| Photo Chemical Process |
The photographic process occurs when visible light strikes a material that has been coated with a photographic "emulsion".
The emulsion is a very thin coating of invisible silver halide crystals. The silver halides can be either silver bromide, silver chloride, silver iodide, or a combination. The emulsion for film is placed on a plastic medium and similarily the emulsion for prints is placed on a piece of paper.
The silver halides crystals change from white to black metallic silver when exposed to light. This change will not be visible, but rather is an invisible change which produces what is called the "latent image". The amount of light striking the emulsion will determine the quantity of the metallic silver developed. The greater the amount of light striking the salts, the greater the amount of metallic silver grains developed. This is called density in the developed film or paper
In order to see the image the print must be developed, "fixed" and washed.
The print developer reacts with the exposed crystals of silver halide, freeing the silver from its compound and depositing it as tiny, irregular grains of metallic silver. These many, minute grains, form the black silver image (this is what you see - the blacks or grays of the image).
After developing to the desired effect, the development must be stopped so that the print will not darken further. This is done by placing the print into an acid stop bath for about 10 seconds. The acid neutralizes the chemically alkaline, photographic developer.
The print is then placed in a fixing bath sometimes called hypo, (active ingredient in fixer is either sodium thiosulfate or ammonium thiosulfate) which will dissolve the remaining unexposed and undeveloped silver halide crystals into the fixing solution. These areas then become the white or gray shaded areas of the print, or in the case of film, the transparent or semi-transparent areas.
The print then must be washed to remove any residual chemicals and dissolved silver salts. This washing away of the remaining silver halide crystals will prevent the film or paper from turning dark when the print is exposed to visible light.
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