Toning is a method of changing the color of black-and-white photographs either to achieve a unique tone or to increase the permanence of the image. This darkroom process cannot be performed with a color photographs.
In analog photography, toning is a chemical process in which the metals which make the image, such as silver prints, iron-based prints (cyanotype or Van Dyke brown), or platinum or palladium prints is either:
- Coated by an inorganic compound (such as gold)
- Converted by reaction into a different salt (sulfide, selenium, or coffee) or replaced by another metal (Iron or copper)
- Dyed with an organic toner that affects the base of the paper
- Total replacement occurs during indirect toning, which as a bleach step first, followed by redevelopment.
Different toning processes, which must include the type of paper, exposure, development, fixing and toner formula, dilution and temperature, give different colors to the final print, as the reacting compounds have different colors and optical characteristics. Different papers react to toners very differently, with some that don't tone at all In some cases, the printer may choose to tone some parts of a print more than others (split print) or tone more than once to have special effects. Some toners also can increase the range of tones visible in a print without reducing the contrast (increase D-Max).
Some toners produce very stable compounds, which can be used for archival and permanence of images with little tonal change. Selenium and sulfide toning are especially effective.
The effects of this process can be emulated with software in digital photography, with large gamut of effects easily avilable. Especially sepia is considered a form of black-and-white or monochrome photography by itself.
Chemical toners must be handled carefully, as some of them are highly toxic, or contain chemicals that are considered carcinogenic. Like with any other chemicals, precautions such as: use in a well ventilated area, wear rubber gloves and eye protection, use thongs must be followed.
- Development: Prints must be developed in a solution without exceeding its capacity. Contaminated or exhausted developer can lead to stains
- Fixing: Prints must be fixed properly, agitating to avoid uneven fixing and avoiding over-fixing. Non-acid and non-Hardening fixers are
- Washing: The use of a Hypo-Clearing Agent (10% Na-Sulphite) is highly recommended before washing to ensure prints are thoroughly washed and avoid stains.
- Avoid reusing the HCA as it might contain traces of toner
- Trays: plastic trays are recommended to avoid metal contamination
- Best results are found in prints that have been dried and then re-wet, it is good practice to soak them in water for 5 minutes (2 for RC paper) to wet the emulsion and allow the toners to act.
- Prints can stain in the edges as toner is hard to wash, giving the prints wide margins (at least 2 cm) allows for trimming the stained part
- Washing after toning is very important. Fiber based prints need a Hypo-Clearing agent to save water while RC paper can be washed with water only.
- Toned prints must be dried face up to avoid contaminating dryer racks
- Dryer racks can be washed with some bleach to ensure there is no residue
Chemical toning of prints is carried out in 2 methods, direct and indirect toning. Indirect toning is also known as bleach and redevelop toning.
Direct toning Chemicals are applied directly to the fixed and washed print. Toners act directly on the image
Indirect toning Prints are first bleached (partially or totally), to convert the silver of the image back into a halide. The toner is applied afterwards converting this halides into other compounds. The effect of this toning includes changes in print density, tonality and shadows change color. The most common of these processes is the Sepia toning which yield a yellow/brown image.
Bleaching the image is generally done with a mixture of potassium ferricyanide and potassium bromide; however other bleach formulas are available and can be used depending on the process and final result desired. Bleaching can also be done to already toned prints for special effects.
When bleaching is incomplete this is known as Split-Toning. Prints that are split-toned are not of uniform coloration, but show several tones in the same picture with traces of intermediate colors in the places where bleaching was more or less complete.  The split toning procedure lends itself for endless experimentation, and it becomes "art" more than science.
Selenium toning has been used for a long time, and it works by converting metallic silver to silver selenide (Ag2Se). It produces a range of colors, from very subtle purple to purple brown depending on the dilution, and paper. 
Selenium toning was popular for image permanence, using a diluted solution that produced little change in tonality. However, research by IPI  has cast doubts on the effectiveness of diluted selenium toner in ensuring print longevity. Selenium is very effective in protecting the shadows, but not the highlights unless print is fully toned. A combination of selenium and sulfide is preferred.
Sulfide toners can be applied directly (brown toners) or using a bleach step (sepia). These toners make use of the reaction between metallic silver and sulfide salts that create a very stable compound (Ag2S). These reactions were recognized early as 1850 by W.H. Fox Talbot. The downside of Sulfide based toners is the smell of rotten eggs as they release Hydrogen Sulfide gas, and the fact that they fog photographic material. They must be used in a well ventilated area, preferably outside and away from any photographic materials.
Direct toners are usually polysulfide (liver of sulfur) based, and produce a range of tones from grey-brown to deep brown to peach depending on the paper and processing. These toners affect all the parts of the print equally, and care must be taken to stop the toning and avoid peach colores stains in the highlight during washing. A HCA bath is recommended.
Sepia toning is one of the traditional processes, in which the prints are bleached (partially or totally) and then "redeveloped" in a sulfide based toner. Sulfide based toners have been used for decades. Hypo-alum toners, in which thiosulfate (Hypo) is aged to the point where is releases sulfur and is "ripened" by the addition of silver nitrate and potassium ioodide. This toner is very slow and yields rich brown-red tones. Modern toners are based in thiourea (Thiocarbamide) and are less smelly than the old versions and depending on dilution they provide a whole range of final tones. These are easy to mix, and act much faster than hypo-alum toners.
