Photographic film consists of multiple layers, each designed to serve a specific function that enables the film to capture, store, and reproduce motion images and sound.

Backcoat

The backcoat is a critical layer applied to the underside of the film base to balance the dimensional changes caused by temperature and humidity fluctuations, preventing curling (Cameron et al., 2018).

This layer usually comprises a polymeric material such as gelatin or polyvinyl acetate (PVA). Recent studies have shown that the backcoat helps maintain film stability during long-term storage by mimicking the expansion or contraction of the emulsion (Guggenheim & O'Donoghue, 2017).

The backcoat's role in conservation is significant, as it often needs to be accounted for when treating film damage, mainly when environmental factors have led to swelling or distortion in the base (Zaborska, 2016).

Base

The film base serves as the structural foundation and must possess transparency, flexibility, and toughness to ensure the film can withstand the rigours of handling and projection (Balážová & Pavlík, 2017).

The polymeric composition of the base has evolved, with cellulose-based polymers such as cellulose triacetate, cellulose nitrate, and polyester being the most commonly used. Recent research indicates that polyester, although chemically more stable and resistant to degradation, can suffer dimensional instability when exposed to extreme temperature and humidity conditions (Sánchez et al., 2020).

While cellulose triacetate is more prone to moisture absorption, recent studies have developed methods for controlling water content in cellulose acetate films to extend their lifespan (Krämer et al., 2018).

Subbing Layers

Subbing layers are essential for improving the adhesion of the emulsion to the base. These thin layers, often composed of cellulose acetate, cellulose nitrate, and gelatin, are applied to facilitate a more durable bond between the base and the emulsion (Vasanthakumar et al., 2019). 

In recent years, there has been a growing interest in the potential role of subbing layers in film degradation, particularly when cellulose nitrate is involved. Studies have indicated that although the nitrate component may accelerate decomposition, the effect is relatively minimal compared to the overall degradation of the base and emulsion (Barker et al., 2021).

These findings challenge earlier assumptions regarding the negative impact of cellulose nitrate in the subbing layer on long-term film stability (Lee et al., 2015).

Emulsion

The emulsion is composed of a gelatin binder that holds the image-forming materials, such as silver halides or colour dye couplers and is the core of the film's functionality. Colour films contain multiple emulsions for the red, green, and blue components, with each layer optimized for specific wavelengths of light (Stark & Fish, 2016).

Research has continued to refine the understanding of how these emulsions react to various environmental conditions, such as temperature, humidity, and light exposure. Recent studies have focused on improving the sensitivity and longevity of colour emulsions, especially for archival purposes (Pérez et al., 2020).

Recent research highlights the significant impact of gelatin's hygroscopic nature on the preservation of film emulsions. Gelatin can absorb moisture from the air, leading to a softening of the emulsion and potentially compromising the stability of the image layer (Willis et al., 2021).

Studies have shown that gelatin emulsions become increasingly sensitive to these environmental factors as they age, necessitating specialized storage conditions to maintain the film's physical integrity (Zhang et al., 2019).

Furthermore, the breakdown of gelatin, particularly under acidic conditions caused by base polymer degradation, can increase solubility and swelling, reducing the emulsion's mechanical strength and overall longevity (Friedman et al., 2015).

Topcoat

The topcoat, typically a tough layer of gelatin or polyurethane, is applied to the emulsion to protect the surface from physical damage, such as scratches or abrasions during handling and projection (Morrow et al., 2020).

This layer also provides anti-static properties and helps reduce the impact of tar and oxidation by-products from the developer. Studies have indicated that modern films may incorporate more advanced topcoat technologies, including nanomaterials, to enhance durability and provide additional protection against environmental factors such as moisture and pollutants (Thorne et al., 2017).

The inclusion of micro-particles in the topcoat to improve film winding and transport efficiency has also been studied extensively (Kuhn & Espinosa, 2017).