The only people that never make mistakes are those that never do anything.

Based on the mistakes I make, I must be a very busy fellow!

When you catch up to my mistake count, let me know. That way I'll know you're busy too!

Best regards - Al

Hello Lawrence, it can happen pretty easily that's for sure. I ran into a similar type thing, however it was a mistake that a student had made. He came up and told me that he just couldn't figure out what was going on with his "welder", he claimed that one minute it was working fine and then the next minute it was welding like pooh. Made a trip to the booth, sat down and ran a bead with the FCAW gas shielded set-up, was like the arc was turbo charged and wouldn't wet out to the toes. Had slag islands and bare spots all over the bead, I thought for a moment and then asked that fateful question: what exactly have you done differently and have you changed anything? Of course, the response was, well I did change the gas, is that something major? I probably don't have to go too much further with this, the bottle was 100% argon, doesn't work too well with the flux-cored wire. Did have a few issues with our argon supply though, we're still using cylinders. In recent months we had 2 bottles of argon that were contaminated and as I believe you eluded to, aluminum will certainly bring out the nasties in poor shielding gas. It was quite apparent after working down the punchlist that we had some contamination going on, changed out the bottles and wah-lah problem solved.
     So you had a little chuckle this morning and likely your students did too(I'm sure they won't soon let you forget it), it's still a great learning moment and it'll provide that special bonding that us instructors so need with our students. Enjoyed the story Lawrence, best regards, Allan