A new study published by CLIR suggests that a type of audio tape commonly used since the 1950s may have greater longevity than previously thought. Researchers from the Library of Congress’s Preservation Research and Testing Division and FUJIFILM Recording Media Products Division did a variety of tests on specific models of open-reel polyester tape produced 10 to 20 years ago. They concluded that the tapes in their research sample that are playable today and are kept at standard room conditions are likely to be playable for as long as 100 years into the future. The findings, presented in Accelerated Aging of Polyester-Based Legacy Audio Magnetic Tape Stock, could have significant implications for preservation planning of audiovisual collections, as they more than double the predicted life expectancy of some types of magnetic tapes.

Commercial polyester tapes were introduced in the 1950s and quickly gained widespread acceptance and use. While the polyester film itself is quite stable over time, the binders used in the magnetic layers of these tapes are susceptible to degradation. Some formulations are known to be widely problematic and have rendered tapes unplayable within decades. However, there remains uncertainty about the stability of currently playable historical magnetic tape recordings. The study addresses that question, which can be pressing for institutions that have large collections of seemingly good magnetic tapes. 

The study focused on two main areas. First, test tapes were subjected to a variety of accelerated temperature and relative humidity ranges, chosen to best predict and assess potential conditions for storage in institutions with and without controlled environmental options. Second, changes to the tapes’ physical, magnetic, and chemical properties were measured to assess their impact on the tapes’ usability. The key findings were as follows:

• Test tapes in playable condition before accelerated aging continued to be easily and cleanly windable after accelerated aging across a range of temperature and relative humidity values.
• Physical and magnetic properties remained effectively un­changed after one year of accelerated aging, even at tempera­tures above common remedial baking temperatures. This magnetic stability suggests that briefly applied baking treat­ments likely have little detrimental effect on recorded content.
• Certain chemical properties decreased after aging, consistent with hydrolysis and lubricant loss, but these changes occurred only at the most extreme aging conditions and not sufficiently to cause playback problems.
• The observations indicated that under standard room tem­perature conditions (20-25 °C), approximately 100 years would be needed for the tested playable tapes to reach the properties measured in unplayable tapes.
• Estimates for magnetic tape longevity provided confidence that tapes currently in good condition are unlikely to rapidly turn unplayable under standard room temperature conditions of 20-25 °C.
• Manufacturing variations appeared more indicative of risk than environmental factors. Environmental controls still benefit magnetic tape preservation, but understanding of collection tapes’ individual histories might be even more beneficial.

“Based on recommendations published 10 to 30 years prior to the time of this writing, the life expectancy for magnetic tapes was predicted to be 10 to 30 years if institutions adhered to published storage guidelines,” note the authors. “This study of legacy tapes at the upper end of this age range offers an opportunity to re-evaluate those blanket predictions.” The authors note that further research would confirm how broadly their findings apply to other models of tape.

Accelerated Aging of Polyester-Based Legacy Audio Magnetic Tape Stock is available electronically at https://www.clir.org/pubs/reports/accelerated-aging-of-polyester-based-legacy-audio-magnetic-tape-stock/.