What masking is used on acrylic plastic, specifically first surface mirrors, but generally?

There’s an instructable post for building an interesting kaleidoscope using first surface acrylic mirrors. The creator suggests to leave the masking in place when cutting the mirrors with a laser.

It’s my understanding that acrylic masking is either paper or vinyl plastic and I posted a recommendation to remove the masking, to avoid the chlorine danger it represents.

I’ve been unable to find a confirmation that the masking on acrylic products are vinyl.

Is there also a means of testing this sort of stuff? I recall something along the lines of heating a copper wire in a torch flame, melting a bit of the material on the wire, then reheating it to see if it turns green. Is this correct?

My limited experience is that the paper masking that’s on some acrylic is great to leave on for protection, but the plastic masking needs to be removed because it melts on to the acrylic at the edges and is a royal PITA to remove. I peel it off first and protect the acrylic with paper “transfer tape”.

I’ve peeled the plastic off partly for the reason you note, the melting of the film into the surface, but also because I don’t care to risk the chlorine danger. So far, no confirmation that there is vinyl involved.

Google “Beilstein Test”. You could use this to test the masking film for the presence of Chlorine.

That’s the one I’ve heard about. It seems easy enough, but I’ll have to wait for the spouse to return, as my red/green color disability will invalidate any testing!

Please let us know what your results are.
I only use Perspex® acrylic and have been told by my supplier that is OK to cut with it left on.


I can’t test the exact plastic masking discussed in the above noted Instructable, but I have some acrylic from a big-box store with plastic masking on it.

As a means of validation, I also performed the test with a section of PVC pipe (polyVINYLchloride) and after a few moments under the torch, we had a bright green flame from the copper wire.

I then applied the plastic masking to the copper and three tries came up green-free. That’s good news in general, but of course, does not change the problem of the plastic melting into the base acrylic.

I don’t think it’s a good idea to leave the masking plastic on the mirrors when cutting is performed, but then again, the edges are not part of the viewed area.

Not a good idea to leave the brown paper on acrylic. The glue will melt / bubble, mar the acrylic surface, and often catch fire :slight_smile:

I have used the acrylic from the big box stores and haven’t really encountered a problem with the plastic film covering. In my experience, it melts and pulls back from the cut edge, rather than melting into the cut. Your mileage may vary, but I wouldn’t be too concerned about it. On the other hand, the paper backing that is on some acrylic sheet does burn and will leave black sooty marks so I would recommend removing it beforehand.