Sepia toning produces a reddish-brown tint on black-and-white photographs.
The term "sepia tone" is used in photography to describe the effect of a black and white print-toning process by means of a substance derived from the dried ink sack of "Sepia officinalis," the common cuttlefish. The term "sepia" has come to apply more generally to the warm reddish-brown color of the sepia pigment.
History
According to Duotones, DePaul University, "Sepia was commonly used as writing ink in classical times...In the last quarter of the 18th century, Professor Jacob Seydelmann of Dresden developed a process to extract and produce a more concentrated form for use in watercolors and oil paints." Sepia ink had been used in the drawings of great artists like Leonardo Da Vinci and Rembrandt. After Seydelmann's invention, the popular sepia tone could also be found in paintings.
Photographic Toning
On black-and-white photographic prints, toning is used for aesthetic effect and for preservation. Silver conversion toners chemically alter the silver bases in the photographic emulsion, changing the image's color and its stability. While some toning processes will make the prints last longer, others can decrease their chemical stability.
Sepia Toner
Sepia is one of the toners that convert the silver in photographic emulsions to silver sulfide, which lasts twice as long as silver and produces archival quality prints. The reddish-brown tint produced by this process, originally a side effect, has become so popular that it has been reproduced by other means than sepia application.
Simple Sepia Toning
Ed Scott describes the process of sepia toning on photo.net. Essentially, the conventional process for sepia toning consists of thoroughly washing the black-and-white print, soaking it in bleach to covert the silver image, rinsing off the bleach, toning the print in sepia solution and, finally, washing it again and drying it.
Split Toning
This process requires a skillful finisher to achieve a more complex, layered sepia tone. Rather than bleaching the photograph completely, as in simple sepia toning, the image is only partially bleached. Only the bleached parts will display the reddish-brown sepia tint while the shadows will remain black. This process can be repeated many times to achieve an even greater degree of subtlety.
Digital Sepia Toning
Modern photo imaging software as well as some digital cameras are able to achieve sepia effects without the use of pigments or chemicals. Adobe Photoshop, for example, offers two options: the generic "sepia tone" setting for a quick and easy touch up as well as a "duotone" setting for the advanced user, which simulates split toning.
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