A large class of toners is based on the replacement of the black silver image by a precipitate of a metal ferrocyanide. The chemistry behind these toners is quite involved. It is described in the second part of the article toner.pdf. This article presents a vanadium toner, based on easy to obtain and cheap chemicals, but the description in the article should suffice for understanding the other metal ferrocyanide toners.
Ferrocyanide toners frequently require an acidic solution for proper working. This is a disadvantage. My experience is that acidic liquids are harder to remove from a print, especially fiber-based prints and that there is strong staining. Stains can be prevented somewhat by keeping the toning time as short as possible and by keeping the print after rinsing in a moderately concentrated solution of sodium sulfate. By means of osmotic processes, the solution penetrates in the paper and while doing so, a lot of the stain is removed. After removal of the stain, the print can be rinsed further in order to remove the sodium sulfate.
The best known ferrocyanide toner is the iron blue toner. Many variations exist for this toner, all of them resulting in a blue image. The variations result in somewhat different hues, ranging from greenish blue to royal blue.
Iron toning requires the use of acidic media. When the liquid is not sufficiently acidic, then the toning action is very slow. Recipes for iron toning are based on
A few recipes for iron blue toners are given here and need no further explanation: http://www.jackspcs.com/toner.htm.
Because of the acidity, the toners have a strong tendency to stain the print. This is one of the most difficult things to control in an iron-blue toner. With the help of some sodium sulfate, as described above, it should be possible to keep the staining at an acceptable level.
Copper toning with strong saturated red/brown color
Copper toning can be used to obtain reddish/pink tones to reddish/brown tones. Standard copper toning is done in near neutral (slightly alkaline) solutions. Copper toning usually also is quite sensitive to staining. For most photographers, it is unknown that copper toning can also be done at a much more alkaline solution than used in the standard recipes for copper toning. Here, a recipe is presented, based on basic copper carbonate, as is available from pottery and ceramics suppliers. This makes the copper toner cheap, because of the low price of the basic copper carbonate. The toner also has very clear highlights. The copper toning can also be done with copper sulfate.
The toner cannot be stored. Slowly, a precipitate is deposited from the liquid and if this occurs, then the toner should not be used anymore. This is why just a few spatulas of chemicals should be used for a single shot solution, suitable for immediate toning of one or two prints. The exact amount is not critical at all. The only thing is that the solution should be clear
The toner has a strong smell of ammonia, so perform the toning in a well-ventilated area, or keep the tray covered by a transparent glass plate.
Toner waste should not be rinsed down the drain. Copper compounds are moderately toxic for the environment.
Vanadium toning and vanadium/silver toning
Vanadium toning, combined with sulfide toning
There is quite some mystique around vanadium toning on the internet. There are recipes around, but some of them look quite strange and their working cannot be understood from a chemical point of view. Sometimes, vanadium (III) chloride is mentioned, but this is a hard to obtain chemical, which is very sensitive to air-oxidation and which is oxidized anyway, when it is mixed with a ferricyanide solution. A lot of trickery is needed to keep such solutions clear and suitable for toning, while on the other hand the toned species (= the toned image) should not dissolve.
A better option is to use vanadium (IV) compounds. These do not suffer from aerial oxidation and when mixed with ferricyanide do not form ferrocyanide already in solution. This makes it easier to keep the solutions clear. I have some experience with vanadyl sulfate and this works fairly well for vanadium toning, but unfortunately, this compound is quite expensive at a price tag of approximately $1 per gram.
After some chemical reasoning and experimenting, I was able to create a nice vanadium toner based on the very cheap vanadium pentoxide, available at pottery and ceramics suppliers. This is available at less than $10 per 100 gram and the vanadium contents is much higher than in the strongly hydrated vanadyl sulfate. The toner is somewhat unusual from a chemical point of view, because it adds a new principle, besides the standard ferrocyanide toning principle. From a user's point of view, the toner also is somewhat unusual, because of the need to fix the image after toning.
The vanadium toner, presented here, is quite versatile: