Kelim, Chapter Thirty, Mishnah Three
This mishnah deals with the purity of glass cups.
1) A cup the greater part of which is broken off is pure.
2) If it was broken in three places extending over its greater part it is pure.
3) Rabbi Shimon says: if it lets the greater part of the water leak out it is pure.
4) If a hole appeared in it and it was mended with tin or pitch it is pure.
a) Rabbi Yose says: if with tin it is susceptible to uncleanness, but if with pitch it is pure.
Section one: If most of the cup is broken off, it is unusable and no longer susceptible to impurity.
Section two: If there are three breaks spread out onto the greater part of the cup, it is also pure. However, if the three cracks are all in the same place and most of the cup is intact, the cup can remain impure or susceptible to impurity.
Section three: Rabbi Shimon adds that if the cracks are sufficient such that it lets most of the water out, then it is pure.
Section four: According to the first opinion, even though he fixed the hole in the cup with tin or pitch it is still pure. According to this opinion, it is not really possible to fix glass cups.
Rabbi Yose holds that if one fixes the cup with tin it is considered repaired and is again susceptible to impurity. He agrees that if it is fixed with pitch, it is still not susceptible. Assumedly, the one drinking from the cup will taste/smell the pitch while he will not taste or smell the tin.