Film base

Film base is a type of flexible material used as a foundation or base for the film emulsion. Film emulsion is made up of light sensitive silver halide grains suspended in a gelatin coating. Film base is sometimes referred to as the "support" as it is the supporting material that the emulsion layer is then coated onto. Throughout history it has been made of different materials, all of which require different handling techniques for projection as well as storage.

Terminology: "film base" vs. "film stock"

A note about the common usage of the term "film stock". People often use the term "film stock" interchangeably with "film base" to refer to the physical material upon which the film is printed as identified by the composition of the "film base". This is a colloquial use of the term.

Film stock properly refers to film manufactured by a specific manufacturer, or that is associated with specific properties of the emulsion like color saturation or development speed etc. For example, you might say "this film was printed on black and white Orwo stock" or "Kodak 2383 stock".

How to tell the difference between the base side and the emulsion side

The base side of a print will look shinier and the emulsion side will appear more matte. Using a flashlight, move the film back and forth and observe how the light reflects off the surface. On the emulsion side you can often see the raised edges within the frame where the emulsion is thickest. It is easier to tell which side is which with certain prints more than others. Black and white prints are often the easiest, very worn prints or newer color prints can be difficult.

Knowing which side is which can help you properly diagnose film damage.

Why is it important to know what type of base a print has?

For projection: Only venues who are properly equipped to project nitrate film should do so. Many projectors require that gate tension adjustments be made depending on the film base material, as polyester base is a little thinner and acetate is slightly thicker. This difference in thickness 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).
For storage: Nitrate and acetate films have very specific needs with regard to long term storage. Improper storage can lead to deterioration of the film base.
Acetate tends to become more brittle with age and may require gentler handling
Knowing the type of film base can help you determine when the print was manufactured.

Types of Film Bases

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 along the edge 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. If you see "safety film" printed in black text that means it is acetate stock. You want to look for black text, text printed in from the negative will appear white. Be very careful when handling a film print you suspect may be of nitrate 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!

Film gauges smaller than 35mm were not made of nitrate stock (with a few very rare exceptions). Films larger than 35mm (like 70mm) made before the mid-1950s may be nitrate, but these are also very rare.
Because Safety film was introduced in 1948 and nitrate was discontinued in 1951 but older nitrate stock may have still been used for prints up until the mid-50s, you won't be able to identify the stock by film production date alone during these "overlap years".
Fun fact: Nitrate is made from cotton!

A more thorough guide to identifying and handling nitrate film stock can be found here on the Association of Moving Image Archives website.

Acetate

Used in the early 1930s mostly for small gauges, acetate film was first used for 35mm print in 1948 in order to replace nitrate prints, and became widely used by the mid-1950s. Often referred to as "safety film" in order to differentiate it from the highly unstable nitrate 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 (it can sometimes be identified by its distinctive "mothball" odor). Older acetate prints tend to be more brittle and prone to warping as they lose moisture. Acetate is thicker than polyester (see below) and tends to break or snap when stressed.

Use only tape or cement splices, it cannot be spliced ultrasonically
Acetate base prints can suffer from vinegar syndrome if not stored at the proper temperature and humidity levels.

Vinegar Syndrome is a type of film decay where heat, water, high humidity cause the acetic acid in the film base to break down and escape. It is something that only happens to acetate film bases.

Signs: Film smells like vinegar. Other things like film cleaner may cause a print to "smell funny" but a vinegary smell is associated specifically with Acetate Film Base Degradation aka "vinegar syndrome". Film begins to shrink and warp, becomes less flexible and more brittle. Note: Acetate film prints may also become more brittle over time, but this does not necessarily mean they have vinegar syndrome.
Handling: Make sure prints 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. Clean projectors thoroughly with 99.9% isopropyl alcohole after screening prints you suspect may have it. There is no reversing the process, but cold storage can halt it. See the "Resources" section of this page for more detailed information about storage of film prints and testing for vinegar syndrome.

Polyester

Developed in the ‘40s (used first in still photography and small gauge motion picture film) not widely used for 35mm motion picture till the mid ‘90s. 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. Nothing pre-1955 will be on polyester base. Unlike acetate or nitrate, polyester is essentially unbreakable and cannot be torn by human hands, it tends to stretch rather rather than snap when stressed. Polyester base is slightly thinner than acetate or nitrate.

Polyester film cannot be cement spliced - it must be tape spliced or ultrasonic spliced.
Not susceptible to vinegar syndrome and does not warp or shrink (though it is not uncommon for the film to have a slight "bow")
Polyester film is often referred to as "Estar base" which is the Kodak trade name for the polyester base that they manufacture.

How to tell the difference between polyester and acetate

Here is 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.

Resources