These silver atoms form the so called “latent” image in the emulsion, that will be developed into the final image Development As further photons are incident on the crystal, the number of metallic silver atoms grows. The halide atoms are released from the crystal and will collect in the emulsion. The electron migrates to an sensitivity site where it combines with a mobile silver ion to form a metallic silver atom. When a photon of light is incident on the silver halide crystals, it can liberate an electron from the halide. ExposureĪs mentioned earlier, the silver halide crystals in the emulsion contain small defects leading to gaps in the lattice, as well as mobile silver ions. Kodak Panalure is an example of the latter. There are also panchromatic emulsions which are sensitive to all wavelengths and thus must be used in complete darkness with no safelight. There are some variations in the choice of sensitive wavelengths, hence the need to use special safelight colours with some papers. Usually paper is designed to only be sensitive to blue and green wavelengths of light, allowing use of a red safelight in the darkroom. Further additives will also be included to control the sensitivity of the emulsion. The emulsions are usually washed to remove reaction byproducts, specifically the salts and nitrates. While the vast majority of photographic paper emulsions are based on Silver Bromide, the choice of other chemicals used during the manufacturing process may vary wildly across manufacturers or product lines. A pH buffer will influence the speed of the crystal forming reaction, a crystal habit former will influence the shape of the crystals, a ripener will encourage the formation of crystals, restrainer prevents the reaction taking place too quickly, surfactants lower the surface tension between the liquids to encourage mixing, a defoamer will hinder the formation of foam, stabilizers will prevent other undesirable chemical reactions taking place, biocides will kill any biological impurities in the mixture. It is these flaws which in fact lead to the photosensitivity of the silver halide.Ī number of other chemicals are added which will influence the chemical reaction, the formation of the crystals and thus the characteristics of the emulsion when later exposed to light. Other active molecules in the gelatin also react with the silver and result in flaws in the crystal lattice. The silver halide molecules form light sensitive crystals, the size & shape of which determine the “grain” of the emulsion. Taking potassium bromide as the example halide, the reaction would be AgNO 3 + KBr => AgBr + KNO 3

The silver nitrate reacts readily with the halides, the silver atoms combining with the halogen atoms, while the salt combines with the nitrate. The emulsion is formed by taking silver nitrate and mixing in a gelatin solution that contains halides (potassium bromide, sodium chloride, or potassium iodide). It also forms a good colloid ensuring an even spread of crystals across the substrate (paper). Gelatin is chosen because it is permeable allowing the chemical agents (developer, fixer, toners) to easily access the light sensitive crystals. In order to figure out what might be happening during the Lumen process, it is important to first understand the chemical processes by which normal silver gelatin images are formed… Traditional Silver Gelatin photographic processĬommon photographic emulsion consists of crystals of one of the silver halides (silver bromide, silver chloride or silver iodide), suspended in a gelatin layer. It did, however, start me wondering what on earth is going on to form such colours from the POV of the chemistry. I had heard before that different papers can give wildly different colours when used with the Lumen process, but the papers I’d tried before this were all reasonably similar, so the results from the Kodak paper was a delightful surprise.

Kodak Autopositive paper after fixing lumen exposure

Lumen print