Identifying A Film Base
There are several factors which can aid in identification of the film base of a roll of film. Many are not 100% conclusive, and it is best to use a selection of these to positively verify a film base.
- Printing along the edge of the film:
- for older films, will often say "nitrate" or "safety" on it, however this text may print through from a negative or other intermediate stock.
- may include a date code (Kodak print films prior to 2001) or an actual printed 4-digit year.
- may include an emulsion number uniquely identifying the print stock (newer stocks, only)
- No Kodak film manufactured after 1951 is nitrate, and no film of any kind is polyester before 1955.
- Deterioration artifacts are distinct between nitrate (noxious nitric acid gas; amber discoloration; soft, sticky, or powdery film) and acetate (acetic acid gas, red or blue discoloration, shrinkage, brittleness, presence of bubbles or crystals).
- Polyester shows red and green interference colors when viewed through cross-polarized filters.
- A solution of diphenylalanine and sulfuric acid will turn nitrate deep blue.
- A highly controlled burn of one frame of nitrate will result in a bright yellow flame which consumes the film almost completely. (ONLY PERFORM WITH THE HIGHEST OF CAUTION)
- Nitrate film is soluble in a variety of solvents - namely methyl alcohol, ethyl, and ether.
- Float testing of the specific gravity of the base in trichloroethylene should cause nitrate to sink, acetate to float, and polyester to remain around the middle. However, this can be complicated by impurities and deterioration factors.
- Light aimed through the side of a roll of film will shine through if it is polyester, but will not if it is acetate.
- Polyester film is very strong and hard to tear off, unlike acetate.
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