Adapted from an entry in the Scientific American Cyclopedia of Formulas, 1932
Solution No. 1 : Dissolve 50 grams of gum Arabic powder in 100 mLs of water. Gum Arabic does not dissolve quickly, but it will dissolve, so have the gum sit in the water over night. The next day it should be completely dissolved. If you already have a commercially prepared gum Arabic solution intended for gum printing, I would suggest you start with this and see how it works.
Solution No.2 : Add 6 mLs of glacial acetic acid to 10 grams of Solution No.1. This will be solution No. 2. Some of the gum Arabic will crystallize out when the acetic acid is added, but stirring will bring most of it back into solution.
Solution No.3 : Dissolve 2.2 grams of silver nitrate in 10 mLs of distilled water. Add this with stirring to Solution No.2, and this will be solution No.3. Stirring this solution will dissolve any undissolved gum Arabic.
Coating the paper : Under a red safelight I use a foam brush to apply the gum-silver to the paper . I let the paper dry in the dark.
The negative : Like salt printing, you need a very high contrast negative. I tried to increase the contrast by adding 1 drop of 5% potassium dichromate solution to the gum-silver solution prior to coating the paper. The increase in contrast was very slight, and approximately 50% more exposure was needed. One positive result was that the image tone shifted from an orange/red color without the dichromate to a nice warm sepia color with the dichromate. This result might be due to the addition of the dichromate, and/or to the paper used. The article says that the image color depends upon the paper used.
Printing : The image appears during exposure. I expose until the image is a bit more darker than what I would want in the finished print.
Processing : After exposure wash the paper with tap water. The unexposed silver nitrate will turn the wash water a milky white. Continue washing with changes of water until the water is clear. Then fix with a 2% sodium thiosulfate solution for about 10 minutes.. Wash the print again to remove the fixer. The print will lighten slightly during the washing and fixing.
Toning : If you wish to tone with gold and/or platinum you should tone before fixing. I did not try toning with gold or platinum, but for the sake of completeness, and to satisfy my curiosity, I did try selenium toning. A toning solution containing 5 mLs of KRST added to 1 liter of water ruined the print in 2 minutes. The print was noticeably bleached, and the color was changed to an ugly slate gray.
For my traditional black and white printing I use Kodak’s T-55 selenium toner, which you have to prepare yourself, and which does not contain any thiosulfates, which I feel are responsible for the rapid bleaching of salt prints with KRST. I added 5 mLs of this to a liter of water and immersed a print in this. After 2 minutes the tone of the color shifted slightly from the warm sepia color to a slightly more yellow sepia color, and after 5 minutes the print started to bleach. I doubt if the 2 minutes in the very dilute T-55 toner added any protection to the silver in the print.