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.

• A note about the use of the term "film stock" in this article. People often use "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. The proper use of film stock is to refer to film manufactured by a specific film lab, 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, the emulsion side more matte and you can often see the raised edges within the frame where the emulsion is thickest. Using a flashlight, move the film back and forth and observe how the light reflects off the surface. It is easier to tell which side is which more on certain prints than others. Black and white prints are often the easiest, very worn prints or newer color prints can be difficult.

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

• For Projection: Only venues who are properly equipped to project nitrate film should do so. Many projectors require gate tension adjustments between prints that are polyester and acetate the latter 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.

Types of Film Bases

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. Nothing pre-1955 will be on polyester base. Unlike acetate or nitrate, 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"). Polyester base is easy to identify because it is slightly thinner than acetate or nitrate. Polyester film cannot be cement spliced - it must be tape spliced or ultrasonic spliced.

• Polyester film is often referred to as "Estar base" which is the Kodak trade name for the polyester base that they manufacture.

Acetate

A handling note found on a 35mm acetate print from Universal

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.

• Introduced in 1948 as a replacement for nitrate film for 35mm prints
• 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 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 fairly 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 guide to identifying and handling nitrate film stock can be found here on the Association of Moving Image Archives website.

How to tell the difference (poly 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.

A polyester print is on the left, acetate on the right (notice the light does not shine through).

• 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.

See Also

• Film stock
• Splicing
• Inspection
• Film damage
• best practices

Resources

• Film Forever: Film Stocks and tips on home storage
• Historical Film Colors
• Make your own film base viewer
• Testing for vinegar syndrome