Mitchell & Kenyon Preservation Process

The Mitchell & Kenyon project has focused on research that has revealed the films' historical and cultural importance and on access outcomes that have made the images widely available. However, at its core - and at the core of all the work of the bfi National Archive (National Archive) - is the physical preservation process that retrieves the images and ensures their long-term survival as part of the nation's heritage.

The preservation of the collection was a very costly, time consuming and painstaking one. Even though the collection's overall condition was reasonably good given its age, all the original camera negatives were fragile and needed individual care and attention.

All 35mm film made before 1951 was produced on highly unstable - and inflammable - cellulose nitrate stock. The chemical instability and the volatile nature of nitrate means that if not kept in a temperature and humidity controlled environment it will deteriorate, and the image will be lost forever. Once acquired by the National Archive the Mitchell and Kenyon original camera negatives were stored safely in optimum conditions in nitrate vaults.

Having spent decades in metal milk churns, the condition of the collection presented many preservation challenges:

• Dealing with non-standard film stock of varying thickness that pre-dated the standardised manufacturing of film stock.
• Tears and broken perforations.
• Some items were variably shrunk, sometimes exhibiting several different rates of shrinkage within a single roll of film.
• Uneven shrinkage of the film base causes the film stock to curve (better known as buckling). In some cases this was severe.
• Brittleness of the film stock due to the loss of water or plasticiser.
• The emulsion was often discoloured to different extents ranging from light grey, or yellow, to deep yellowish brown, to red.
• In some severe cases the emulsion layer of the film was only tenuously adhering to the base.

The images contained on the film emulsion are typically of high quality - and we are very fortunate to be working with camera original negatives rather than with prints (they have not been damaged by constant projection, and as 'first generation' material, contain the maximum visual information). The challenge was therefore to find a way to duplicate these images from the nitrate originals to new 'safety' film masters.

In duplicating this material the aim was to retrieve the maximum photographic information while limiting the use of invasive or irreversible preservation techniques.

Instead of modifying the sometimes damaged and deteriorated film material to run safely through the printing equipment. The equipment was modified to accommodate the range of flaws in the negatives.

To enlarge shrunken images to their original size and standardise the frame-line positions optical duplication rather than contact printing was necessary and a liquid gate was employed. The gate of an Acme optical printer was re-engineered. The modifications included a subtractive light filter system to neutralise the discolourisation, and adjustable transport and registration pins to enable precise image positioning.

There were some items too problematic for the Acme to deal with however. The Sigma, a gentle, continuous printer rather than 'step' printer was used. This could deal with thickness of stock and had a gentle sprocket mechanism to deal with particularly problematic items. The Sigma also needed to be modified to reduce the risk of tearing and counteract contrast and definition problems. Extensive tests were carried out to find a film stock able to cope with the heavy density of originals. In order to enable some items to be duplicated at all a small amount of repair was needed.

From the duplicate negatives made during this process a high quality Digital Betacam video of each film was produced for broadcast and from which video cassettes could be made for ease of viewing. Modern day film is shot and projected at twenty-four frames per second unlike these films that were shot with a hand-cranked camera, and shooting speeds varied across the collection and sometimes within the film. At the transfer stage considerable care was taken to recreate the shooting speeds, which varied across the collection from twelve frames per second and sometimes as low as eight. For the DVD release and public screening Electric Edwardians: The Films of Mitchell & Kenyon the duplicate negatives were transferred onto a digital high definition format.

Copying over 800 films has presented many challenges but the result has justified the effort. Preservation has enabled these film treasures to be seen for the first time by a new generation who can be rest assured that all of them will be preserved in optimum conditions for generations to come.