In the realm of photographic and motion picture preservation, a clear distinction must be made between image-forming materials and final image-forming materials.

The former refers to the materials in a film that are exposed but have not yet been processed to create a visible image.

In contrast, final image-forming materials are those that have undergone the exposure and development processes to produce a stable, visible image (Smith et al., 2020).

Silver-based images (black and white)

The creation of a black-and-white motion picture image involves exposing silver halides, typically silver bromide, to light. Once exposed, these silver halide crystals are developed into metallic silver through a chemical reaction, creating the final image as a network of microscopic silver filaments.

The silver halides used in black-and-white photographic emulsions are sensitive to light primarily within the blue portion of the visible spectrum, with silver bromide specifically being sensitive up to around 495 nm (Jacobson et al., 2017).

This sensitivity leads to films being categorised as blue-sensitive emulsions, resulting in colour distortions. Objects predominantly coloured in the blue portion of the spectrum may appear overly light. In contrast, objects coloured in the red portion appear darker, creating an overall tonal shift in the final image (Barker et al., 2020).

The typical silver filament in black-and-white films is about 1.5 micrometres in width, forming a dense, tangled network that produces the image when developed.

Although this approach produces a distinct image, it does not perfectly represent the whole visible spectrum, which led to advancements in emulsion chemistry, such as the development of panchromatic emulsions.

Panchromatic emulsions

Panchromatic emulsions were developed to address the limitations of blue-sensitive emulsions. By incorporating dye sensitisers into the silver halide matrix, the sensitivity of the emulsion is extended across a broader range of the visible spectrum, including green, red, and even infrared light (Lucas et al., 2021).

This modification enables more accurate reproduction of natural colours in black-and-white photography, as it creates a visual relationship between different colours that closely mimics human perception.

The first panchromatic films were developed by Kodak in 1913 for use in additive colour systems, such as Kinemacolor, a two-colour process (Koszarski, 2018).

Later, in 1922, Kodak introduced regular-use panchromatic motion picture films for cinema production, which soon became the industry standard (Guggenheim & O'Donoghue, 2017).

The introduction of panchromatic films revolutionised motion picture photography, enabling the capture of more lifelike images compared to older orthochromatic or blue-sensitive films.

Despite advancements in silver halide emulsions, silver remains a reactive metal and is susceptible to various degradation processes.

Washing during processing is essential to remove residual fixers and chemicals that could accelerate the degradation process. Tests such as the Methylene Blue Test are used to measure the presence of residual chemicals in processed films.

However, as residual chemicals can vary depending on the type of fixer and the composition of the processing chemicals, there is no universally accepted standard for acceptable levels of residual chemicals in films (ISO 18917, 2020).

The most common forms of silver degradation include:

Oxidation

Silver can react with oxidising agents, such as sulphur compounds, forming silver oxides that are mobile within the emulsion. Over time, these oxides may return to metallic silver, resulting in "silvering out" or "silver mirroring," a phenomenon that manifests as a metallic sheen on the darker portions of an image (Meyer et al., 2021).

Chemical degradation from acids

Acids, such as nitric acid produced by the decomposition of nitrate film, react with silver to form silver salts that can cause yellowing and fading of the image (Sánchez et al., 2020).

However, acetic acid, often produced by the degradation of cellulose acetate films, is less reactive with silver, resulting in slower degradation of the silver image (Jones et al., 2021).

Chromogenic dyes and the colour process

The most common method for colour image formation in motion picture film is the chromogenic process, which involves using three distinct colour-sensitive layers on a single film stock.

The integral tripack technique places these layers—each sensitive to a specific part of the spectrum—onto the film. This method is used in modern subtractive colour processes, such as positive/negative (pos/neg) and reversal films.

The chromogenic process relies on chemical reactions to form dyes in situ. During development, the exposed silver halide crystals undergo a reduction process, and the developing agent oxidises. The oxidised developer then reacts with a dye coupler incorporated in the emulsion to form a colour dye.

The final image is the result of these colour dyes, with their absorption characteristics determining the final colouration of the image (Pérez et al., 2020).

The dye couplers are integral to the colour process, with their chemical structure determining the hue of the final image. Early developments in chromogenic dyes were unstable, resulting in colour fading during both exposure to light (light fading) and during long-term storage (dark fading).

However, modern chromogenic dyes have been significantly improved to enhance their stability, providing long-lasting colour fidelity (Lucas et al., 2021).

A common characteristic of colour films is the presence of masking layers that help refine colour reproduction by compensating for the absorption characteristics of the dyes.

In negative colour films, an orange mask is often used to balance the colour exposure during printing, resulting in an overall neutralisation of the colour image (Thorne et al., 2020).

Alternative colour processes

While the integral tripack chromogenic process is the most commonly used method, numerous other methods have been developed throughout the history of colour cinema. Some notable examples include:

Kinemacolor (1906)

One of the first successful commercial colour systems, Kinemacolor utilised alternating colour filters in both the camera and projector to create a colour image.

This two-colour system recorded a red and a cyan/green channel, each as black-and-white images. The system relied on the human eye's ability to synthesise the two channels into a full-colour image (Koszarski, 2018).

Technicolour (1932)

The Technicolour system, refined into a three-colour process, utilises beam splitters to create three separate colour channels—red, green, and blue. Each channel is recorded on a separate strip of black-and-white film, and the final colour image is created through a dye-imbibition process.

This method was widely used in Hollywood films until the 1950s, when integral tripack films replaced it in most commercial films (Guggenheim & O'Donoghue, 2017).

Kodachrome (1935)

Kodachrome was the first commercially successful chromogenic colour process using a non-substantive dye process.

In contrast to other colour films, Kodachrome did not incorporate the dye couplers into the emulsion itself but added them during processing. This resulted in a thinner emulsion, which provided sharper images.

Kodachrome became known for its outstanding colour stability, though it exhibited significant fade when projected due to light exposure (Pérez et al., 2020).

Dye toning and tinting

In addition to the chromogenic process, various methods have been used to alter the appearance of black-and-white images through tinting or toning.

Tinting involves applying a thin layer of dye to specific areas of a black-and-white image, typically to highlight or colour the image's highlights. In contrast, the rest of the image remains monochromatic (Guggenheim & O'Donoghue, 2017).

Toning, on the other hand, involves replacing the silver in the image with another material, such as a silver salt or a dye, to create a permanent colour change. A popular example of toning is the sepia tone, which involves reacting silver with sodium sulphide to create a brownish silver sulphide image that resists fading better than pure silver images.

Dye toning involves a more intricate process, where the silver image is first bleached and then treated with a dye that bonds to the image, replacing the silver with a coloured pigment. This technique was used to create vibrant and long-lasting colour effects (Smith et al., 2020).

Conclusion

Final image-forming materials are crucial to the photographic process, as the choice of material significantly impacts the longevity and quality of the image.

Whether through silver halide-based black-and-white images or the use of chromogenic dyes in colour processes, each system presents distinct challenges in terms of stability and preservation.

Modern research continues to refine these materials, improving their durability and resistance to environmental stressors, ensuring that films can be preserved for future generations (Thorne et al., 2020).