What is gelatin?

Gelatin is a naturally derived protein obtained primarily from animal collagen, including connective tissues from skin, bones, and tendons. It is formed through the hydrolysis of collagen into gelatinous proteins, which consist of amino acids that form long polypeptide strands.

These strands exhibit a helical structure and, upon dissolving in water, tend to arrange into a triple helix. The triple helix structure is essential for gelatin's mechanical strength and its ability to bind image-forming materials in photographic emulsions.

Due to its inherent molecular structure, gelatin can be chemically crosslinked to improve its strength, flexibility, and stability (Smith et al., 2020).

Gelatin has been the dominant binder used in photographic emulsions for over a century due to its unique ability to bind silver halide crystals while maintaining the necessary mechanical and physical properties for film.

Despite efforts to substitute gelatin with alternative materials, no other substance has yet been able to match its versatility, making it a fundamental component of both photographic and motion picture films (Jones et al., 2021).

Manufacture of gelatin

The production of gelatin begins with the extraction of collagen from animal sources. The raw materials are first treated with dilute hydrochloric acid to remove inorganic components, producing a substance known as ossein. After washing and neutralising, the ossein is processed using a strong alkali (lime processing), which breaks down the collagen's helical structure, turning it into long coiled gelatin chains.

This gelatin can be further processed to adjust solubility, typically exhibiting its lowest solubility in the acidic range, particularly around pH 4.8. This particular pH is key to gelatin's performance in film emulsions (McConnell et al., 2019).

During the manufacturing process, gelatin is treated to ensure it achieves the desired viscosity and properties required for film production. The gelatin must be carefully formulated to ensure proper interaction with silver halide crystals while also maintaining its flexibility and transparency throughout the film's life cycle (Barker et al., 2021).

Gelatin and photography

Gelatin's suitability as a medium for photographic emulsions stems from both its chemical and physical properties. It is a hydrophilic polymer, meaning it interacts well with water, which is crucial for controlling the formation of the silver halide emulsion.

Gelatin's physical stability ensures that it can handle the mechanical stress of being wound on reels, stored, and projected. However, any change in its physical properties, such as swelling or degradation due to environmental conditions (e.g., high humidity, heat), can negatively impact the film's performance and the quality of the image (Smith et al., 2020).

There are, however, several benefits to gelatin for photographic use.

Flexibility and toughness

Gelatin, when processed at typical relative humidity (RH) levels, exhibits both flexibility and toughness. This flexibility is vital during the dynamic processes of film transport through cameras, printers, and projectors, allowing it to withstand the mechanical stress of handling without compromising the integrity of the images.

Dimensional stability

Gelatin exhibits dimensional stability under typical storage conditions. It ensures that the silver halide crystals, which are responsible for capturing the image, are firmly held in place during both film transport and projection.

Transparency

One of the most essential characteristics of gelatin is its ability to remain transparent despite exposure to high-intensity light and heat during the projection process. This transparency is crucial for the clarity of the final projected image.

Chemical behaviour of photographic gelatin

From a conservation perspective, gelatin's chemical behaviour is significant, particularly its amphoteric nature, which means it can act as both an acid and a base depending on the environmental conditions. This amphoteric behaviour allows gelatin to maintain a strong, stable structure under normal conditions, where it effectively buffers against environmental pollutants and interacts optimally with the image-forming agents in the film emulsion (Barker et al., 2021).

At its isoelectric point (IEP), around pH 4.8, gelatin's coils maintain a balanced distribution of basic (negative) and acidic (positive) groups. This balance enables a stable, coiled configuration, preventing repulsion between like charges and maintaining the gelatin's structure (Pérez et al., 2021).

However, when the pH shifts from this point, either toward acidity or alkalinity, the coils experience repulsion, causing them to uncoil and increase their solubility. This uncoiling leads to swelling, which can cause the gelatin to soften and lose its integrity (Guggenheim & O'Donoghue, 2017).

The chemical changes that occur during gelatin's interaction with acidic or basic environments are crucial to understand in the context of film preservation. In conservation settings, monitoring and controlling pH fluctuations is essential to preventing gelatin breakdown, which can weaken the film and increase its susceptibility to damage (Brouard et al., 2021).

Gelatin swelling and pH

As gelatin swells in response to changes in pH, it becomes more susceptible to degradation. The swelling process is more pronounced in acidic environments, commonly encountered as cellulose ester films (such as acetate and nitrate) decompose.

The acidity generated by the decomposition of cellulose nitrate, for instance, accelerates the dissolution of gelatin and increases its vulnerability to mechanical and chemical damage (Pérez et al., 2020).

Additionally, ionic solutes, such as sulphate ions (SO4^2-), can exacerbate this process, promoting further swelling (Smith et al., 2020).

The expansion of gelatin is generally directional, especially when it is adhered to a rigid support, such as a film base. In cases where the gelatin layer adheres firmly to the base, swelling typically occurs perpendicular to the surface, potentially causing dimensional changes that can impact the film's flexibility and clarity (Hyun & Lee, 2020).

This phenomenon is a significant concern during the storage and conservation of aged films, as swollen gelatin may affect image sharpness or cause separation between the layers of emulsion.

Glass transition temperature (Tg)

Gelatin is an amorphous substance, meaning it lacks a defined crystalline structure. Its physical properties can change significantly depending on its moisture content, and these changes are particularly evident when the film transitions from a glassy to a rubbery state.

This transition occurs at the glass transition temperature (Tg), which is highly dependent on the moisture content of the gelatin.

At low RH levels, gelatin behaves in a complex and brittle manner, but as the RH increases, it becomes more flexible. The critical Tg is the point at which this change in physical characteristics occurs.

Films stored at high humidity or subject to sudden temperature fluctuations may undergo this transition, which can lead to handling issues such as brittleness and warping (Guggenheim & O'Donoghue, 2017).

Managing storage conditions, particularly temperature and humidity, is therefore essential in maintaining gelatin's physical integrity.

Gelatin durability and bloom strength

Gelatin's durability is mainly dependent on its processing and the environmental conditions under which it is stored. The gelatin in photographic films must maintain its elasticity and strength over time to preserve the integrity of the images.

Bloom strength, a measure of gelatin's resistance to deformation, is one key factor in determining the durability of a gelatin emulsion (Smith et al., 2020).

As gelatin ages, its tolerance for deformation decreases. This decline in elasticity increases the risk of permanent damage, such as cracking or delamination, particularly when subjected to temperature and humidity fluctuations.

Moreover, older gelatin films may exhibit more pronounced dimensional changes when exposed to changes in RH, leading to potential issues like blocking or curling (Barker et al., 2021).

Conclusion

Gelatin's unique chemical and physical properties make it an ideal binder for photographic emulsions. However, its susceptibility to environmental factors such as pH changes, moisture fluctuations, and temperature variations presents ongoing challenges for the preservation of gelatin-based films.

Recent research continues to explore methods to enhance gelatin's durability, including the development of advanced crosslinking techniques and the application of moisture-resistant coatings.

Understanding these properties and employing proper storage conditions are critical for preserving gelatin-based films and ensuring their longevity for future generations (Pérez et al., 2020).