Measurement of Decomposition

Two main approaches are of interest to archivists:

  • Physical means, such as optical characteristics, viscometry, cohesion/shedding, adhesion, buildup of sticky residue of heads and guides;
  • Chemical analysis, inc chromatography, infra-red or mass spectra, nuclear magnetic resonance (NMR), solvent extraction, and pH determination.

The main drawback with relying on visible indications of decomposition or physical symptoms evident during playback or spooling, is that they often don’t show up until it is too late - once binder failure has occurred. Despite this fact, it is still too often the primary or only means of detecting binder decomposition. Indeed, manufacturers still advise clients that this is the simplest and most reliable means of determining the state of tape collections 6. Certainly it is the ability of magnetic tapes to retain their physical integrity for running on a tape transport to reproduce a faithful signal which is of ultimate concern.

Chemical condition and quantitative measurement of binder condition are of more than just academic interest, however; they serve as an early warning of problems which axe building up, and allow time for planning and implementing the task of treating and dubbing original or historical tapes, particularly when work on large collections must be prioritised. In conjunction with accelerated aging, stability testing is an important means of selecting formulations for recording and acquiring new material, as well as dubbing the sonic content of older, unstable carriers.

Of all the available options, solvent extraction (SE) and pH are of particular interest, and have been used in recent years at the NFSA. Spectral analysis and chromatography are powerful tools for those fortunate enough to have the facility, although results often need careful interpretation. They can be useful for initially testing new or unknown formulations to help in analysing the different constituents. Binders in the 1970s have failed prematurely due to the inclusion of low - as well as high- molecular weight polymers, despite an apparently acceptable average molecular weight. Manufacturers have cited more accurate analysis of the spectrum of molecular weights in quality control of raw materials as a means of improving stability of binder formulations 7,8. SE and pH measurement can be routinely implemented more readily with simpler equipment and less skilled personnel, provided that they are adequately trained.