Many factors affect the storage life of radiographs. One of the most important factors is residual thiosulfate (from the fixer chemicals) left in the radiograph after processing and drying. For archival storage1, ANSI PH1.41 specifies the amount of residual thiosulfate (as determined by the methylene blue test) to be a maximum level of 2 micrograms/cm2 on each side of coarse-grain x-ray films. For short-term storage requirements, the residual thiosulfate content can be at a higher level, but this level is not specified. Washing of the film after development and fixing, therefore, is most important. The methylene blue test and silver densitometric test are laboratory procedures to be performed on clear areas of the processed film.
The following ANSI documents2 may be used as an aid in determining storage conditions:
1. ANSI PH1.41, Specifications for Photographic Film for Archival Records, Silver Gelatin Type on Polyester Base
2. ANSI PH1.43, Practice for Storage of Processed Safety Photographic Film
3. ANSI PH4.20, Requirements for Photographic Filing Enclosures for Storing Processed Photographic Films, Plates, and Papers
4. ANSI PH4.8, Methylene Blue Method for Measuring Thiosulfate and Silver Densitometric Method for Measuring Residual Chemicals in Film, Plates, and Papers
5. ANSI N45.2.9, Quality Assurance Records for Nuclear Power Plants, Requirements for Collection, Storage and Maintenance of Commercial Keeping
Since definite retention times for radiographs are often specified by applicable codes, archival keeping may not always be necessary. Recent studies3 have indicated that industrial x-ray films, with a residual thiosulfate ion level of up to 5 micrograms/cm2, per side (as measured by the methylene blue method described in ANSI PH4.8) 4, should retain their information for at least 50 years when stored at 0 to 24°C (32 to 75°F) and a relative humidity of 30 to 50 percent. Peak temperatures for short time periods should not exceed 32°C (90°F) and the relative humidity should not exceed 60 percent. Storage conditions in excess of these ranges tend to reduce image stability. The extent of reduced image stability is very difficult to define, due to the great number of conditions that could exist outside of the above suggested storage condition ranges. It should be noted that this does not imply that industrial x-ray films with a total residual thiosulfate content of 5 micrograms/cm2, per side, will have archival keeping characteristics. It does, however, suggest that these films will fulfill the needs of most current users of industrial x-ray film requiring a storage life of 50 years or less.
Additional Storage Suggestions
Regardless of the length of time a radiograph is to be kept, these suggestions should be followed to provide for maximum stability of the radiographic image:
1. Avoid storage in the presence of chemical fumes.
2. Avoid short-term cycling of temperature and humidity.
3. Place each radiograph in its own folder to prevent possible chemical contamination by the glue used in making the storage envelope (negative preserver). Several radiographs may be stored in a single storage envelope if each is in its own interleaving folder.
4. Never store unprotected radiographs in bright light or sunlight.
5. Avoid pressure damage caused by stacking a large number of radiographs in a single pile, or by forcing more radiographs than can comfortably fit into a single file drawer or shelf.
Other recommendations can be found in ANSI PH1.43.
1A term commonly used to describe the keeping quality of x-ray film, defined by the American National Standards Institute as "Archival Storage--Those storage conditions suitable for the preservation of photographic film having permanent value." This term is not defined in years in ANSI documents, but only in residual thiosulfate content (residual fixer) for archival storage.
2These documents may be obtained from the American National Standards Institute, Inc., 1430 Broadway, New York, NY 10018.
3These studies, called Arrhenius tests, are relatively short-term, elevated-temperature tests conducted under carefully controlled conditions of temperature and humidity that simulate the effects of natural aging.
4The methylene blue and silver densitometric methods produce data as a combination of both sides of double-coated x-ray film.