To obtain beautiful rich brown, sepia-like tones, the print is first bleached using a ferricyanide-bromide bath then redeveloped in a sulfide bath. In this process, the silver metal in the print is first converted to silver bromide than to brown silver sulfide. The end result can give the impression of print made in the late 1800's. The thiourea toner is actually a sulfide toner In alkaline solution, thiourea (thiocarbamide) reacts to yield sulfide ions, the active ingredient in the toning bath. The advantage of this process is that the foul smelling sodium sulfide bath is avoided. An additional advantage of thiourea toner is that it is odorless. The chemicals are used to prepare 1 liter of working bleach solution that can be reused. A total a little less than 2 liters of toning bath can be mixed with the specified chemicals. Thiourea Print Toner is a similar thiourea based toner.
CHEMICAL SAFETY
Thiourea is not toxic nor corrosive but,
unfortunately, it is a potential carcinogen. Use rubber gloves when mixing or
handling this compound or its solutions. Wash the work area, trays, and all
mixing utensils with water followed by soap and water. Should a solution
containing thiourea be spilled on the skin, wash immediately with water
followed by soap and water. Dispose of residual solid or spent solutions
containing thiourea down a drain with copious amounts of water.
Potassium Ferricyanide. In spite of the
fact that this compound contains cyanide, it is not particularly toxic. The
reason is that the cyanide groups are bound to the iron atom and are not free
to act as a poison. The cyanide groups can be released as hydrogen cyanide
gas if the potassium ferricyanide is placed
in a strong acid solution, however the bleach bath in this toner does not
call for the use of acid. To dispose of excess
potassium ferricyanide (solid or in solution), wash the material down the
drain with excess water.
MIXING THE SOLUTIONS
Solution A (The Bleach Working Solution)
| Chemical | Amount | Units |
|---|---|---|
| Water (125°F/52°C) | 750 | ml |
| Potassium Ferricynide | 50 | g |
| Potassium Bromide | 50 | g |
| Cold water to make | 1000 | ml |
You will need a dark brown bottle with a capacity of at least 1 liter to hold the bleach solution. You will find it convenient to first mix the solution in a temporary mixing bowl then transfer the solution to its storage container. Place the warm water in the temporary container (or storage container) and add the potassium ferricyanide. Stir the solution until the solid dissolves. Next, add the potassium bromide and again stir the solution to dissolve the solid. After the potassium bromide is in solution, add the sodium carbonate. As before, stir the solution to dissolve the solid. Finally add cold water to bring the final volume of the solution up to 1000 ml. Stir the final solution to ensure it is homogeneous. It is best to store this working bleach solution in the dark. Potassium ferricyanide solutions are somewhat light sensitive. Should the stock solution turn blue (from photo-formed ferrous ions reacting with the ferricyanide to form Prussian blue) the bleach has been ruined and must be discarded. Stock Solution B (Thiourea for the Toning Bath) You will need a mixing bowl and a storage container each with a capacity of at least 100 ml a spoon and a graduated cylinder to mix this solution.
Solution B (The Redevolpment Solution)
| Chemical | Amount | Units |
|---|---|---|
| Thiourea | 2 | g |
| Sodium Carbonate (mono) | 100 | g |
| Cold water to make | 1000 | ml |
Place the water in the mixing bowl and add the thiourea. Stir the solution until the solid goes into solution then transfer it to the storage container. Be sure to wash all utensils after use.
USING THE TONER
Correctly exposed and fully developed prints work best with this toner.
Bleaching the Print
Stock Solution A is used at room temperature without dilution as the
bleach. The print to be bleached must be thoroughly washed and wet before it is
immersed in the bleach solution. If residual hypo is left on the print, the
hypo combined with the ferricyanide in the bleach will cause permanent loss of
the image. Pour just enough Stock Solution A to cover a print into a plastic
or glass tray. Immerse the print into the bleach solution and gently rock the
tray. The black image will be converted to a light brown or straw colored
image in about one minute. Wash the bleached print in running water
(68°F/20°C) for 15-20 minutes. During the wash, the yellow ferricyanide color
on the print will be lost. The bleach solution can be reused therefore,
return it to its container.
REDEVELOPMENT
Place the working solution in a plastic tray and immerse the well-washed,
bleached print in it. Work at room temperature. Redevelopment of the image to
a rich sepia tone will occur in 2 to 4 minutes. After redevelopment, wash the
print in running water for about 30 minutes.
After Treatment
The thiourea toning bath will soften the emulsion of the print. Therefore,
it is advisable to harden the print before it is dried. The chemicals for a
hardening bath are not contained in this kit. Should you wish to prepare a
hardening bath, use the following formula:
| Chemical | Amount | Units |
|---|---|---|
| Water (125°F/52°C) | 750 | ml |
| Sodium sulfite | 15 | g* |
| Acetic acid (28%) | 40 | ml |
| Potassium alum | 15 | g |
| Cold water to make | 1000 | ml |
REFERENCE
Thiourea Toner, Kit No. 06-0045,
Photographers' Formulary (800-922-5255).
Photo-Lab Index, Supplement No. 193, p.123