We learned yesterday that the Rabbis instituted Tumah on glass bc it comes from sand and resembles earthenware. Earthenware accepts Tumah only from the inside and not from its outside and yet we find that glass accepts tumah from its outside. This is because glass is also like metal in the sense that it can be fixed if it becomes broken, and the Rabbis therefore gave it the law that it accepts Tumah from the outside like metal. A second answer from Rav Ashi is that since glass is clear, the “outside” is treated like the “inside” bc the outside is visible from the inside.

When a tamei vessel breaks and is unusable, it loses its Tumah status. Even if it is repaired, by Torah law the old Tumah does not return. However, the Rabbis added a stringency that the old Tumah comes back if the vessel is fixed. The reason for this decree is that the Rabbis observed that if a vessel would become in contact with a dead body and require 7 day wait for purity, people would seek the alternative option of breaking the vessel and repairing it in order to expedite its purity. This would eliminate the usage of the Parah Adumah waters that are usually used for purity. A second concern was that people would not “break” the vessels and wold simply make small holes in them before fixing them. A third concern was that people would think that a vessel immersed in a mikveh is immediately pure, when, in fact, it requires “haarev shemsesh”-setting of the sun, before it can be used.

Even though glass is somewhat similar to metal in the sense it can be fixed if it breaks, it does not have the Rabbinic law that old tumah returns when it fixed, bc that would be a double D’rabanan. (glass is rabbinic tumah to begin with, in addition to the variable of the fixed vessel)

Earthenware accepts tumah only if it has some hollow part. Metal accepts tumah even if it is flat. The Rabbis were lenient with glass and said that flat glass does not accept tumah so that people would remember that glass is only tamei from the Rabbis and not burn terumah that came in contact with impure glass.

In summary- the Rabbis added tumah to glass because it resembles earthenware. It still accepts tumah from the outside bc 1.it also resembles metal bc it can be fixed or 2. its inside and outside are all the same. Flat glass does not accept Tumah. Glass does not have the stringency that old Tumah comes back if its fixed.

Another one of the 18 decrees is regarding the laws of drawn water and that way they invalidate mikveh waters. If they are caught by any sort of vessel, they becomes “drawn waters.” However, this is only if the vessels were placed intentionally to catch the waters. In random buckets in a courtyard caught water, the water would not be considered “drawn water.”
If it was cloudy and someone put out buckets under a drain, then water that falls in would be considered drawn water, and even if the clouds had scattered before they subsequently returned and it rained.

Another of the 18 decrees is about blood of Niddah. The law is that even a baby girl who has menstruated is tamei. The Kussim did not observe this law and the Rabbis therefore said that all Kussi girls are automatically Niddas.