Wind tension

Along with storage temperature and relative humidity, wind tension plays a significant role in the rate of decomposition of cellulose based films. The correct wind tension allows greater diffusion of decomposition gases from the film during storage, which in turn has an effect in reducing the rate of decomposition. The ideal wind tension is referred to as a preservation wind.

For long term storage films should be evenly wound to a tension whereby they just hold their shape in a pack, Fig 8.3.

Fig 8.3 Correct wind tension, the film pack holds its shape.

Too loose, the pack can distort and the core may drop out while handling.

However, the film should not be so loose that it can move easily against itself during normal return to storage handling. This tension is really a compromise between so loose that damage can occur if the film is moved and so tight that the decomposition gases cannot readily diffuse from the film.

To hold the film in the correct wind tension a small piece of 'photographically safe’ adhesive tape can be used to secure the end of the leader.

Films wound to a preservation wind must be rewound before transporting or being used on projection equipment. The lower tension will be taken up by machine transport mechanisms causing the film to pull against itself. This can potentially cause scratching. Similarly, during physical movement from location to location the lower tension permits the film to move against itself, again potentially causing scratching.

Other advantages of the looser preservation wind are that any dimensional change that may occur within the film, due to fluctuations in temperature or relative humidity (RH), will have less effect on the film. One of the major causes of blocking is the cross linking that occurs between the gelatin emulsion and backing layers under high RH. This is assisted by the pressure exerted by the gelatin swelling. A preservation wind reduces significantly the pressure that the swelling causes and reduces the degree of cross linking that can occur. Similarly, with changes in temperature the preservation wind can accommodate the dimensional changes that may occur.