For starters, testing wouldn't provide the least bit of evidence if the plastic was responsible, or not. It's no different than looking at the horizon and concluding that there must be a creator. Looking at the horizon proves there is a horizon, nothing more.
Regardless of why it's there, it's obviously not a good idea to use plastic, particularly if high values are involved. I've been through that process with cemented silver----and recovered the film by incinerating the plastic. Not a good idea in the scheme of things when the problem can be avoided.
My point was directed to gold----likely misdirected in this case. Still, a film such as you described is almost guaranteed to be metallic in nature, and conductive. I can think of no other reason for a metallic appearing film to be created under like circumstances. Why it deposited on plastic I can't say, but that's the very reason why I have never used plastic in any operation that was critical. That, and the appearance of a bush-league operation. I don't mean to sound like I'm trying to be rude----but I had an image to uphold, so my customers had confidence in my work. They were free to inspect my facility--so I tried to present a professional image. I don't think working out of buckets projects that image very well. YMMV.
On another board where I spend my time, one of the contributors is a retired college dean. His sig line is "You are what you write".
I firmly believe that to be the case, and also believe that the work you do represents you. I have a hard enough time passing myself off as a human being without going out of my way to prove I'm not. I highly recommend proper labware when dealing with values----as well as tried and proven methods, assuming your goal is extracting and refining precious metals for gain (I see a difference if your objective is research)----but then that's me------and I don't profess to speak for anyone but myself.
Harold
