Analogue Film Types Used for Still and Moving images

Last updated: 27 January 2010
Published in: Digitising analogue media | Creating new digital media
Tags: analogue collections | digitisation | negatives | photographs | restoration | scanning

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Summary

This document looks at the history of photographic and movie film and how and why the film type has altered over time. The document also contains some frequently asked questions on film types and handling.

Introduction

In the early days of photography, solid materials such as glass plates were used to support the light sensitive emulsion. While glass plates offered high quality they were expensive to produce, fragile, heavy and not very portable. There was a widespread demand for a flexible lightweight film base which could be transported rapidly through a stills or film camera or projector to replace the heavier materials used up to this date.

Cellulose nitrate film provided a flexible transparent substrate onto which the light sensitive emulsion would adhere. It was perfect apart from the fact that it was highly inflammable. Film projectors have to transport the film at speed past a very bright and hot bulb, if the film should jam then it was likely to ignite rapidly and uncontrollably. After a few fatal accidents with nitrate film, projectors and projection rooms had to be modified to contain any fire damage. The projectors had fire traps, which limited the fire to a short length of film. Storage of film beside hot projectors was prohibited and the projection rooms had to be built from solid concrete with metal doors and steel shutters that would close automatically if the temperature in the room got too high.

Nitrate film was used for both still and moving images - while still images were normally kept in open storage, movie films were stored in sealed metal cans. As the film ages, hazardous fumes are released - in open storage the fumes disperse gradually into the environment, however film stored in cans are sealed in with the fumes which further accelerates the decay. As nitrate film degrades it becomes more hazardous and so there is a real urgency to digitise it and permanently remove it from collections.

Even during the height of nitrate film manufacture there was an urgent search for a more stable film base. Cellulose acetate ‘safety’ film was introduced in the early 20th century and while it is much safer than nitrate film it may still degrade under certain conditions. Polyester film may also be referred to as safety film and be marked as such. In the 1950s still photographers started to use polyester film rather than cellulose acetate based material however polyester is difficult to splice together and so it wasn’t until the 1990s that it was adopted by the moving image industry.

We have found some old black and white film negatives in our collection, how can we identify the type of film used?

Nitrate film will normally have NITRATE or the letter N at the edge of the image or alongside the sprocket holes. However some negatives originally captured on acetate safety film were duplicated onto available film, which may have been nitrate and so may have the letter S or SAFETY along the edge though the film is far from safe. The images below show negatives with the manufacturers film type identification printed on the edge.

   
Left, ‘Migrant Mother’ by Dorothea Lange 1936. Right, detail of Lange’s image showing NITRATE. Image has been rotated and flipped to aid viewing. Photo from The Library of Congress on Flickr: The Commons - No known copyright restrictions.

 
Left, James Lynch by Jack Delano 1942. Right, detail of Delano’s image showing SAFETY. Image flipped to aid in viewing. Photo from The Library of Congress on Flickr: The Commons - No known copyright restrictions.

Simple tests can also be used to identify film types. Polyester film can be identified using two polarising filters, one above the other below. It will produce a rainbow colour effect not seen in the other film types. Nitrate film is soluble in methyl alcohol while, acetate and polyester film are insoluble.

We have a collection of still images which includes acetate and nitrate negatives, can we digitise the collection?

First identify the film type and its condition. If the negatives are in a good state then technically you should be able to digitise them. If nitrate film has been positively identified it is recommended that you seek specialist advice on handling, digitising (if safe to do so), storage, or more commonly disposal of the film.

The existence of a nitrate film collection should be brought to the attention of the insurance company otherwise the policy may be rendered invalid and the building and its contents are essentially uninsured.

Nitrate film can spontaneously combust at 49 degrees centigrade so care should be taken to avoid exposing the negatives to heat. Scanners normally use low energy lamps and so are unlikely to generate enough heat to ignite nitrate film though projectors with bright halogen lamps produce significantly more heat.

As cellulose acetate film starts to degrade it releases acetic acid, this then attacks the film releasing more acetic acid, accelerating the process. Eventually the film will become brittle and as the film base shrinks it separates from the image layer and becomes unusable. A vinegar smell is one of the first tell tale signs of deterioration the decay of acetate film is known as ‘vinegar syndrome’. Film showing signs of vinegar syndrome should be separated from the rest of the negatives to contain the damage.

If the decision were made to scan the negatives it would be wise to look at how the scanner digitises film. Some scanners have special film holders, which position the film over the scanning bed, most flatbed scanners press the image down onto the scanning bed. While typical objects can withstand this type of handling it may damage or destroy fragile acetate or nitrate film and so it would be wise to run tests to identify potential risk.

Is our colour film safe?

Colour film regardless of the base layer is prone to fading. Cold storage is the most effective way of slowing or preventing this.

Will our polyester based film degrade?

While the polyester film base is more stable than acetate or nitrate film the emulsion layer may contract over time and potentially detach from the base. If polyester film is tightly bound for extended period it can become difficult to uncurl and risks damage.

We have a large collection of film negatives, they appear to be in good condition and we are unable to digitise them at present, how should we store them in the meantime?

If the collection contains nitrate film (in any condition) then it you should seek specialist advice and contact your insurance company as soon as possible. Some valuable or historically significant nitrate film collections are stored in specialist facilities but this would be prohibitively expensive for most collections.

To slow or prevent deterioration it is essential that all types of film including polyester based stock are kept in a cool dry environment. The minimum requirements vary between film types, colour film which is prone to fading should be kept at 0 degrees centigrade or below and at the lowest possible relative humidity. This should be seen as the optimum conditions for a collection containing black and white and colour film.

If the film is refrigerated it should be stored in sealed containers to avoid contact with condensation. Movie film should be kept in sealed chemically inert containers and stored flat. If stored at room temperature the containers should be vented to allow acetic acid fumes to vent off.

Last updated: 27 January 2010
Published in: Digitising analogue media | Creating new digital media
Tags: analogue collections | digitisation | negatives | photographs | restoration | scanning

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