Table 2 Traditional methods used by cultural heritage institutions for identification of photographic plastic film bases
Cellulose nitrate | Cellulose acetate | Polyester | Evaluation of technique | |
|---|---|---|---|---|
Contextual information | Non-invasive No WHS Concerns Not always available | |||
Date [6] | ? | ? | ? | Requires verification: periods of use overlap. |
Visual examination | Non-invasive No WHS Concerns Physical characteristics not always present | |||
Edge printing [6] | ✔ ‘Nitrate’ | ✔ ‘Safety’ | ✔ ‘Estar’ ‘Cronar’ | Requires verification: film duplication (for example from nitrate to polyester) can include previous edge printing |
Notches [47] | x Usually ‘V’ | x Usually ‘U’ | ? | Requires verification: different manufacturers used notches to mean different things |
Deterioration [48] | ? | ? | x | Requires verification: some deterioration is common to both |
Polarisation test [48] | x | x | ✔ Shows interference colours | Only identifies polyester Requires verification: does not distinguish between cellulose nitrate and cellulose acetate |
Chemical testing | Invasive & destructive (requires sample) WHS concerns (solvents or by products) | |||
Float test [48] | ✔ | ✔ | ✔ | Requires verification: results can be difficult to interpret, since there are many different types of cellulose nitrate and acetate manufactured, and deteriorated film behaves diversely |
Diphenylamine spot test [49] | ✔ | x | x | Requires verification: cellulose acetate and polyester sometimes have a subbing layer of cellulose nitrate —can create a false positive. |
Burn test [48] | ✔ | x | x | Requires safety measures: must be performed outdoors, requires bucket of water, and is generally no longer conducted |