Biological damage

Moulds
Bacteria
Insects
Control

It is well known that gelatin is a good food source for a variety of micro and macroscopic life forms. It is also entirely possible that cellulose ester base polymers can also be a target.

Moulds

Moulds are microscopic, plant-like organisms, composed of long filaments called 'hyphae’ with cell walls made from 'chitin’, the same material as the hard outer shells of insects and other arthropods. However, because of the filamentous construction, lack of chlorophyll and the fact that plants do not make chitin, moulds are considered by most biologists to be separate from the Plant Kingdom and members of the Kingdom of Fungi. They are related to mushrooms and toadstools, differing only in not having their filaments united into large fruiting structures.

When mould hyphae are numerous enough to be seen with the unaided eye they form a cottony mass called a 'mycelium’.

Moulds feed by absorbing nutrients from the organic material in which they live. Since moulds do not have stomachs they must digest their food before it can pass through the cell wall into the hyphae. To accomplish this the hyphae secrete acids and enzymes that break the surrounding organic material down into simple molecules they can easily absorb.

The hyphae grow in the search for food, apparently randomly spreading across the surface of the gelatin. The furrowing or channelling marks are left where the gelatin has been digested.

Fig 6.6 Mould damage on the surface of a film

Mould damage is apparent on the emulsion surface as a dendritic pattern that looks a bit like a fern leaf, Fig 6.6. It is more usual for the mould to start at the edge of the film and work inwards, but it is not unheard of for most of the damage to appear in the centre of the film. This may have something to do with the tension under which the film has been stored.

Fig 6.7 Mould furrows in a colour emulsion

Sometimes the hyphae will furrow into the emulsion rather than just spreading across the surface. When this occurs on a colour film the mould damage might appear as a different colour to the surrounding areas depending on how far through the colour layers of the emulsion the mould has digested (Fig 6.7).

Moulds reproduce by spores. Spores are like seeds; they germinate to produce a new mould colony when they land in a suitable place. Unlike seeds, spores are very simple in structure and never contain an embryo or any sort of preformed offspring. Spores are produced in a variety of ways and occur in a vast array of shapes and sizes. In spite of this diversity, spores are quite constant in shape, size, colour and form for any given species. Accordingly spores are very useful for mould identification.

Research carried out in Australia, Vietnam and Europe has identified two main genus of moulds that affect film, 'Aspergillus’ and 'Penecillium’.

Bacteria

Bacteria are among the simplest, smallest, and most abundant organisms on earth. All bacteria are unicellular, single celled organisms. Most bacteria are only 1×10-6m in diameter.

The most readily identifiable damage caused by bacterial action occurs if the film is soaked in water (e.g. a flood) and left for a length of time without drying. Due to the microscopic size of bacteria they cannot be seen, only the damage caused after an infestation has occurred is noticeable.

The damage caused by bacteria is more of an altering of the gelatin structure rather an obvious physical sign such as the channelling left after a mould has digested a section of the emulsion. Bacterial attack can sometimes be seen by small bubbles of a black tarry substance on the edges of the reels. In very bad examples the bacterial action can cause a film to block together, once this degree of bacterial attack occurs a film is past salvation.

Insects

Cockroaches, silverfish and beetles (both larvae and adult) all find gelatin a good food source. Being large these vermin can be easily seen, as can the damage they do as they feed. The marks left by insects look like irregular holes or channels, but these tend to be wider than mould marks and have a more irregular edge.

Control

The biological action of moulds and bacteria is rate dependant upon three factors, food supply, water and energy (as heat). By controlling any of these factors the rate of activity can be reduced. In reality we can only control two factors, heat and moisture. By storage under controlled low temperature and low relative humidity the rate at which biological factors attack the film can be minimised.

Moulds are particularly sensitive to relative humidity. Research performed in Vietnam has shown that moulds will cease to be viable below 60% RH.

Larger vermin can be controlled by an integrated pest management approach.