I agree with Roger. Fixer is the devil reincarnate! When I was printing the 16x20s that I whined about on page 1 of this post my tongs started leaving fingerprints. I have a set of stainless steel tongs, one for each chemical, careful not to cross contaminate. But the demon fixer still managed to possess my developer tongs. To finish the series I resorted to going upstairs to the bathroom and washing my hands between each print, drying my hands on a clean towel, making the print, using my clean hands to move the prints from solution to solution, and repeating the process. This was with Ilford fiber base warm tone semi matte paper. I think warm tone paper is more susceptible to contamination than, for example, MGIV. After a thorough decon of my tongs they are safe to use again.