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Film base
Film base is the material that film emulsion is printed on. It is sometimes referred to as the "support" as it is the supporting material that the emulsion layer is then coated on. You may need to handle prints differently in exhibition settings and in long term storage depending on the material their base is made of. See Inspection for instructions on how to tell the difference between base side and emulsion side of a print.
Polyester
With rare exceptions, all 35mm release prints made since the late 1990s have been printed on polyester base. Many 16mm prints from the mid 80s onward are also printed on polyester stock. Nothing pre-1955 will be on polyester base. Unlike acetate or nitrate stock, polyester is essentially unbreakable and cannot be torn by human hands. It is also not susceptible to vinegar syndrome and does not warp or shrink (though it is not uncommon for the film to have a slight "bow"). It is believed that a film printed on polyester stock will last several hundred years because the base does not deteriorate [cite]. Polyester base is easy to identify because it is slightly thinner than acetate or nitrate, which also means adjustments in focus must be made if acetate and polyester films are spliced together in a single reel (such as a trailer reel). Polyester film cannot be cement spliced - it must be tape spliced or ultrasonic spliced.
- Polyester stock is often referred to as "Estar base" which is the Kodak trade name for their polyester stock.
Acetate
Used in the early 1930s mostly for small gauges, then was widely used post 1950s to replace nitrate base. Often refered to as "safety film". Acetate" base in modern exhibition settings usually refers to triacetate base. Diacetate was used on some early safety film, but you won't see it very often under normal circumstances (whatever THOSE are!). Older acetate prints tend to be more brittle and prone to warping as they lose moisture. Acetate is thicker and tends to break when stressed.
- Use only tape or cement splices
- Acetate base prints can suffer from vinegar syndrome if not stored at the proper temperature and humidity levels. Make sure they are kept cool and dry, and separate out any prints you suspect of having vinegar syndrome (even the beginning stages) as it can spread from one print to another.
Nitrate
Nitrate base is the infamously unstable and flammable stock that most release prints were made of until the early 1950s. Discontinued in 1951 and replaced with acetate "safety film". Look for the word "nitrate" written on the side of the film, though make sure it's not printed in from the original negative, as many nitrate prints were later re-struck on acetate or polyester stock. It's considered a hazardous material and becomes more dangerous as it deteriorates. Make sure you know the proper storage specifications for nitrate if you have any!
How to tell the difference (poly and acetate)
- A guide for making a film viewer for easy identification of acetate or polyester film base. Older polarized 3D glasses (they use spherical polarization) work great for this if you can find some. Real-D glasses do NOT work. Polyester base will create a sort of "rainbow" effect when viewed between the cross polarized lenses, acetate base will not.
- You can also differentiate between the two by holding up the reel of film to a strong light. With polyester base prints the light will shine through, with acetate it will appear opaque. I find this is true most of the time though with certain film stocks it will be much less obvious than the photo example to the right. The polarized lens test is more definitive.
- Polyester is very difficult to tear with your hands, it stretches rather than snaps. Obviously this test is not recommended as it requires you to damage the film, though an accidental film break, or a tearing of the leader or countdown (if there is any) may give you an answer. Just make sure the leader is original to the print, as sometimes a leader or countdown may be spliced onto a print of a different base.
- A poly print may have "Estar" written along the edge of the print, acetate may have "safety". Although some new prints struck from acetate negatives may still have "safety" written on them. So it's helpful thing to check for, but use it along with other identifying qualities.