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.
- ANSI Standard IT9.2-1991 Photographic Processed Films, Plates and Papers – Filing Enclosures and Storage Containers.
- ANSI Standard IT9.16-1993 Photographic Activity Test