To prepare the toning baths, dissolve the ounce of uranium nitrate in 10 ounces of water. The water should be distilled if this is easily obtainable, and the solution should be kept in an orange-glass bottle or an ordinary bottle protected from light by a non-actinic paper wrapping. Dissolve the ounce of potassium ferricyanide in 10 ounces of water. Keep this also in an orange-glass bottle, well corked. There are many cautions about this particular salt, and it has been said that it will not keep in solution. In my practice I find no difficulty whatever in the use of a solution six months old, despite the difficulties mentioned in the text-books.
To tone the bromide prints, first note that the prints should have been developed and fixed and washed just as usual. It is necessary that prints to be toned shall contain no trace of hypo. To secure this, the prints should be specially prepared for toning by being again thoroughly washed, as any hypo remaining in the print will cause spots and streakiness. With care at this stage the toning will give clean and bright prints, which should be as permanent as the original bromide print.
I cannot give the reason why, but, as a general rule, bromide prints tone better if the print has been dried after washing and rewet just before toning. There may be a chemical reason for this, but I am inclined to think that it is a physical one, viz., that the emulsion is softer after its first washing than after having been dried and wet, so that it allows toning solution to get into the film more quickly. This naturally results in more rapid toning, and quick toning does not yield as good prints as a slower and more gradual building up of the color image.
Having the print ready for toning as here outlined, take 1 dram of the uranium solution, add 1/2 dram of acetic acid and then 5 ounces of water. In a separate graduate put 1 dram of ferricyanide solution and 5 ounces of water. Just before toning, pour these two solutions together into the third graduate and use immediately. To proceed, lay the rewetted print face up in a clean tray and flow the freshly made toning bath (the two solutions combined) over the print. The print and tray must be kept in motion by gentle rocking during the toning operation. The toning solution tends to throw a red precipitate as it works. This precipitate should not be permitted to settle on the face of the print. Some workers tone their prints face down, but I do not advocate this, as it is important to take the print from the toning bath at just the right moment, and, as the toning process is short (six or seven minutes is usually sufficient even for the deepest red) you need to watch the print all the time. In the toning operation note that a constant quiet motion of the tray, to keep the solution moving over the print, is essential to success.