Not only does sepia toning protect the longevity of a darkroom print, it also helps to control tones and add a warm vintage effect. They process is extremely simple. Unlike other toners, sepia is a two bath toner.
The first bath bleaches the print until almost all the silver is gone, starting in the highlights then moving to the shadows. The effect of the tone is controlled in the bleaching process. The longer you bleach, the more prominent the effect.
After washing off the bleach, the print is places in the toner bath. The amount of time in here does not make a difference, as the print should be toned to completion. The process works by turning the silver halide in the print to silver sulfide, which is much more stable.
You can see in these photos that the toner brings out a lot of detail in the shadows. When you are printing to tone, you should print about a half contrast grade higher than intended, as it will be taken away with toning.