The Sepia process consist of:
- Redevelop in toner
- HCA bath to stop toning
Partial bleach is called split toning in which the shadows of a print are a combination of silver and sulfide and the highlights are fully toned.
Precious metals, Gold and Platinum have been used since the beginning of photography. In 1841 it was used to improve the color and permancence of Daguerrotypes  The resulting color ranges from an almost imperceptbile warm tone to brown to red, depending on the whole toning process.
This toner coats the silver grains in a photography, which should make it impervious to the elements and extending its life. However for image permanence the toning must be complete.
Blue, red and green toning
Blue toning is achieved by using ferric ferrocyanide salts, to obtain "Prussian Blue" compounds, similar to a cyanotype. The final color depends on the process and the toner used, and can vary from a deep blue, to a green blue to a "porcelain blue" obtained by redevelopment in blue toner.
Red toning is obtained by a copper ferrocyanide and the process is similar to blue toning. The resulting color varies form a red-brown to a "rusty"-red depending on the process.
The conversion of silver to ferrocyanide is not always complete, thus a short fixing bath is recommended to avoid stains. These types of toning produce images that are less stable than the original silver prints, unless followed by a sulfide or gold treatment which again changes the color.
Green toning is a combination of blue toners, either with split toning or with sepia toning. Techniques to obtain these colors are difficult.
Organic toners, usually act by staining both the paper base and image.
Among these toners tea and coffee take a special place due to their ease of use. Organic dyes can also be used. Prints are stained with either to obtain a series of effects
Tea and coffee take a special importance with cyanotype toning, as both of them work very well and can be used directly or with a bleach step to obtain a large range of tones. Tea toning of cyanotypes is also known to make them more permanent.
Combination of toners
Toners can be used in combination, either as a single bath (sulfide and selenium) or as several steps for different effects.
Single bath sulfide-selenium toners such as Kodak Poly-Toner and AGFA Viradon were common in the 70s and 80s and were used for general purpose photography as well as permanence of the image. The color change ranged from a subtle grey-brown to a full pink-brown.
Double Toning allows for especial effects in prints, depending on the order and intensity of toning the combinations of colors are different. Sepia and selenium, sepia and gold, selenium and gold, and brown and gold have been used for permanence and effects directly or indirectly.
Blue toners have been used with sepia, selenium to obtain blue and brown or blue and purple images. Blue, sepai and gold produce orange browns to greens, etc.
Toning for permanence
Toning has the side effect of creating stable compounds which replace the original silver salts in negatives, microfilm and silver-halide papers. Selenium and sulfide are the 2 best known toners, where silver sulfide and selenide (Ag2S and Ag2Se) are less prone to oxidation than silver. Kodak used to recommend the use of KRST while Agfa recommended the use of Viradon. Latest research  has shown that sulfide toner, even partial provides longer lasting protection
Recent research at the Image Permanence Institute has shown that sulfiding treatments give excellent protection for microfilm against red spots, and that gold and selenium treatments only are effective (in the absence of sulfiding action) to the degree that the silver image is converted to gold or to silver selenide.
Other photographic images, such as cyanotype can be made permanent by toning with tannins (coffee, or tea).
Digital photography can bring all these techniques to the computer realm. Software like Photoshop, GIMP, Darktable, etc have built in modules that will emulate the wet process.
GIMP has a function called "Sample Colorize" in which an image is recolored as per a sample gradient. The color gradient of a chemically toned print can be extracted and applied to a variety of images.
- Anchell, Steve, The Darkroom Cookbook, 4th Edition. Focal Press, 2016.
- Rudman, Tim. The Photographer's Toning Book: The Definitive Guide. Amphoto Books, 2002.
- Split-Toning part I: Background and Historical Antecedents by Jonathan Bailey at Unblinking Eye.
- Split-Toning part II: Processes and Procedures by Jonathan Bailey at Unblinking Eye.
- Testing Kodak Selenium Toner by Liam Lawless at Unblinking Eye.
- Not Fade Away Silverprint magazine #7 archived
- Testing Gold Toner by Liam Lawless at Unblinking Eye.
- Double toning by Ed Buffaloe Unblinking Eye.
- Wilhelm, Henry, and Brower, Carol, The Permanence and Care of Color Photographs: Traditional and Digital Color Prints, Color Negatives, Slides, and Motion Pictures, Preservation Publishing Company, 1993 Book available form the authors
- Stability of Black-and-White Photographic Images, with Special Reference to Microfilm by Reilly et al. 1988
- AGFA-Ansco Corp, AGFA FORMULAS FOR PHOTOGRAPHIC USE, Binghamton, NY, 1937.
- Anchell, Steve, The Darkroom Cookbook, 4th Edition. Focal Press. 2016
- Rudman, Tim. The Photographer's Toning Book: The Definitive Guide. Amphoto Books, 2002.
- Kodak Corp, Processing Chemicals and Formulas for Black-and-White Photography-Kodak publication J-1, 7th edition, 1977
- Kodak Corp, Toning Black-and-White Materials-Kodak publication A-1671, 2001
- Kodak Corp, Use of KODAK Brown Toner to Extend the Life of Microfilm-Kodak publication G-23, 2006
- Wilhelm, Henry, and Brower, Carol, The Permanence and Care of Color Photographs: Traditional and Digital Color Prints, Color Negatives, Slides, and Motion Pictures, Preservation Publishing Company, 1993 Book available from publisher