Question: If Chachamim made glass vessels like Klei Cheres, why are they Mekabel Tum'ah from the back?!

(Mishnah): The Tum'ah of Klei Cheres and vessels of nitre (white earth) is the same - they Mekabel Tum'ah from their inside and from [the interior of] their outside (e.g. if a receptacle was hollowed out in its outside), but not from their exterior [surface].

The only way to Metaher them is to break them.

Inference: The Tum'ah of Kelim of Cheres and nitre is the same, but nothing else is like them!

Answer #1: Because a glass vessel can be fixed (melted down and made into a new Kli) if it is broken, Chachamim equated it to a metal Kli.

Question: If so, it should have Tum'ah Yeshanah (return to its old Tum'ah if it is broken and fixed)!

(Mishnah): A metal vessel receives Tum'ah, whether or not it holds anything;

If it was broken, it becomes Tahor; if it was fixed, it is Tamei like before [it broke].

(Mishnah): A vessel of wood, leather, bone or glass receives Tum'ah only if it can hold something;

If it was broken, it becomes Tahor; if it was made into a Kli again, it is Mekabel Tum'ah from now and onwards.

Inference: it is Mekabel Tum'ah from now and onwards, it is not Tamei like before!

Answer: The Tum'ah of glass vessels is mid'Rabanan, Tum'ah Yeshanah is mid'Rabanan - Chachamim only decreed on glass vessels Tum'os that are mid'Oraisa in other vessels.

Question: Glass vessels that cannot hold anything should be Mekabel Tum'ah, for this is mid'Oraisa regarding metal vessels!

Answer: Chachamim decreed that they are not Mekabel Tum'ah, to show that the Tum'ah of glass vessels is only mid'Rabanan, so people will not burn Terumah or Kodshim on account of them.

Answer #2 (Rav Ashi): Really, Chachamim made glass vessels like Klei Cheres;

They are Mekabel Tum'ah from the back because the inside can be seen like the outside (it is as if there is no distinction).

(Beraisa): Shimon Ben Shetach enacted [a lien on all a man's property to pay a] Kesuvah and decreed Tum'ah on metal vessels.

Question: Tum'ah of metal vessels is mid'Oraisa - "Ach Es ha Zahav v'Es ha'Kesef..."!

Answer: He decreed Tum'ah Yeshanah on them.

(Rav Yehudah): A case occurred, Shel Tziyon ha'Malkah made a party for her son; all her vessels became Teme'im, she broke them and gave them to a smelter to solder them to fix them. Chachamim said that they are Teme'im like before.

Question: What is the reason?

Answer: This is so people [who do not want to wait seven days to Metaher vessels] will not cease to use Mei Chatas to Metaher from Tum'as Mes.

Question: We understand this according to the opinion that Tum'ah Yeshanah applies only to Tum'as Mes;

According to the opinion that it applies to all Tum'os, how can we answer?

Answer #1 (Abaye): He decreed lest they were not broken enough to Metaher them.

Answer #2 (Rava): He decreed lest people will say that a Kli is fully Tahor the same day it is immersed.

Question: What is the difference between these answers?

Answer: If he mashed them [surely they were broken enough, but Rava's decree still applies].

Question: What were some other decrees [of the 18]?

Answer (Mishnah - Beis Shamai): If one put Kelim under a pipe to catch rainwater, or forgot and left them there, whether they are big or small, or even if they are made of stone, dirt or dung (which are not Mekabel Tum'ah), the water caught is Posel a Mikvah (it is She'uvim) (Decree #12);

Beis Hillel say, if he forgot them the water is Tahor (not Posel);

R. Meir says, they counted, and Beis Shamai outnumbered Beis Hillel (Beis Hillel ceased to argue about this).

Beis Shamai agree that if he forgot them in the Chatzer the water is Tahor (it is not clear that he wanted them to gather water).

R. Yosi says, Beis Hillel did not retract.

(Rav Mesharshiya): All agree that if he left the Kelim when clouds were gathered (but the rain delayed coming, and he forgot them) the water is Teme'im (he initially wanted them to collect water);

All agree that if he left them when clouds were dispersed, the water is Tahor;

They argue about when he left them when clouds were gathered, then dispersed, then gathered again:

Beis Hillel say that his initial intent was Batul (once the clouds dispersed), Beis Shamai say that it is not Batul.

Question: (We count 18 decrees, there was consensus on all of them -) according to R. Yosi, Beis Hillel still argue, he is missing one of the 18 decrees (surely, all agree that there were 18)!

Answer (Rav Nachman bar Yitzchak): [He holds that] the decree of Benos Kusim (the people Sancheriv settled in place of the 10 exiled Shevatim, who later converted; they decreed to consider Kusi girls Nidos from birth) was on that day (13). (On 17B, all count this among the 18, we give another answer for R. Yosi.)