How do you tell PMMA from Pyrex?

[bobrob]

This may be a dumb question but...

How do you tell PMMA from Pyrex? I was at a B&M shop the other day, looking at tank that I thought normally came with Pyrex, but the salesman (who I trust) said it was PMMA.

Of course, that was a guy I trust. But what happens with a guy that I don't trust?

Is there a good way to tell the difference PMMA (or any plastic) and pyrex on the spot?

 

[twgbonehead]

The only way I know it is to try to scratch it with the tip of a knife. If you can feel a scratch with your thumbnail it's not glass (or at best, it's coated glass).

If a vendor tells you a tank is glass, you ought to be entitled to do this. But, of course, if the vendor says it's PMMA he might not appreciate you scratching up his inventory!

 

[p7willm]

If the glass/PMMA has threads they were probably molded in the glass and cut in the plastic. They will be much sharper on the plastic.