When toning blue with iron-salt, the metallic silver is transformed into a dye called berlin blue. Talking in terms of print permanence, this dye is sometimes more sometimes less stable than silver. The reaction to light and environmental gasses depends on the method that you use. The well known toner formulae differ not only in the intensity of the blue and blue-green they produce, but also in the colour fastness when exposed to strong light. Some of the known formulations have unacceptable disadvantages. The colours are fading and the dye diffuses into adjacent (white) areas. This is especially evident on the unexposed image border.
Basically, we have two methods to our disposal, direct and indirect toning. Toning indirectly, you have to bleach first and tone subsequently. Toning directly, bleach and toner are mixed in one bath. Both methods have advantages and disadvantages.
When toning directly, you can observe the toning process and stop it when the desired intensity is reached. Without having a lot of experience, the hue itself cannot yet be judged as the white areas are still in a yellow cast, which is produced by the ferricyanide (red prussiate of potash, potassium ferrocyanide). Only when this yellow cast is removed in after-treatment, does the picture show its final hue.
When toning indirectly, the yellow colour that comes from bleaching is completely removed by rinsing the print thoroughly. The image whites should be absolutely clear after toning. Some formulations, however, produce a yellow fog, especially when the acidity of the toner is not well adjusted. The bleached silver image has to be transformed completely into a colour image. If the toning process is stopped too early, because the desired image tone is already reached, the picture will darken when exposed to light. To uphold the shadows in a diluted bleach bath, does not always lead to the result that you hoped for. Sometimes solarisation effects occur on the transitions and the gloss of the surface can appear non-uniform. If the print is bleached completely it shows an intensive hue. Depending on the toner formulation that you use, density and contrast range can decrease significantly. In my opinion, this procedure is only suitable for papers with a high silver content. Especially warmtone papers with a high content of chloride-silver appear too light after toning. For that reason they recommend to overexpose the prints. If you do so to the required degree you are likely to get undifferentiated, obscure shadows. This will not be improved by toning. Therefore, I do not believe in such recommendations. Pre-toning the shadows in selenium can prevent a loss of contrast. However, when toning directly this results in a split tone. After a couple of hours in daylight this effect abates or even vanishes completely.
When toning directly, you notice a different shortcoming. This concerns self made recipes as well as ready made formulations of any brand. Apart from the desired change in colour - thus the transformation of silver to prussian-blue - a blue coloured residue of oxidation falls out and deposits on both sides of the print as well as the inner walls of the dishes. On RC paper this dye can be removed mechanically. On fibre based paper it accumulates due to its open surface. One manufacturer (Tetenal) explicitly indicates the problem in the instructions and recommends the use of its toner only for RC paper. Other manufacturers do without such hints and leave their users wondering. The first, the second and maybe also the fourth print come out clean, but after that, the dye that developed in the working solution of the toner contaminates the gelatine. This causes blue stains or a continuous dyeing of all white areas in the photo or of the paper base. If you only have the problem of bluish staining on the image borders, you can remove this by treating the image partially with a piece of cotton wool soaked in a mild alkali. If the contamination is finely spread all over the image surface, it is also possible to remove it, but will result in a deterioration of the hue and its intensity. When toning directly, the prints should only be handled on its edges. Any contact with your fingers before and during toning, especially with higher pressure, can lead to (toned) fingerprints.
