Art 50 & 150
Black&White Photography

Basic Black&White Chemistry

Black&White photography is based on the chemistry of silver halides. Silver bromide is most commonly used today. Both photographic film and photographic paper consist of a suspension of silver bromide micro crystals in gelatin coated onto clear plastic (films) or white paper (print papers), respectively. Of course, these are formulated in quite diverse ways by different companies and for different purposes.

The basic process of converting silver bromide emulsions to film negatives or prints rests on the reduction of silver ions to metallic silver. The fundamental key to this process, however, lies in the light sensitivity of silver halides. The second key lies in the chemical kinetics of reduction. Imperfections in the silver bromide micro crystals allow halide ions to jump to higher energy levels when struck by photons. The higher the density of photon bombardment, the larger the number of excited cites per micro crystal. The emulsion is said to have been "exposed" and it can remain in this state for a reasonably long period of time.

When the exposed emulsion of silver halide is "developed," it is placed in a solution of some chemical reducing agent. Many modern developers are based on hydroquinone (p-Dihydroxybenzene). If enough time is provided, all silver ions will be reduced to metallic silver and the negative or print paper will be uniformly black. B&W photography is based on the differential kinetics of photon-excited silver halide cites versus the kinetics of ground-state silver halide ions. The photon-excited cites, as you might imagine, react more rapidly with the reducing agent to produce metallic silver micro crystals. Imagine two ascending curves of silver deposition versus times. Both reductions are taking place but the curve for excited halides ascends much more rapidly at first. The reduction of unexcited halides results in what is called "fog." Clearly, if development is carried along for a short period of time and then stopped, the deposited silver will have come primarily from the photon-excited cites and will be densely deposited in proportion to the amount of photo excitation. In other words, the properly developed negative or print will be a "record" of the light intensities in whatever image was focused on the film or print paper originally. It should be obvious, at this point, that solution concentrations, mixing, temperatures, and contact times are all extremely important in delivering the best image possible.

While controlled development has produced a fine image record in the negative or print, the process is by no means finished. Silver halides are basically unstable in light and will undergo spontaneous, though slow, reduction. Even though development has been stopped by removing the developer and washing the negative or print, long exposure to light will eventually spoil the image by allowing a cloud of silver to form from the remaining undeveloped silver halide. To render a negative or print permanent, the undeveloped silver halide must be completely removed from the emulsion, leaving only the inert metallic silver. This is the chemical job of the "fixer." Since silver halides are all water insoluble, they have to be removed by chemical reaction. Sodium thiosulfate and Ammonium thiosulfate are both commonly used active ingredients in fixing baths. (The ammonium salt gives a faster reaction.) Silver forms complex anions with thiosulfate and these are highly soluble, allowing the silver to be washed away. Fixing baths also usually include "hardening agents" that turn the emulsion hard to prevent scratching as the negative is further handled.

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