Latent Image Bleaching
Latent image bleaching has the opposite effect from bleaching a developed-out silver image—instead of increasing contrast, it reduces it. This is fortuitous since it gives the photographer an additional means of controlling gradation.
Standard Bleaching Technique: After Development
When bleaching a developed-out silver image, the bleach typically acts first on areas containing the least amount of silver; or in any case the effect of the bleach is much more obvious in the areas of low silver, which are the high values (whites) of a print and the low values (clear, or less dense, areas) of a negative.
Latent Image Technique: Before Development
The original technique, known as the Sterry Method, was used to enable very high-contrast negatives to be printed. The method involved soaking the print in a potassium dichromate bleach immediately after exposure and just prior to development. David Kachel’s technique, as outlined in the articles listed at the bottom of the page, uses a highly dilute solution of potassium ferricyanide for the same purpose, and extends the practice to negatives as well.
Potassium Ferricyanide Stock Solution
To make a 10% solution of potassium ferricyanide, add 100 grams of potassium ferricyanide to 800 milliliters of water at room temperature, stir until dissolved, then add water to make 1 liter. For negative bleaching, distilled water may be preferable.
Some negatives require the addition of potassium bromide to the solution to reduce fog. Kachel recommends 1 part potassium bromide to 3 parts potassium ferricyanide. This would require the addition of 33.3 grams of potassium bromide to the above stock solution.
You may wish to mix both solutions, the first (without bromide) for use on prints and the second (with bromide) for use on negatives. Both chemicals are relatively cheap, and the solutions will keep for a year or more in brown glass bottles.
Latent Image Bleaching of Negatives
Potassium ferricyanide typically acts as a contrastwise bleach with latent images in negatives and almost always requires the addition of potassium bromide for elimination of fog. The effect is to keep the high values of the negative from being overdeveloped, hence unprintable, by reducing them before development. This is appropriate for very high -contrast subjects, to reduce the high values without affecting shadow detail. Experimentation is necessary to determine the dilution required for a particular reduction.
1. Presoak the exposed negative in water (preferably with a drop or two of Edwal LFN low-foam wetting agent added).
2. Start with a .1% dilution for 5 minutes with continuous but gentle agitation. (To make a .1% dilution, mix 10ml of the 10% solution with 990ml of distilled water.)
3. Rinse in water.
4. Develop for the normal developing time.
5. Adjust the dilution of the bleach up or down as necessary. For greater reducing action, try a .2% dilution; for less reducing action, try a .05% dilution.
6. Adjustment of film speed may also be necessary.
Potassium ferricyanide also acts as a contrastwise bleach for latent images on printing papers. Our initial impression is that the greater the reducing action required, the more likely one is to require the addition of bromide. The effect is to reduce the contrast of whatever paper is being used by cutting the blacks before they develop. This is appropriate when one does not have the correct contrast grade of paper or the correct variable-contrast filter, or when there is a very high-contrast negative that cannot be printed on a grade 1 paper, when one desires a between-grades contrast, or if one wishes to change the gradation of the resulting print.
2. Place the exposed print in the chosen solution of potassium ferricyanide for one minute. The time may be increased as necessary. Agitate continuously. Do not re-use the solution for a second print--mix it fresh for each print (at these extreme dilutions, why take a chance on the solution losing its potency?).
3. Move the print directly to the developer and develop normally. The solutions are so dilute that a rinse is not necessary.
4. Paper speed will be reduced by at least a full stop.
Experimentation is necessary to determine the correct dilution for each paper. Bromide papers in general seem to require more bleaching than chloride papers (bromide papers generally have larger grains), but there are notable exceptions. Most paper emulsions today are chlorobromide mixtures.
Everything we have been able to gather regarding latent image bleaching comes from two articles by David Kachel in Darkroom and Creative Camera Techniques magazine: “Zone System Contraction, Part III: Selective Latent Image Manipulation,” Vol. 11, No. 5, September/October, 1990 and “Variable Contrast Control From Graded B&W Papers,” Vol. 13, No. 3, May/June, 1992.