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P.SO I'l.XEMATOGRAPHIC ANNUAL
The method of compounding this bath is very important. Each of the solid chemicals should be dissolved separately in a small quantity of warm water, the solutions allowed to cool, filtered into the tank strictly in the order given, and the whole diluted to the required volume. The bath should be a pale yellow color, and perfectly clear.
Time of Toning: Two to ten minutes at 70° F. (21° C.) The color of the image varies from a light bluish-grey for short time toning (about 3 minutes) to a deep blue for maximum immersion.
Time of Washing: Ten to fifteen minutes, until the highlights are clear. A very slight permanent yellow coloration of the clear gelatin usually occurs, but should be only just visible. It has no effect on projection. Washing should not be unduly prolonged.
Life of Bath: If acid is renewed to the extent of the original amount after toning each 5,000 feet, the bath is capable of toning 15,000 feet per 50 gallons of solution.
A very pleasing tone is obtained by first toning in the Uranium bath for about 3 minutes and then in the iron bath for about 2 minutes.
Dye Toning
As the number of suitable colored metallic compounds is rather limited, other methods of toning have also been evolved. Certain inorganic compounds have been found to have the peculiar property that when immersed in a solution of a basic dye the dye comes out of the solution and attaches itself to the compound. The dye is then said to be mordanted, and the inorganic compound is called a mordant. Silver ferrocyanide is a typical mordant. Therefore if a silver image is converted into a silver ferrocyanide image, and then immersed in a solution of a basic dye, a mordanted dye image is produced.
The Mordanting Bath
Uranyl (uranium) Nitrate 10^ ounces 320 grams
Oxalic Acid 5 !4 ounces 160 grams
Potassium Ferricyanide _. 5 \4 ounces 160 grams
Water to 50 gallons 200 liters
The uranyl nitrate should be of good quality, and should not contain an excess of free nitric acid. First dissolve the chemicals separately in small quantity of water, then add the oxalic acid solution to the uranyl nitrate solution, and finally add the potassium ferrocyanide solution. After mixing, the bath should be light yellow and perfectly clear. Expose the solution to light as little as possible, as light causes the precipitation of a brown sludge of uranyl ferrocyanide.
Time of Mordanting: Immerse the film until a very slight chocolate colored tone is obtained. When the bath is new this will take from \y2 to 2 minutes, but as the bath ages, this time will be prolonged. If a concentrated stock solution of the mordanting bath is kept on hand a little of it may be added to revive the bath.
After mordanting 10,000 feet of film per 50 gallons, the bath should be thrown away.