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Motion picture film is inherently unstable, consisting
as it does of many different and complex layers. Black
and white film has four to five layers: a gelatin
layer to protect the image against abrasion, the image
layer of light-sensitive emulsion consisting of silver
salts in gelatin, an adhesive
layer that binds the image to the support, the support
made from acetyl-cellulose
(before 1951 all support was made from nitrocellulose
or nitrate),
and generally another coating of gelatin as a dimensional
stabilizer.
Color film is much more complex. It consists of nine
layers: the gelatin layer to prevent abrasion, blue-sensitive
gelatin with silver salts and color couplers, a yellow
filter that allows the passage of green and red rays
but absorbs any blue rays not absorbed by the blue layer,
a green-sensitive layer, a gelatin layer to prevent
dye migration, a red-sensitive layer, an adhesive
binding layer, the support made from
cellulose triacetate or polyester,
and a gelatin dimensional stabilizer.
Identification
Before the early 1950s all 35-millimeter film was produced
on nitrate base or support, although acetate or polyester
was used for 8-, 16-, and 70-millimeter film. Because
nitrate film is very unstable, it should be identified
and appropriate steps taken to segregate it from other
types of film.
Nitrate
deteriorates in a way that is dangerous to other materials.
In some instances motion picture film has burst into
flames at temperatures above 40 degrees Celsius. Nitrate
film has five stages of deterioration:
1. An acidic odor comes from the film, the silver image
starts to fade, and the color of the emulsion changes
to amber or brown.
2. The acidic odor becomes more pronounced, and the
emulsion becomes sticky, with the result that negative
layers adhere together and to their container.
3. The acidic odor strengthens, and portions of the
film become soft, with bubbles forming.
4. The film continues to soften and congeals into a
solid lump, sometimes with a froth.
5. The solid lump collapses into a strong smelling powder.
Cellulose
acetate, or "safety" film,
was manufactured in response to concerns about the inherent
instability of cellulose nitrate. Used mainly for 16-millimeter
film after about 1923, cellulose acetate is made from
various forms of cellulose fiber combined with acetic
acid, acetic anhydride, and, as a catalyst, sulfuric
acid. Cellulose diacetate was first made by Kodak, from
1925 to 1940, and Agfa continued to make it until 1955.
Cellulose triacetate, more durable than diacetate, gradually
replaced it in the late 1940s. Most commercial motion
picture film is still produced on triacetate.
Polyester-based
film is strong, flexible, and durable and is not dimensionally
reactive to changes in temperature and relative humidity.
The emulsion
layer is composed of gelatin, which is
more vulnerable to change than the polyester base as
it is hygroscopic and can mold
in conditions of high relative humidity.
• Diagnostic Tests
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readings.
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Film
