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Methods of identification

There are several methods that can be used to identify cellulose acetate within collections.

Non-destructive identification methods include:

  • visual and contextual information; and
  • the polarisation test.

Destructive tests include:

  • the burn test;
  • the float test; and
  • the diphenylamine test.

It is strongly recommended that destructive tests be avoided wherever possible. Identification through visual or contextual information is preferable.

The method commonly used to identify cellulose nitrate within collections is the "float" test. This test can be unreliable and hazardous, and is best avoided.

Identification of sound material

An easy way of distinguishing cellulose acetate sound material from polyester is to hold the reel up to a strong light source and place an object (eg your fingers) between the light source and the reel. If the material is cellulose acetate, you will be able to see the 'shadow' of your fingers through the tape.

Commonly, sound reels are not stored in their original boxes - therefore information provided on the box should be treated with scepticism.

Because of a tendency to re-use audio tape, date information should be treated with caution also.

Identification of film bases

Positive identification of the film base is not always possible; some ambiguity will always exist. It may be necessary to use more than one of the following tests. Film identification may need to be treated in general terms, rather than on an individual basis.

Identification of film by date is one possibility, permitting a remote and rapid evaluation of collections. Cellulose nitrate has not been produced commercially since the early 1950s. Many institutions holding large quantities of film have already identified the cellulose nitrate in their collections, and segregated it from other film. In these institutions, identification of cellulose acetate should be made much easier, as differentiation will only be needed between acetate and polyester.

It is suggested that identification be attempted by the following steps:

1. Visual / contextual information.

  • Date Range
  • Notch code
  • Edge markings
  • Deterioration characteristics

2. The polarisation test.

These steps are discussed in the following sections.

Date Range

The date of manufacture of a film may be estimated by:

  • Date of development - for example if a film is still in a dated developer's wallet;
  • Date of subject-for example a photographic negative depicting a scene from WWII;
  • Edge markings-some films may have the date of manufacture / date of development printed in the margins.

In some institutions, the date may be recorded in the cataloguing information. How a film has been catalogued will govern how useful the date information is. A film that has been catalogued by date of acquisition will not be immediately identifiable by date range. A film that has been catalogued by date of subject, on the other hand, could well be quickly identifiable.

In most cases, the date of development of a film will be quite close to its date of manufacture (within a year or two).

The date range table provides information that can be used to help identify films by their date of manufacture.

If a film lies close to a changeover period for base types, the information should be treated as particularly ambiguous, and further identification may be necessary.

The "most probably" rows in the table indicate which base type the film is likely to be. The "could be" rows give the next most probable possibility. This information is included to increase awareness of the other base types used, and to prevent over confidence in film base identification by date range. Even the 'could be' row does not encompass all possibilities; the film could be any of a number of base types at any given date.

When ascertaining the date of a film, be wary of duplicates. Edge markings and other date clues are often transferred to duplicates; a 1980s polyester film could therefore be mistaken for a 1940s acetate.

Some examples of film identification are given below:

  • An original sheet film negative shows an image of a soldier in the Second World War. Its base type, according to the table, is almost certainly diacetate, although there is some chance that it might be nitrate.
  • The cataloguing system of a collection records the date of production of a motion-picture film as 1985. The film is therefore most probably cellulose triacetate, although it could be polyester.
  • The cataloguing system records the date of acquisition of a still negative as 1989. The film does not bear any notch code or edge marking. The film is in good condition. The subject matter is of an unidentified landscape with some cattle. No identification by date range is possible.
  • A 35 mm amateur roll film is marked as having been developed and printed in 1952. This falls close to a changeover period on the date range table, and so the film's identity is somewhat ambiguous, although it is most probably acetate.

Notch Codes

The notch code is a small, specifically shaped nick in the margin of some sheet films. The notch code exists partly to help developers find the emulsion side of a film in the dark.

The notch code also identifies the film type. If a notch code can be recognised, the film base can be identified.

The lack of availability of information on notch codes is a major hindrance to this type of identification. A major source of notch code identification information is David Horvath's Acetate Negative Survey: Final Report (Horvath, 1987) which provides information principally about Kodak film.

Notch code information derived from Horvath's Acetate Negative Survey is given in Appendix 2.

Information on Kodak Sheet Film Notch codes can be found at: http://www.kodak.com/global/en/professional/support/techPubs/f3/f3.jhtml.

Edge Markings

Edge markings may provide some clue to identification.

Edge markings usually consist of words or numbers printed in the emulsion of the margin of a film. They are found on most film formats, including motion picture and sheet film.

Edge markings may include clear information on the base type, such as:

  • Nitrate (on nitrate film)
  • Safety Film or Safety (on acetate film)
  • Estar (on polyester film)
  • Cronar (on polyester film)

In most cases this information can be relied upon. However, it should be noted that some edge markings are false or misleading, and "safety film" has been known to be printed on both acetate and polyester films.

