Sepia tone
In photography, toning is a method of changing the color of black-and-white photographs. In analog photography, it is a chemical process carried out on silver-based photographic prints. This darkroom process cannot be performed with a color photograph. The effects of this process can be emulated with software in digital photography. There is debate whether a toned black-and-white photograph should be considered to still be black-and-white, as simply being monochromatic is not a sufficient condition for an image to count as black-and-white.
Most toners work by replacing the metallic silver in the emulsion with a silver compound, such as silver sulfide (Ag2S) in the case of sepia toning. The compound may be more stable than metallic silver and may also have a different color or tone. Different toning processes give different colors to the final print. In some cases, the printer may choose to tone some parts of a print more than others.
Toner also can increase the range of shades visible in a print without reducing the contrast. Selenium toning is especially effective in this regard. Some toning processes can improve the chemical stability of the print, increasing its potential longevity. Other toning processes, such as those including iron and copper, can make the print less stable. Many chemical toners are highly toxic, some even containing chemicals that are carcinogenic. It is therefore extremely important that the chemicals be used in a well ventilated area, and rubber gloves and face protection should be worn when handling them.
Selenium toning is a popular archival toning process, converting metallic silver to silver selenide. In a diluted toning solution, selenium toning gives a red-brown tone, while a strong solution gives a purple-brown tone. The change in color depends upon the chemical make-up of the photographic emulsion being toned. Chloro-bromide papers change dramatically, whilst pure bromide papers change little. Fibre-based papers are more responsive to selenium toning.
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