Found it. See

A quote from the article: "Recent research at the Image Permanence Institute has shown that sulfiding treatments give excellent protection for microfilm against red spots, and that gold and selenium treatments only are effective (in the absence of sulfiding action) to the degree chat the silver image is converted to gold or to silver selenide. Planned new research may result in a redefinition of "archival processing" for all types of silver images to include sulfiding treatments to resist oxidation."