What is the Toning?
Toning changes the black image into a color, by either coating the silver or converting it into another, colored chemical, or dye. The paper base remains unchanged. Some toned images (sepia, red) are at least as permanent as the original silver. Others (blue, green) are not.
You may want to tone your print to subtly improve tonal richness and increase its permanence, using selenium (this chemical is carcinogenic and must be used in a well-ventilated area) or perhaps gold. Or you might sepia-tone, either to create an antique-looking image, or as a preliminary to tinting.
Two of the most common colors, sepia and blue, are sold as prepared chemicals, or can be made up. You can also buy ready-to-use comprehensive kits such as ‘Fotospeed Pallette Toner’ or a selection of package chemicals from ‘Speedibrews’, offering a whole range of colors. By mixing these chemicals in different proportions, different toning colors are formed. Stronger toning colors should be used with restraint unless you need a gaudy effect.
Some toners require two stages, e.g., sepia. First, you bleach the area you want to tone in a ferricyanide solution (without fixer), and then you redevelop this bleached image as a colored chemical image in the toner. Redevelopment can take place in normal room lighting because only halides representing the image are present; so, fogging is impossible.
Others are single solution toners and gradually displace or form an amalgam with the black silver, starting with palest tones first. Yet others (typically those in kits) use dye-coupled development, in which the existing image is first bleached, then redeveloped in a developer plus a chosen color coupler, and finally bleached again to remove the black silver simultaneously reformed during the redeveloping stage. This leaves an image in dye alone. Notice the similarity with color film processing.
Whichever toning formula or kit you use, you can choose to change the whole image to a colored form or, perhaps by means of a paint-on resist, tone just selected areas only. Unaffected parts which remain as black silver can next be toned a farther color. Another way of working is to duotone, meaning that shadows and dark tone values in your picture remain black while mid-tones and paler parts take on color.
The way you achieve this two-tone effect will depend on the formula and color. With sepia tones, for example, you dilute the bleach bath, which then allows you time to remove your print before silver from the darkest shadow areas has been affected.
Then in the toner, only paler, bleached tones become fully sepia. In single-bath toner you treat the print just long enough to start affecting the lighter tones.