Since iron toning was invented before RC paper was introduced, we have to raise the question what people did about this problem back then, when there was only (less suitable?) fibre based paper available. There is only one explanation: In contrast to today, knowledge about the effects within this process was there. They used formulations developed with their own acquired know-how or they used compiled and published information containing the experience of other users. Who puts in this kind of effort today, who has access to the publications of that time? Today, we rely on the blessings of the world wide web and are doomed by the fact that we find third hand information, which is shortened or even copied wrongly. Nonsense is still nonsense, even if it was passed on by many people. The know-it-all-internet type of person outshines the fertility of rabbits and for the absence of any authority like that of a moderating teacher, the blabber is regarded as an authority just for his inflationary number of postings. Unfortunately, the reader is inclined to believe nonsense that has been copied many times and is likely to look for an explanation for failures in his own inadequacy. This might lead to him throwing in the towel and walking off to Photoshop. If you work digitally, you don't make that choice and there is even little to object about. But what is it that makes an analogue-worker, who tries to get ideal negatives for processing, present his work to the public with the notion: negative-scan toned in Photoshop? Note here, that this is not the question, if we are talking about a real life print on just any given carrier. A photograph has to be touchable, regardless on which carrier no matter if with pixel or grain.
New literature to this topic is available.
Tim Rudman, The Photographer's Toning
Book Tony Worobiec and Ray Spence, Monochrome
Both books on this topic are excellently illustrated and offer a wide range of suggestions for your own work. Something that is often held back concerning example pictures is the information which paper was used. However, this can be as important as the brand of the toner or its recipe. Different papers can lead to very different results in identical toner solutions. I am not only talking about the image tone, but also about the contrast and effects of the toner on the image surface. The user asks himself, why his results are different from his expectations. Often he searches for reasons in his own work flow instead of in the material he used.
If you want to tone silver prints in iron-salt, you have to develop them absolutely free of fogging. Any fogging however fine and invisible to the eye prior to toning will be apparent after toning. You have to make sure that your darkroom light is safe for the paper and also that you use a developer that creates fog-free photographs. Usually conventional developers are suitable. You absolutely have to use an acidic stop bath before fixing, even if you use an acidic fixer. The print has to be well agitated in both stop bath and fixer. If you only use one fixing bath, the fixer ought to be freshly prepared. You are on the safe side if you fix in two baths. All silver-salt has to be completely removed from the emulsion. The toner would transform any residue into prussian- blue. It is also important to wash out all thiosulphate. Any residue of it in combination with either the bleach or the ferricyanide of the toner would act as Farmers Reducer. This would lead to eroded highlights. If the whites on the test strip are coated blue, a couple of causes are possible. Even if you take utmost care when processing, a grey fog can occur due to the material used. Some papers - the ones with a high silver content - can have a measurable base-fog after development. If the density of this fog is above 0.02 logD you have to expect it to take on colour. In such cases I recommend to remove this fog in highly diluted bleach (ferricyanide) with subsequent fixing for a short duration (before any toning!). To use highly diluted Farmers Reducer for that purpose is only an option, if you want to treat only a couple of prints in it. If you dilute the reducer substance to the degree that it is suitable for this process, it would not be any stable.
One of the oldest writings about blue toning is the one by Vogel. You prepare two separate solutions using distilled water, one with 1% ammonia ferric citrate and one with 1% potassium ferricyanide. To use it you put 200ml of each solution together and add 40ml of a 10% solution of either acetic acid or citric acid. Like all direct toning substances, this toner solution has limited durability.
All iron toners require a specific amount of acid. Without acid nothing works. If the content of acid is increased above the required amount, the effect of toning increases as well. However, this also produces more of the undesired turnbull blue that does not dye the silver but the paper fabric and the gelatine. To increase the acidity can be a necessity if the toner is diluted more than recommended in the instructions. As a general rule for adding acidity, it can be assumed that you can increase the initial amount of acid proportionally to the increase in volume of the working solution.
There are different opinions about the durability of blue toned photographs. From my own research into the subject, I am inclined to assume that they compare apples and oranges. It is simply wrong to conclude that blue toning is of inferior quality in terms of print permanence, just because of improper treatment by the user or because of suboptimal methods and the resulting faults. You have to note though, that hue and density can change due to strong exposure to light. This can happen, but it does not have to happen. For example, in some formulations, indirectly toned prints appear to be too light when dry, but darken down through exposure to light. This phenomenon is well known. It is better known in any case, than the fact that this phenomenon is unknown for other methods of treatment.