D'oh!!!! Thanks for pointing out my error. I now see that I made a typo in my first post, then it got perpetuated. I use POTASSIUM cyanide (KCN), NOT hydrogen cyanide (HCN). I should have spelled it out rather than fat-fingering the abbreviation. So, Donald, perhaps that's what you meant by being confused. I'm not confused, but I am a crappy typist.

In any case, I suppose I should emphasize that no-one should delve into processes like wet plate, daguerrotype or anything involving dichromates without understanding the hazards and proper handling of these materials. I apologize for any confusion I may have induced.