I am having a hell of a time with my argon tubes lately. I got pure mercury and I changed out my electrodes and I still have no idea what is going on. Ugh. 

Mercury staining could be happening for a variety of reasons. I was having a problem with some units staining last year. In my case, I think it was the electrodes I was using (Brillite 10mm) and that I was re-using my mercury traps. I've since switched to Masonlite (Voltarc) electrodes and using a fresh mercury trap for each unit, and the staining problem has gone away. Incidentally, I used to love Brillite electrodes but I think something has changed in their production over the last year or so because they don't process like they used to. I hope you get it figured out. I know how frustrating that can be!

Staining can be caused by so many different things that it can be hard to narrow down.  Ultimately, upon close inspection, you can sometimes see faint nuances between different stain types which will help pinpoint a causation.  The differences can be so close, however, that online posting of photos is often not adequate due to the differing color renditioning of the various cameras, monitors, etc involved.  Voltarc published a technical document several years ago that did a pretty good job of describing and outlining the types of stains and their causes.  I believe I put a copy of this in the library, if not, please let me know and I will upload it.