Permanence of an ambrotype made in plexiglass

I'm learning how to made ambrotypes using black plexiglass. I made the one attached in a workshop held recently in Oaxaca. The conditions of the darkroom and equipment were less than precarious, and I haven't develop the skills needed, so don't judge the piece. My concern is what could be the archival life of a piece made in plexiglass using the traditional wet collodion technique, fixed with a cyanide based fixer. Would it really last? there is a way to know if an ambrotype or tintype is correctly fixed?. I would love to know that I can offer a lasting image to the probable customers.

Regardless of fixer used, there are real concerns around using plastic subtrates versus traditional glass and tin/ferro types. Mark Osterman who is a conservator at the Eastman House has evidence that plexitypes are not archival. While they certainly look very comparable to black glass, there could be long term issues with them breaking down. What that looks like is something I do not know unfortunately.

Plexiglass is not UV stable. You will have to store the work in total darkness, which is pointless if your purpose is to display it. UV causes embrittlement and opaqueness or a milky discolouration of plexiglass (aka perspex or acrylic). For industrial use, such materials are usually coated with a UV screen of some sorts, but even then it has a limited practical lifespan. So in short, no acrylic materials are archival, and they are much more prone to chemical attack than many other plastics, too.

The manufacturer tells something different and even gives guarantees on that.

What is a "practical lifespan", what is "archival"?

Most manufacturers of Perspex/plexiglass will guarantee the quality for 10 years. That's it. If you see frames, art and furniture from the 60'ies you'll see how the material has yellowed or cracked. It's possible that the lifespan has been improved, but if you want truly archival standard it's a dubious material to use.

i used to use plexiglas when i framed prints, but discovered they quickly turned brown -- outgassing from the plastic.

don't use it.

OK, no plexi, alternatives?, color anodized aluminum?, colored tin?... IMHO the thing is that the presence of a "fresh" ambrotype made in plexi surpasses tintypes, the silver really shines!!

(black glass, off course)

Plexiglas is a trade name for acrylic. It shouldn't "turn brown" for generations. What does discolor is styrene which is sometimes sold as a cheap picture framing substitute. Nor does it outgas like vinyl. It would take a lot of UV to spoil it within any reasonable period of time. What does happen is that acrylic can warp (bow) due to temperature or humidity issues. It is not permanent in the sense that glass is, but neither is any kind of photographic medium ever invented. Even glass is not utterly permanent. It hydrates enough to spall off its surface every few thousand years. But someone might break it next week anyway. "Archival" is a relative term; and in this respect, the main advantage of glass would be its reliable flatness. But there are also kinds of coated acrylics and crystalline plastics like polycarbonate you could also experiment with.