Kelim, Chapter Thirty, Mishnah Two
1) A mirror is pure.
2) A tray that was made into a mirror remains susceptible, but if it was originally made to serve as a mirror it is pure.
3) A large [glass] spoon that has been placed on a table is susceptible to uncleanness if it can hold anything whatsoever.
4) But if it cannot do so:
a) Rabbi Akiva says that it is susceptible,
b) And Rabbi Yohanan ben Nuri says that it is pure.
Section one: A mirror is pure because it is a glass vessel without a receptacle.
Section two: A glass tray is susceptible to impurity. Therefore, even if he backs it up with some metal to be able to use it as a mirror, it is still considered a tray and it is still susceptible to impurity. However, if he made it to be a mirror from the outset, it is pure, even if it could be used as a tray.
Reading this mishnah made me a bit curious concerning the history of mirror making. I found the following in Wikipedia:
Metal-coated glass mirrors are said to have been invented in Sidon (modern-day Lebanon) in the first century AD, and glass mirrors backed with gold leaf are mentioned by the Roman author Pliny in his Natural History, written in about 77 AD. The Romans also developed a technique for creating crude mirrors by coating blown glass with molten lead.
It would seem that our mishnah would serve as additional evidence for the existence of glass mirrors during the first two centuries C.E. If any of you out there are Wikipedia editors, maybe you should even write this reference in!
Section three: The test of whether this spoon is susceptible is whether it can stand on a table and still retain its liquid. If it can, everyone considers it a vessel and it is susceptible to impurity.
Section four: If the spoon cannot hold liquids when placed on the table, Rabbi Akiva still considers it impure. Rabbi Yohanan ben Nuri says that it is pure.