Read the Essay "Dangers in the Dark" on the late Bill Jay's website. There apparently is no reason to use potassium cyanide to fix wet-plate images. Plain old sodium thiosulfate will work, and was available to photographers in the 19th century. Some "authority" said cyanide was superior and everyone jumped on the bandwagon to the detriment of many. A very interesting read with Jay's sense of humor included.
I will note that I do not play with wet-plate chemicals and processes and see no reason to do so. Experienced users of these processes may tell me I'm full of crap, but that's okay. My kids tell me that all the time.