Approximately half of the global silver production is utilized by the photographic industry.
Bars of silver are dissolved in nitric acid to form silver nitrate, which then reacts with a halogen element (commonly iodine, bromine, or chlorine in the form of alkali salts or halides such as potassium iodide, potassium bromide, or potassium chloride). Following the removal of by-products, the resulting compound consists of silver halide crystals, which exhibit light sensitivity. To ensure these finely divided silver halides can be evenly coated onto the film base, they are mixed with gelatin to form a creamy silver halide emulsion.
Gelatin is selected due to its high transparency and lack of visible texture. It becomes liquid when heated, making it ideal for coating, and sets (‘gels’) upon cooling or drying. This property allows it to hold the silver halides in a firm, even coating across the film surface, while swelling just enough in processing solutions to permit the entry of chemicals, which interact with the halide crystals without displacing them.
Modern film manufacturing is intricate and demanding. Mixed emulsions are treated with additives and maintained at controlled temperatures for specified periods to ‘ripen.’ This process allows some crystals (‘grains’) to grow larger, enhancing light sensitivity (greater ‘speed’) and producing less extreme contrast. Initially, crystals are uniformly small and equally sensitive to light, resulting in consistent light effects. However, when an emulsion comprises different-sized grains (mixed sensitivity), low-intensity light affects only the large crystals, moderate light affects both large and medium crystals, and the brightest light impacts all crystals, including the smallest. Consequently, when developed, these variations in light intensity are recorded as a range of grey tones rather than mere extremes of black and white.
Further modifications to the emulsion can alter its sensitivity to colored light. In its raw state, the emulsion responds only to blue and ultraviolet (UV) light, but this sensitivity can be extended to additional bands or the entire visible spectrum. Concurrently, the film’s base (usually polyester or tri-acetate) undergoes several preparatory coatings, including an anti-curl gelatin layer on the back to prevent shrinkage or curling when the emulsion is applied to the front. An additional layer of dark ‘anti-halation’ dye prevents light from reflecting off the base and creating ‘halos’ around images of bright highlights.
This anti-halation layer can be positioned between the emulsion and the base or coated on the back of the film. Additionally, 35 mm film includes grey dye in the base to prevent light from entering the cassette and being transmitted along the film’s thickness like a fiber-optic cable. Like the anti-halation material, this dye vanishes during processing, sometimes appearing as a darkening of the used chemicals.
The emulsion coating process itself is highly critical and conducted in ultra-clean conditions. Black and white films may require between one and four layers, while most color films necessitate more than ten layers with various color sensitivities. The final layer is a clear protective gelatin coating. For specific details on the structure of instant-picture materials, see Advanced Photography. The film is coated in large rolls, typically 1.5 meters wide by 900 meters long. After drying, these rolls are cut into various standard film sizes and edge-printed with frame numbers and other information.
Specialized types of film are also produced for X-ray, infrared, lithographic, and other applications. It is often possible to shoot on one type of film and achieve results typically produced by another—for example, black and white prints from color negatives. However, the most straightforward process, involving the fewest stages, usually yields the highest quality. Therefore, it is crucial to know in advance what is required from a job to select the appropriate materials from the outset.