Edge markings may also include the date of manufacture and / or date of printing of a film; they might therefore be used in conjunction with the date range table on page 8 of this document.

A useful website containing Kodak 'Date Code' symbols can be found at http://www.kodak.com/US/en/motion/support/h1/identificationP.shtml#p.

In many cases, however, edge marking information can be even more difficult to access than notch code information. Some edge markings are simply numbers or letters. Decoding these markings may be complicated and time consuming, particularly with older films. Manufacturers' websites occasionally provide information on some of their more recent codes.

Edge markings that plainly give the base type (such as Nitrate, Safety or Estar), or a date are the most useful.

Use of Cross-Polarising Filters

The polarisation test is a straightforward and non-destructive test that distinguishes polyester-based films from cellulose acetate and cellulose nitrate.

"when viewed between cross-polarised filters, polyester…exhibits red and green interference colours [while]…cellulose nitrates and the cellulose acetates do not"

Polarisation tests for polyester-based film may be conducted by placing a sheet of Polaroid or other polarising material on both sides of the film; when these are rotated the film should exhibit interference colours.

This will only apply in cases where the film is sufficiently transparent to allow a good deal of light to pass through. In films with dense, opaque image area, it may be difficult to see the interference colours. However, some result is usually visible in the margins where the emulsion is often clear.

Figure 1: Cross Polarizing filters showing:

a) Polyester based film b) Cellulose acetate based film

In institutions where all cellulose nitrate films have already been located, the polarisation test may be an effective means to distinguish between the remaining polyester and acetate films.

Polarizing film is available from photographic shops, or online1.

Deterioration Characteristics

In some cases it may be possible to use deterioration characteristics to distinguish cellulose acetate from cellulose nitrate. It is possible, however, that acids from nearby deteriorating nitrates may cause the deterioration characteristics of nitrate bases to appear on acetate or polyester bases. If nitrate and acetate/polyester material are stored together it is therefore necessary to be cautious in interpreting deterioration symptoms.

Polyester base is generally regarded as stable and not subject to deterioration. However, at least one study has found evidence of deterioration of the gelatin emulsion layer on polyester base. Characteristics include ink smudges, image transfer to adjacent materials, discolouration, blistering, separation of the emulsion, and 'soapy' odours (Szczepanowska & Wilson, 2000, p.379).

Both acetate and nitrate film bases will become embrittled and shrink when deteriorating. Nitrate film is more likely to show yellow or amber discolouration than acetate, and the emulsion is more likely to become soft and/or sticky.

Deteriorating nitrate has a sickly sweet odour, which in severe cases is perceived as choking or suffocating. Deteriorating acetate may smell of vinegar (cellulose acetate) or rancid butter (cellulose acetate butyrate and cellulose acetate propionate).

Acetate film is more likely to show blue or pink colours than nitrate. These are due to the re-formation of anti-halation dyes.

The following table summarises the deterioration characteristics of cellulose nitrate and cellulose acetate.

Symptom Nitrate Acetate
Yellow/amber base Common Rare
Brittle base Yes Yes
Base shrinkage Yes Yes
Warping Yes Yes
Vinegar smell   Yes
Rancid butter smell   Yes
Sickly sweet smell Yes  
Choking, chemical smell Yes  
Blue or pink colours Rare Common
Soft, sticky emulsion Common Rare
Syrupy emulsion Yes  
Buckled emulsion or backing layer   Yes
Crystals under gelatin   Yes
Bubbles Yes Yes - liquid filled
White circular plugs of crystals   Yes
Image relief Yes  
Mirroring with purple, green, blue or bronze interference colours Yes  

(Nishimura, 1995, p.368)

Deteriorating cellulose acetate and nitrate can release substantial quantities of acid vapours. It is very important for personnel to be familiar with relevant Occupational Health and Safety Guidelines when exposed to these vapours, and to use protective equipment when necessary.

Destructive Tests

Several common tests exist to identify film base types. These are:

  • the burn test,
  • the float test and
  • the diphenylamine test.

As previously mentioned, it is not recommended that these tests be carried out. All three tests are destructive, and can be misleading in their identification of the film base type. For information on to how to carry out these tests, see Fischer and Robb (1993), available online at http://palimpsest.stanford.edu/byauth/fischer/fischer1.html.


(1) One outlet selling polarising film 'by the foot' for US$0.07 per square inch can be found at: http://www.polarization.com/shop/catalog/index.html. Another possible source can be found at http://www.edmundscientific.com (search for 'polarizer'). At the time of writing, they are charging US$58.80 for 17"x 15" polarizing film.


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