Yellow filter layer of Kodachrome
Thanks to PE and his tip concerning how to bleach out the yellow filter layer in Kodachrome. Of course this layer is bleached and fixed out when Kodachrome is developed into color images, and presumably when Kodachrome is developed as a Black and White reversal. But the yellow layer remains when Kodachrome is developed as a Black and White negative.
Kodak published a document TECHNICAL DATA/COLOR FILMS in April 1999 which gave instructions on removing the yellow layer. It is called "Printing Color Films Developed as Black-and-White" and can be found on the internet at
This yellow layer is composed of colloidal silver; that is silver grains so small that they can go into colloidal suspension. At this small size they interact more directly with light rays, apparently absorbing blue light more efficiently and thus appear yellow.
The instructions that Kodak gives are simply to mix 1 oz, 28 grams, of anhydrous citric acid into one gallon of Kodak Rapid Fixer which has been mixed to the proper concentration to fix films. The Kodachrome photos are fixed in this for 7-14 minutes at 75-80 degrees f, after having been soaked in Photo-flow and rinsed. The ingredients for Rapid Fixer can be found at https://www2.itap.purdue.edu/msds/docs/9712.pdf
The main ingredients for the Rapid Fixer are: ammonium thiosulfate, sodium acetate, boric acid, sodium bisulfite, aluminum sulfate and sulfuric acid. The ph of this material must be low. The sulfuric acid probably has enough potential to oxidize the colloidal silver particles, especially since these are so small. I'm guessing that these will be oxidized first and then fixed out by the thiosulfate. The instructions do warn that this Rapid Fixer can reduce the metallic silver forming the image.
I'm guessing also that the citric acid provide citrate ions causing the yellow silver to form silver citrate. Perhaps this salt is less soluble than the acetate salts and will isolate the silver atoms and drive the reaction to dissolve out the colloidal silver. Would these citrate salts be less soluble?