Throughout our travels we have come across a few archives that were unaware they had cellulose nitrate in their collection or that film based photographic material can be made of cellulose nitrate. Cellulose nitrate is flammable and releases hazardous nitrogen oxide gases as it deteriorates. It is extremely important to identify any cellulose nitrate in your holdings as this material is unstable and requires special storage and environmental considerations.
There are many ways to identify nitrate; however, few of the methods are absolute. It is beneficial to utilize a combination of methods to identify if a negative is nitrate:
- Edge printing: Many, but not all, manufacturers identified the type of film along one border of the film with nitrate or safety. Safety indicates the negative is acetate. It is important to note that some early nitrate does not have edge printing.
- Notch codes: A notch code is a group of indentations or recesses on the edge of a piece of film to help identify the film type and brand. If there is a ‘V’ notch code first from the edge of the negative, it is nitrate, and if there is a ‘U’ notch code first from the edge of the negatives, it is acetate. Notch codes are not always accurate as the photographer may have cut the film sheet for various reasons and removed the notch code closest to the edge.
- Dates of negatives: Kodak started selling cellulose nitrate negatives in 1889 and the last year of nitrate manufacture was 1950. These dates are only applicable for Kodak and there are no other dates for other manufacturers. Also, photographers may have purchased film and used it much later than the purchase date. The dates of the negatives are not always the most reliable method for identifying negatives.
- Level of deterioration: There are six levels of deterioration for nitrate negatives. The negatives start to yellow and mirror, then become sticky, then the image begins to fade before turning into a brownish powder. Please visit the NEDCC for images of all the different levels of deterioration: https://www.nedcc.org/free-resources/preservation-leaflets/5.-photographs/5.1-a-short-guide-to-film-base-photographic-materials-identification,-care,-and-duplication
- Testing: There are a variety of tests available to identify nitrate; however, many of these tests are damaging to the negative. The diphenylamine, burn and float test can damage the record and should all be conducted by a professional. A non-destructive test is viewing the negative through a polarizer. Polyester negatives show red and green interference colours through a polarizer whereas nitrate and acetate do not. Please visit the NPS for more information about creating a polarizer: http://www.nps.gov/museum/coldstorage/pdf/2.3.1b.pdf
If available, use duplicate access copies of the negatives. Allow material stored at cool temperatures to acclimatize before handling. Wear cotton or nitrile gloves when handling nitrate negatives. This prevents oil from transferring to the negatives and acts as a heat buffer. Negatives at a higher level of deterioration ignite at lower temperatures and body heat may trigger a reaction. Try to handle negatives by their edges and avoid touching the emulsion layer. Do not expose the negatives to any heat sources. If projecting or digitizing negatives, consider using a lower watt lightbulb in the projector or scanner.
Storage and Environment
It is important to isolate nitrate from other material is the collection. These negatives off-gas nitric oxide and nitrogen dioxide, which is harmful to humans and other items in the collection. For example, gases can cause skin and eye irritations as well as respiratory issues and cause paper to become discoloured and brittle and metal to corrode. Keep the storage space well ventilated.
Nitrate negatives should be housed in PAT approved negatives sleeves or envelopes and placed in an acid-free box. If freezing the negatives, the box should be sealed in a polypropylene bag. Freezing the negatives in a sealed bag counteracts the advice to keep the storage well ventilated; however, freezing is more beneficial to negatives in good condition than well ventilated storage.
Temperature and relative humidity levels are significant factors in slowing the deterioration of nitrate negatives. Cool to cold temperatures and 30-50% RH are recommended. If possible, store negatives in a cool – cold storage vault or freezer. Freezers must be frost-free and have the ability to monitor temperature and RH. Some freezer brands are also explosive-proof, which may be beneficial for these flammable records.
Consider digitizing the nitrate negatives prior to freezing them. Digitization prior to the start of or further deterioration ensures that an accurate representation of the record is being kept. It decreases use of the original, which prevents damage caused by handling. It also ensures the original is kept in a stable environment to prevent further degradation.
Due to the flammable nature of this material, it is important to include cellulose nitrate in your disaster plan. Train your staff on potential scenarios involving cellulose nitrate and advise your staff on its location within your collection. Set up a monitoring schedule to check for any signs of deterioration. Ensure that there are no heat sources, such as light bulbs or radiators, near this material. It is recommended to advise your local fire department where this material is located within your facility. This is extremely important as ignited cellulose nitrate creates its own oxygen and cannot be put out using water.
Disasters may also speed up the deterioration of nitrate negatives. While we have been helping archives across Alberta recover from the June 2013 floods, we have seen nitrate negatives’ deterioration accelerated due to their time in water. It is important to identify vulnerable media in your collection and plan for evacuations, if possible.
If you decide to dispose of the cellulose nitrate in your collection, be advised that it is considered a hazardous material. Check with Alberta Environmental Protection (http://environment.gov.ab.ca/info/library/7423.pdf) or with your local environmental agency for more information on safe disposal methods for nitrate film.
The Association of Moving Image Archivists. “Identifying and Handling Nitrate Film.” The Association of Moving Image Archivists December 2008. Accessed on April 2, 2015. http://www.amianet.org/groups/committees/nitrate/documents/NitrateIGNov08.pdf
Fischer, Monique. “5.1 A Short Guide to Film Base Photographic Materials: Identification, Care and Duplication.” Northeast Document Conservation Centre. Accessed on April 2, 2015. https://www.nedcc.org/free-resources/preservation-leaflets/5.-photographs/5.1-a-short-guide-to-film-base-photographic-materials-identification,-care,-and-duplication
Kodak. “Storage and Handling of Processed Nitrate Film.” Kodak. Accessed on April 2, 2015. http://motion.kodak.com/motion/Support/Technical_Information/Storage/storage_nitrate.htm
Messier, Paul. “Preserving Your Collection of Film-Based Photographic Negatives.” Rocky Mountain Conservation Centre. Access on April 2, 2015. http://cool.conservation-us.org/byauth/messier/negrmcc.html
National Park Service. “Cold Storage: A Long-Term Preservation Strategy for Film-Based Photographic Materials.” National Park Service. Accessed on April 2, 2015. http://www.nps.gov/museum/coldstorage/NPSColdStorage.swf
Reilly, James M. “IPI Storage Guide for Acetate Film.” Image Permanence Institute. Last modified 1996. Accessed on April 2, 2015. https://www.imagepermanenceinstitute.org/webfm_send/299
Williams, R. Scott. “Display and Storage of Museum Objects Containing Cellulose Nitrate – CCI Notes 15/3.” Canadian Conservation Institute. Accessed April 2, 2015. http://www.cci-icc.gc.ca/resources-ressources/ccinotesicc/15-3-eng.aspx