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Generally, a conservator considers a photographic object as three components comprising of carrier (the base), binder (the emulsion) and the image material (silver or dye). However, there are several other components that can affect the outcome of any conservation treatment.
The topcoat is a layer of gelatin that covers the surface of the emulsion. This layer is a slightly tougher form of gelatin and has small granular particles embedded in it that give the emulsion surface a slightly matt finish. This assists in winding an evenly wound reel and also has some slight optical benefits.
The binder component, a gelatin layer that contains the image forming materials. In a colour film the emulsion can consist of many image forming layers, while black and white film usually has no more than two distinct layers. Fig 2.3 shows a cross section of colour negative emulsion before and after processing. The emulsion layer is very thin and readily susceptible to physical damage such as abrasion or scratching.
Gelatin is quite hydroscopic and will readily absorb moisture from the air. It is also sensitive to changes in pH. If the film has started to decompose, or has been affected by mould it may become very water soluble and aqueous treatments cannot be used.
A thin layer that allows the emulsion to better adhere to the base. The exact composition of the layer is proprietry information, but in general subbing layers are a mixture of cellulose acetate, cellulose nitrate and gelatin. The cellulose nitrate in the subbing layer has been linked by some researchers to accelerated decomposition but this is still an area of contention.
When a film shrinks to a significant degree, it is the adhesion between the subbing layer and the emulsion or the subbing layer and the base that fails.
During film repair the subbing layer needs to be completely removed when using film cement. If this is not done the subbing layer will interfere with the strength of the adhesion of the cement repair.
The carrier layer, with the important characteristics of transparency, toughness and flexibility. The most common film base polymer is cellulose triacetate with most collections holding lesser amounts of cellulose diacetate, cellulose nitrate and some polyester base.
Any treatment given to a film must not compromise the important characteristics. The polymers used in bases can retain considerable quantities of water even after drying. Cellulose acetates can potentially hold up to 6 per cent water by weight, although polyester is much lower at 2 per cent. With aqueous treatments the impact of this quantity of residual free water as a source available for decomposition reactions and mould needs to be considered.
A performance enhancing layer that is applied to the underside of the base. Usually a gelatin layer, although sometimes other polymers such as poly vinyl acetate (PVA) may be used. This layer is designed to counteract any curl that is caused by a change in dimension of the emulsion due to temperature and humidity by responding in a similar way to the changes in environment.
During conservation treatments the gelatin in this layer will behave in a similar fashion to the emulsion.