How can we test for degradation?

From the moment a film is produced it begins to degrade slowly. During the time period prior to auto-catalysis there is a gradual build up of acid in a film. It is possible to identify the acidity threshold to auto-catalysis by means of an indicator which changes in colour, when the acidity threshold is crossed.

A number of such indicators are currently commercially available. For acetic acid these include IPI's A-D Strips and Dancan's Dancheck. For nitrate film: the Alizarin red-dye test is available. In this test a hole punch of film is placed inside a test-tube. Filter paper soaked in a solution of the Alizarin dye is then placed in the neck of the test-tube. The sample is then heated and the time for the red dye colour to be bleached is noted. A film is considered to be in good condition if little change in colour has taken place after one hour.

Generally, such colour indicators are not stable to light or require long exposure times (24 to 96 hours or, at the best, 1 hour) before a colour change is noticeable. Irrespective of these problems, it is envisaged that archivists may require more than one type of indicator.

  • On accession of a film by an archive, an instant-response indicator is required to give immediate feedback on the condition of the film.
  • When the film is shelved in the archive we require an indicator which responds to gradual build-up of acid within the film container. Here it must be appreciated that when a film is taken from the vault and the lid removed acids will escape and it will be necessary to replace the lid and leave for a further period of time when an indicator is put in the can to allow acids to build up again, in any event this will not reproduce the acidity levels which built up over years in the can. The indicator should be in constant contact with the film in the can in the archive and its change of colour should not be reversible.
  • A very sensitive on-line sensor placed in the storage area capable of monitoring minute fluctuations in acidity during the course of a day would maintain optimum humidity and temperature corresponding to minimum degradation.

With this in mind indicator tests are being developed for acetate film at the Centre for Archival Polymeric Materials. One test uses an indicator dye which changes colour from red to yellow; here a hole-punch film sample does not need to be heated but left for 1-2 hours at room temperature. After this time dyed filter papers which show a linear colour change from their base of 1mm height or more require copying. This test method is rather labour intensive and time consuming so another indicator was developed by absorbing the dye onto a solid support.

A number of trials were carried out on films supplied by the North West Film Archive. A small quantity of the indicator was placed in the film can and examined periodically and the results are given in Table 4. The results provide a graded indication of the presence of various levels of acidity within the film can which apparently correlates with the tested pH of the film samples. A sample with pH<4 is yellow within 4 days of exposure whereas a sample of pH 5.74 remains red even after 14 days; between these two extremes a sample of pH 4.88 takes 14 days for 90% of its area to become yellow whereas a sample of pH 4.44 takes only seven days for the same amount of change. Therefore, the indicator is capable of operating over a long period when a film can is placed in a vault.

Table 4 Colour Change of Indicator Exposed to Acetate Films
Sample pH 1 Day 4 Days 7 days 14 Days
3 <4.00 50% Yellow 100% Yellow 100% Yellow 100% Yellow
28 4.44 10% Yellow 50% Yellow 90% Yellow 100% Yellow
21 4.88 100% Red 100% Red 5% Yellow 90% Yellow
18 5 100% Red 50% Pink 5% Yellow 100% Pink
5 5.74 100% Red 100% Red 100% Red 100% Pink
38 n/k 100% Red 100% Red 50% Pink 50% Pink
42 5.7 100% Pink 100% Pink 100% Pink 100% Pink

The test is relatively insensitive to nitrate film and the dye used is relatively light and pollutant insensitive: A sample place near a window in Manchester city centre had not changed colour after 6 months. It is possible to modify the response time of the indicator, and this has recently been achieved. On contact with severely degraded films the indicator changed colour irreversibly in less than one minute; so it may also be used as an instant response indicator.

In conclusion, though marked progress has been made towards preserving our audio-visual heritage there is still much to be done. Only by a continued effort on the part of archivists, scientists and manufacturers can we extend these horizons.