High humidity and tight wind

Blocking caused by high relative humidity and wind tension

Extended periods of high humidity will cause the emulsion to absorb water from the air and swell. If the film is wound tightly then the swollen gelatin will press against the adjacent layer in the film reel. In time the softened gelatin will start to acquire the surface characteristics of the surface it is pressed against. This effect was used in the past to produce highly glossy surfaces on still photographic prints and was known as ferrotyping.

Motion picture films have a topcoat that contains small granules of hardened gelatin that provide a slightly matt surface. This is used as an aid to handling and gives certain light transmitting characteristics. If the swollen gelatin presses against a smooth surface, such as the base of an adjacent layer of film, then this layer is pressed into the emulsion and the emulsion surface takes on a shiny appearance. This will affect the light transmission characteristics and be visible on projection (Fig 12.12). This effect, which is undesirable in motion picture film preservation, is also known as ferrotyping.

Fig 12.12i Ferrotyped image under transmitted light

Fig 12.12ii Ferrotyped image under reflected light

Humidity blocking

Under conditions of high humidity and tight wind the gelatin can start to crosslink from the emulsion to the backing layer of the adjacent layer in a reel. This adhesion can form a bond stronger than the bond between the emulsion and the subbing layer.

Consequently if a film is carelessly unwound then the emulsion will tear from the subbing layer. In lesser instances the emulsion may lift away from the subbing layer but then release from the adjacent layer. This leaves a space between the emuslion and the base which causes a blemish that is highly visible. Locally this is referred to as antiferrotyping. An example of this is visible as the thin white line on the image in Figure 12.12 i.

Unblocking humidity induced blocking

Soaking a blocked film in a slightly alkaline solution can reduce the degree of adhesion. The alkaline solution (given in Table 12.3) is around pH 8 which swells the gelatin and creates a negatively charged surface which slightly repels the adjacent gelatin layer.

Chemical Weight (g/litre)
Sodium polymetaphosphate 15

Table 12.3 Unblocking Solution

The process is very slow and can take many weeks to reach the stage where the film can be unwound without significant damage. During treatment the solution needs to be changed on a daily basis as this prevents too much biological action affecting the film. However, biological action still remains a problem.

While undergoing treatment the film needs to be checked regularly. If some of the film can be unwound, do so and wash and dry it immediately. Identify the sections and splice them together at some later date.

A particular problem is with A and B rolls of 16mm film, where often the black spacing has been spliced in with the spacers, emulsion reversed to the image emulsion. This creates parts of the roll where there is emulsion to emulsion contact, which is often exceedingly hard to unblock without damage.

Fig 12.13 Unwinding a treated film

To unwind the film place it on a spindle while still wet and kept it wet while unwinding carefully (Fig 12.13). Placing the film in an ultrasonic bath can help to release the film. This is a slow and painstaking process that cannot be hurried.

After unblocking, the film needs to be rewashed to reduce any blemishing that may have occurred and to thoroughly remove any treatment chemicals.