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. Sepia is considered a form of Black and White or Monochromatic Photography.
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.
Selenium toning may not produce prints quite as stable as sepia or gold toning. Recently, doubts have surfaced as to the effectiveness of selenium toner in ensuring print longevity.
In most applications, selenium toning was not used for its sepia or red tone by fine art photographers. In fact it was used for the opposite of those effects
Agfa Portriga Rapid and Agfa Record Rapid (the highest silver content black and white photographic paper made for the mass market) had a brownish green tone. Depending on the strength of selenium tone mixed in water, a three- to four-minute tray bath, with continuous circulation, removed the brownish green tone and provided a deep rich black.
Higher concentrations of selenium toning and longer tray baths produced the red effect. 99% of selenium-toned vintage prints in museums and sold at galleries and auction are toned to remove the brownish green tone and provided a deep rich black.
Most reported tests over the years in technical photography publications gave selenium-toned prints twice the longevity of untoned prints.
Sepia toning is a specialized treatment to give a black-and-white photographic print a warmer tone and to enhance its archival qualities. The metallic silver in the print is converted to a sulfide compound, which is much more resistant to the effects of environmental pollutants such as atmospheric sulfur compounds. Silver sulfide is at least 50% more stable than silver.
There are three types of sepia toner in modern use;
Except for polysulfide toners, sepia toning is done in three stages. First the print is soaked in a potassium ferricyanide bleach to re-convert the metallic silver to silver halide. The print is washed to remove excess potassium ferricyanide, and then immersed into a bath of toner, which converts the silver halides to silver sulfide.
Incomplete bleaching creates a multi-toned image with sepia highlights and gray mid-tones and shadows. This is called split toning. The untoned silver in the print can be treated with a different toner, such as gold or selenium.
Fred Judge FRPS made extensive use of sepia toning for postcards produced by the British picture postcards manufacturer Judges Postcards.