Tig welding with C25 instead of Argon?

Has anybody here ever tried (probably by mistake) TIG welding Aluminum with C25 instead of Argon? If so, what did the results look like?
The reason that I am asking is that I just swapped a gas cylinder and my welds went from OK to awful looking. I am welding 6061 aluminum and the weld pool gets a big hollow space in the center and the hollow space has some light brown crud in it. The rest of the weld zone is kind of gray. The crud brushes off with a wire brush.
I suspect that the vendor filled a cylinder labeled argon with C25. As a test, I welded the same pieces of aluminum on the other side with gas from a different cylinder and it looks fine.
What really stinks on this (beside the trashed work) is that I got the cylinder back in July and I am going to be really surprised if they do anything but charge me for another cylinder.
Thanks, BobH
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I did some web searching, and there were some photos that someone took from trying to TIG weld steel with C25, and there was a LOT more crud on everything than what I saw. This probably isn't C25. I guess it doesn't matter what the contamination is.
On 11/4/2015 7:50 PM, BobH wrote:

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On Thu, 5 Nov 2015 19:08:11 -0700, BobH

A few years ago I was having a hell of a time getting good aluminum welds. Kinda like air was getting sucked into the argon hose. I had the cylinder replaced and was told by the welding supply place that sometimes people will completely empty a gas cylinder and leave the valve open and this lets water condense in the cylinder. They are supposed to check that but sometimes I guess they don't. Sure enough with the new cylinder installed my welds came out looking the way they should. Eric
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On 11/6/2015 10:27 AM, snipped-for-privacy@whidbey.com wrote:

I just replaced the gas cylinder and all seems better. I did several test welds on the same material that I had the problem with, and they are as good as my welds ever are.
I bought the cylinder in July, and they still took care of it for me, I am really pleased with those guys at Airgas in Tucson. I would have been happier if this hadn't happened at all, but their willingness to fix it makes a big difference to me.
BobH
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On Fri, 6 Nov 2015 22:10:10 -0700, BobH

Well, when you have a nearly full cylinder with bad gas in it there can really only be one reason-that when purchased it had bad gas in it. It is unlikely that a customer would have the ability to fill a cylinder with bad gas. Eric
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