(Beraisa): To build the Mizbe'ach, we bring whole, smooth stones, plaster, Kunya and pitch, mix them and pour it into a frame...

Avodah Zarah 33b (Rav Zvid): Black or white Klei Kunya (earthenware vessels covered with lead or glass) are permitted. If they are green, they are forbidden, for they are from Tzarif (earth from alum ore).

(Mereimar): They are permitted, whether they are black or white or green.

Question: Why is the law (of Kunya) different regarding Chametz?

Question: May Klei Kunya (that were used with Chametz) be used on Pesach?

Surely, green Kelim absorb, and they are forbidden. The question is only about white or black Kelim. If they have cracks, they absorb, and they are forbidden. The question is only if they are smooth.

Answer (Mereimar): The outer walls give off moisture. This shows that they absorb, so they are forbidden.

Question: Why are they (permanently) forbidden?

Answer: The Torah testified that absorptions in Cheres (earthenware) never leave the walls. ("U'Chli Cheres Asher Tevushal Bo Yishaver" - if it could be Kashered, we would not need to break the Keli.)

Summation of question (4): Mereimar permits all Klei Kunya of Nochrim. Regarding Chametz, he forbids all Klei Kunya!

Answer #1: He is stringent about Chametz, for it is a Torah prohibition, but Yayin Nesech (wine offered to idolatry) is mid'Rabanan.

Objection: Chachamim enact like Torah law!

Answer #2: The Kelim are used with hot Chametz (so they absorb). Wine is (normally) cold.

Kesuvos 107b: The Halachah follows Rav Zvid.

Rif and Rosh (Avodah Zarah 12b and 2:24): White and black Klei Kunya are permitted. Green Kelim are forbidden. If they have cracks, even black and white Kelim are forbidden. The Halachah follows Rav Zvid, like it says in Kesuvos. This refers to Yayin Nesech, which is cold. Regarding Chametz, which is hot, all are forbidden. The outer walls give off moisture. This shows that they absorb, so they are forbidden. Absorptions in Cheres never leave the walls.

Rosh: Rashi says that Klei Kunya are plated with lead. R. Tam says that they are covered by melted glass.

Rambam (Hilchos Ma'achalos Asuros 11:18): One may drink from a cup of Cheres that a Nochri drank from only if he rinsed it three times, for the drops of wine are gone. This is if it is plated with lead the way potters make them.

Rambam (19): If Yayin Nesech was used in a Keli Cheres smoothed with lead, if it is white, red or black, it is permitted. If it is green, it is forbidden, for it absorbs. If the Cheres is exposed in some place, whether it is white or green, it is forbidden, for it absorbs. This is only if the wine was put in for a long time. If not, one rinses it and it is permitted, even if it is of Cheres.

Rambam (Hilchos Chametz u'Matzah 5:25): Any Keli Cheres used for hot Chametz, whether a Keli Rishon (pot) or a Keli Sheni (bowl), whether it is smoothed with lead that they make like glass, or plain Cheres, one may not use it for Matzah. One leaves it for after Pesach.

Shulchan Aruch (YD 135:5): Kelim of Neser, i.e. they are made from earth from which alum is mined) absorb very much. They can never be Kashered, even if they were not used for 12 months, and wine is not put in them for a long time, and a Yisrael used them first.

Shulchan Aruch (6): Klei Cheres plated with lead or with Resuchei glass (Rema - a kind of glass Keli that was not fully cooked. It is from sand), if they are green they are like Kelim of Neser, for they have earth from alum ore. If they are white or black, if they are smooth without crevices, they are like metal Kelim. If they have crevices, they are Klei Cheres that are not smooth.

Beis Yosef (DH u'Mah she'Chosav v'Im): If they have crevices, if they were first used by Nochrim, Iruy (Kashering by leaving water in a Keli for 24 hours, three times) is needed. If they were first used by Yisre'elim, Shichshuch (washing and scrubbing them with water) suffices.

Taz (8): The Shulchan Aruch connotes that this refers to our Stam Klei Cheres, unlike the Tur in the name of the Rosh (that it is only Kelim made from a special earth). If so, one may not put our wine in a thin-necked Keli Cheres of a Nochri, for it was first used for Asur wine, and rinsing does not help. However, barrels used also for water are permitted, for perhaps the first use was for water. We are lenient about a Safek for mid'Rabanan laws.

Shulchan Aruch (OC 451:23): Klei Cheres plated with melted glass are like Klei Cheres.

Beis Yosef (DH Mani): The Mordechai (Pesachim 584) says that R. Tam says that this is only if glass was melted on, for glass is from sand, like Cheres. If they were plated with lead, they are Kashered through Hag'alah (putting in boiling water), like metal Kelim. Also Semag says so.

Magen Avraham (45): Hag'alah helps if they are plated with real lead, but not if they merely get the appearance of lead.

Mishnah Berurah (137): This is if they are plated on every side. Even though the Mechaber (Sa'if 26) holds that glass does not absorb at all, here it covers the Cheres and was fired with it in the kiln, so it absorbs more.

Hagahah (in Mishnah Berurah): The Chasam Sofer said that Hag'alah does not help for iron pots on which a white, smooth plating was melted on. Some Gedolim are stringent about this only regarding Chametz, when it is not a Ben Yomo (used for Isur in the last 24 hours) and there is only a tiny amount (of Isur) in the plating.

Gra (72): Zevachim 54a connotes that Kunya is not lead.

Mishnah Berurah (138): The Acharonim are stringent even if it is plated with lead or tin, for Chametz is absorbed in the entire Keli, including the Cheres, and it never leaves.

Kaf ha'Chayim (278): The Tur is stringent about lead. Ma'amar Mordechai says that perhaps also the Mechaber is stringent.

Mishnah Berurah (139): If the Keli is used for hot food, it is forbidden in every case. If it is used only for cold, if it is from white or black earth, Hag'alah or Iruy permits, or even Shichshuch if it has no crevices.

Rema: In some places the custom is not to use glazed Klei Cheres on Pesach, even if they are new. One should be stringent only in a place where there is such a custom.

Taz (29): The reason for the stringent opinion is because some use bran (to make them). Maharil (47) says that it is because we cannot tell whether the Nochri used them for Chametz. This is difficult. If so, we should be stringent the entire year, lest he used it for Isur! We must say that it is a mere stringency because one can use other Kelim.

Magen Avraham (46): Bran is used to strengthen them, and they are put in a mere oven. A kiln would burn the bran and permit them, but this is only if it was heated for two or three days consecutively. Some forbid a Keli that received some Kunya from another Keli. If it was returned to the kiln it is permitted. This does not permit a fully glazed Keli, for the owner fears to ruin it and does not fire it enough. A Ga'on says that we should forbid every glazed Keli, lest it was used beforehand; we are lenient because perhaps it is not a Ben Yomo. We are more stringent about Pesach. In some places a Yisrael oversees the making of the Keli. This does not remove the latter concern.

Mishnah Berurah (141): Even where there is a custom, if we know that no bran was put in, one may be lenient.

Kaf ha'Chayim (282): Even where there is a custom, we do not forbid the food b'Di'eved.