Originally Posted by Mark Fisher
First of all, let me say that my understanding of photochemistry is nowhere near that of some of our members, so I can only guess at the reason. I suspect that the inclusion of thiosulfate in off-the-shelf KRST is a form of insurance against getting inconsistent toning results and staining in prints.
I think that using KRST in toning prints (silver gelatin primarily) involves two simultaneous processes similar to that of monobaths in film developing. In monobaths, the developer acts on the exposed silver, while the fixer simultaneously dissolves the unexposed silver. For monobaths to work properly, a balance must be struck between the speed of development vs. the speed of dissolution of the unexposed silver.
In a KRST solution, the selenium probably works on areas where silver compounds are present the most (shadow densities), and proportionately less on the midtones and highlights. Meanwhile the thiosulfate simultaneously works to remove any "free-floating" silver remaining in the print. If you recall, there is the standing rule that prior to any form of toning, the print should be thoroughly fixed, as well as thoroughly washed. I presume this is to remove all traces of non-image silver, and to remove any compounds formed by the fixing process. The presence of thiosulfate in the KRST solution is probably designed to ensure that the selenium gets to work cleanly.
As an aside: A long time ago while I was working for a media house in Asia, we were able to order selenium powder from a chemical supplier. We would mix a selenium solution and apply it locally to portions of B&W prints with watercolor brushes. The result was the most 3-D B&W prints you ever saw! Prints that were impossible to create by conventional darkroom methods. All this before the advent of unsharp masking or Photoshop.
But since selenium has been classified as toxic (as is many other photography-related chemical compounds), I doubt if it is still possible for individuals to purchase selenium powder to use directly as a toner.