What is meant by toning ?
Not to be confused with toning through the use colour development where the silver is replaced with a dye, or the silver is stained, Toning is a chemical reaction that changes the silver of a black and white photographic image into another compound. In some cases, the metallic silver of the print is changed into an another form of silver, like silver sulphide, in the case of sepia toner. In other methods of toning the silver becomes an in insoluble and colourless silver salt while at the same time another metallic salt is deposited on top of the silver, and it is this second metallic salt that affects the colour change. 

While the toning effect can be controlled by time, temperature, dilution etc, there are many variables that affect the final outcome, and some of these like the type of paper, type and concentration of the developer, temperature, length of time in the developer, are preconditioned before the toning process begins. As any of these can influence the exact colour of the final image, it is the chemical composition of the toner that has the main affect. As a means of repeating some toning affects some photographers keep accurate records of all these factors.

Although a particular toner can be chosen because of the colour it produces it is worth noting that this second metallic salt relates to a range of compounds that are either higher or lower on the nuclear scale. This means that while some toners increase the permanence of the image, others will decrease the permanence of the original silver image. For instance Gold is higher on the nuclear scale than silver while Ferric (iron) toners are lower on than silver on the scale and are less stable.

The nature of the original silver salt is very important: it represents the mordant and in some processes it is inclined over time to turn brown in daylight. In these processes to keep the colour of the toned print, the silver salt must be removed by quickly refixing the print in a very diluted fixer bath.