By Kate Jacus, The Photo Curator, LLC
If you work or volunteer at a museum, library, or historical society, you know that objects should be displayed and stored in archival-quality materials. “Acid-free,” “lignin-free,” and “photo safe” are terms that get used a lot, but what do they really mean when it comes to your photography collection? There is more to a material being archivally safe than its pH measurement and lignin content. Understanding the science of what makes something archival can help you make informed decisions about the materials you use to protect your institutions’ cultural heritage. Photographs are the standard baseline material for measuring archival quality.
ISO 18902 is an international standard that covers archival materials. You may have heard of ISO numbers in relation to film speed, or in a manufacturing setting. The International Organization for Standardization develops specifications for products, services, and systems, to ensure quality, safety and efficiency. There are over 22,000 standards ranging from railway engineering to food technology that answer the question: “what’s the best way of doing this?”
The standard’s full title is ISO 18902:2013 Imaging materials — Processed imaging materials — Albums, framing and storage materials. The standard is overseen by a Technical Committee, TC-42, Photography. Because photography is a sensitive medium, the standard is a good baseline from which to judge archival products. If a material is safe to use with photos, you can assume it will be safe for other, more stable, objects. The ‘2013’ in the title refers to the date of the last revision; further information on the ISO website shows that the standard was confirmed in 2018 after a five year review.
The standard covers the entire range of products that you might use for storage, display, or labeling, including papers, plastics, adhesives, and printer ink. Each type of material must meet a standards set with different combinations of tests and requirements. Only materials that meet ALL specifications of this standard can be considered photo-safe, and thus archival.
It’s worth noting here that the standard’s definition of photo-safe refers only to the chemical reactivity of a material and does not apply to how it might interact physically with a photograph or object.
Image Permanence Institute
We are lucky in Rochester to be the home of the Image Permanence Institute (IPI), an academic research center at Rochester Institute of Technology. They have long been involved in developing international standards, including part of ISO 18902, the Photographic Activity Test. Among other things, they are an independent laboratory providing testing for ISO 18902. Companies send samples of their products to IPI where they run a series of five tests related to the standard.
This image is from an IPI chart, showing the five tests that make up ISO 18902, what they test for, what part of a photograph is affected, and the damage that can be caused to each layer of the photograph. Uncovering the science behind these tests gives high level overview of what it means for a material to be archival.
The Five Tests
The PAT, or Photographic Activity Test, is actually an ISO standard in and of itself, developed by IPI. It’s a very good measure of archival quality, but not the full measure. IPI tests samples of materials by layering them in a very particular order in little jigs, or casings. These are placed in an incubator for 15 days to simulate aging in high heat and humidity. After this tropical sojourn, scientists test those materials for oxidation and reduction reactions, which cause fading, spots, and silver mirroring, and also for chromophores (the part of a molecule responsible for its color) which cause yellowing.
All of the materials covered in ISO 18902 must pass the PAT.
Next is the pH test only performed on paper and adhesives for ISO 18902 Acids, are naturally occurring in wood pulp used in paper, and are damaging to many kinds of materials. Highly alkaline environments can also cause decay. IPI does a simple pH test on materials, looking for the pH to be in the neutral range of 6 -7 for unbuffered paper and the alkaline 8 - 10 range for buffered paper.
The bump into alkaline territory for buffered paper is due to the alkaline reserve, or amount of calcium carbonate (CaCO3) embedded in the paper during production. This calcium carbonate neutralizes acids either present in the environment or created through deterioration in storage, acting as a buffer and protecting your objects. The third ISO 18902 test, for alkaline reserve, only applies to paper. IPI soaks a sample in water, then measures the amount of acid necessary to neutralize the alkaline buffering; looking for an alkaline reserve of at least 2% CaCO3.
The fourth test, the Kappa test, also only applies to paper. Lignin is the “glue” that holds the wood together, and during the pulping process, it gets removed from the wood fibers. The most common measurement of lignin is the Kappa number, which IPI measures by breaking down the paper sample back into pulp form, soaking the pulp in a chemical solution, and measuring the results. . To be considered lignin-free, paper has to have a Kappa number of less than 5, on a scale of 1 - 100.
The final test of ISO 18902 is the colorant bleed, and it applies to both paper and labeling materials. IPI tests if dyes or pigments used to either color paper or to write and print on it, have the potential to stain things touching them. The test material is held next to some bond paper and soaked in water to see if anything transfers when it shouldn't.
Get The Numbers
So what can you do with this newfound knowledge? Be an informed consumer for your institution. If a product is labeled “acid-free” or “photo safe” but does not give you any additional specs to back up that claim, you can’t be positive it’s archival quality. Reputable archival suppliers will list detailed specifications about their products that include these test results.
With the best quality archival products your institution can get, the objects in your care will have the best possible shot at longevity, and visitors decades from now will have you to thank!