I bought some circuit board material sold as fr2 so I would not dull my milling bits. I think it is actually fr4. It looks like fiberglass, as you can see the woven cross hatch pattern, which looks exactly like fiberglass. Is there a way to tell for sure, short of dulling my brand new carbide bit? I tried breaking a piece of the board apart to extract a fiber, but after a half hour with a pair of pliers all I could manage to do was crumble it into little bits. I thought if I could get a fiber out I could see if it burns or melts, but no such luck so far. Thanks in advance for any help.