided are those that contain chlorine, such as PVC, or nitrate.
Metals should be non-corrosible, such as anodised aluminium and coated or stainless steel. The coating used for metals should be checked to make sure it is not a source of harmful chemicals. If a metal can starts to corrode it should be replaced, especially if the corrosion is occurring on the inside of the can.
To check the compatibility of some materials with photographic items a Photographic Activity Test (PAT) can be performed. The test is useful for papers, cardboard, adhesives and some plastics.
The PAT uses elevated temperature and relative humidity to accelerate any decomposition reactions in the materials being tested. The detector used to measure any pernicious agents is unprocessed collodial silver on a polyester base. At the completion of the test any significant change in density of the detector indicates some incompatibility between the product and photographic materials.
No paper should be in the can when the film is returned to long term storage. This is especially the case if aluminium cans are used. Many papers contain high levels of acids and peroxides which will damage the film.
While high purity (high alpha cellulose) papers are considered suitable for use with photographic materials these are expensive. Managing the types of papers that may be placed in a film can becomes difficult. It is better to link any paper documentation to the film via a clear cataloguing entry and store the papers separately.
Trapping decomposition gases around the film can rapidly accelerate the decomposition reaction. For this reason it is not longer recommended practice to wrap films in plastic bags for storage.
Punching holes in the side of the film can to enable the easy diffusion of these decomposition gases from within a film can during long term storage would seem to be a good idea. However research has shown that there is very little difference in the acid concentration of decomposing films stored in ventilated and non-ventilated film cans.
It is feasible to reuse films cans. However, when reusing an 'old’ film can for a new film, the can should be thoroughly cleaned to remove any residues of decomposition or mould. Do not reuse any can with signs of rust.
For long term storage, films must be stored horizontally. This evens the weight of the film over the whole surface area of the side of the film pack. The horizontal orientation also ensures that the tension in the film is even throughout the film pack.
Access materials are often stored vertically, usually because of space restrictions and ease of access requirements of the materials. However, this is not ideal from a preservation point of view.
If a film is stored vertically there is only a small area of the film pack supporting the weight of the entire film.
This often results in accelerated dye fade or a higher degree of decomposition occurring in such an area.
- ANSI Standard IT9.2-1991 Photographic Processed Films, Plates and Papers – Filing Enclosures and Storage Containers.
- ANSI Standard IT9.16-1993 Photographic Activity Test