Photographic emulsion
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Photographic emulsion is a light-sensitive colloid, such as gelatin, coated onto a substrate. In silver-gelatin photography, the emulsion consists of silver halide crystals suspended in gelatin, and the substrate may be glass, plastic film, paper or fabric.
Photographic emulsion is not a true emulsion, but is a suspension of solid particles in a fluid. The word emulsion is established usage in photography and photographic science. Sensitizing solutions for non-silver gelatin processes, such as dichromated colloid processes, cyanotype and kallitype are sometimes called emulsions.
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[edit] Components
Photographic emulsion is fine suspension of insoluble light-sensitive crystals in a colloid sol, usually containing gelatin. The light-sensitive component is one or a mixture of silver halides: silver bromide, chloride and iodide. The gelatin is used as a binder since it acts as a semi-permeable membrane, allowing processing agents (e.g., developer, fixer, toners, etc.) in aqueous solution to enter the colloid without dislodging the crystals. Other polymer macromolecules are often blended, but gelatin has not been entirely replaced. The light-exposed crystals are reduced by the developer to black metallic silver particles that form the image. Colour films and papers usually have multiple layers of emulsion, with dye couplers added. Layers of dye are coated between emulsion layers to act as optical filters.
[edit] Manufacture
A solution of silver nitrate is mixed into a warm gelatin solution containing potassium bromide, sodium chloride or other alkali metal halides. A reaction precipitates fine crystals of insoluble silver halides that are the light-sensitive. The silver halide is actually being 'peptized' by the gelatin. The type and quantity of gelatin used influences the final emulsion's properties. A pH buffer, crystal habit modifier, metal dopants, ripener, ripening restrainer, surfactants, defoamer, emulsion stabilizer and biocide are also used in emulsion making.
Most modern emulsions are "washed" to remove some of the reaction byproducts (potassium nitrate and excess salts). The "washing" or desalting step can be performed by ultrafiltration, dialysis, coagulation (using acylated gelatin), or a classic noodle washing method.
Emulsion making also incorporates steps to increase sensitivity by using chemical sensitizing agents and sensitizing dye.
