We learned yesterday that many ancient storage vessels were porous. Some were so porous, in fact, that they were used to intentionally soak up wine which was subsequently extracted when shards of the saturated vessel were submerged in water. For this reason, a recent mishnah forbids Jews to drink their own wine when it has been stored in a non-Jew’s wineskin. The presumption is that non-Jewish wine, which has been offered as a libation to a foreign god, remains in the vessel and leaches out into the Jewish wine, rendering it unfit as well.
On today’s daf, the rabbis ask a series of questions about which materials are so porous that they are of concern (most are) and whether vessels that have stored forbidden wine can be cleaned to the point that they are ready to again store permitted wine (most can). These are, in other words, kashrut questions. Kashrut is an extensive and complex field, and today’s daf makes no attempt to cover it comprehensively. However, today’s discussion introduces us to some principles, so let’s explore a few passages, starting with this one:
Bar Adi the Arab seized certain wineskins from Rav Yitzhak bar Yosef, placed wine in them, and then returned them to him. Rav Yitzhak bar Yosef came and asked in the study hall what course of action he should take. Rabbi Yirmeya said to him: This is what Rabbi Ami instructed me to do in terms of practical halakhah: One fills the skins with water for three days, and then pours the water out. And Rava says: In addition, one must pour out the water in them every 24 hours.
One kashers wineskins by filling them with water and letting that water absorb the flavors that have gathered in the wineskin over the course of three days. There is a debate about whether that water needs to be changed every day, but both authorities agree that three days of soaking will render the wineskin once again ready to receive permitted wine. Even some porous and absorbent vessels, therefore, can be kashered.
Rav Yitzhak bar Bisna had certain vessels made of cow dung (that had contained wine of gentiles), which he filled with water and placed in the sun. As a result, they burst. Rabbi Abba then said to him: You have rendered them prohibited to you forever. You may say that the sages said that one should fill the containers with water, but did anyone say that you should place them in the sun?
In this case, Rav Yitzhak bar Bisna, who is particularly zealous in cleaning his vessels not only soaks them, but also puts them in the hot sun. This causes the vessels to explode and earns him a teasing jibe from Rabbi Abba. Cleaning the vessels of the flavor of non-Jewish wine is important, but it’s not necessary to go beyond what is required.
As we proceed through the daf, we learn further that the cleaning procedure for a given vessel depends entirely on the material of the vessel, what was stored in it and for how long. Vessels of natron (a kind of soda ash) are so absorbant they are impossible to clean of wine. Others, like dark brown Aramean jugs, are impervious enough they require only a quick rinse after use. Cups, which hold wine only for a short period, do not need to be cleaned before they can be used for permitted wine — according to some. Others hold that they do. But all agree that whatever is first drunk from a brand new cup is definitely absorbed.
Even the color of glaze used on a piece of pottery can affect what is absorbed:
Rav Zevid said: With regard to glazed earthenware vessels, white and black ones are permitted. Green ones are prohibited, because they contain alum. And if any of these vessels have cracks in them, they are all prohibited.
Mareimar taught: Glazed earthenware is permitted, whether it is black or white or green.
Glazed vessels are generally assumed not to absorb the flavors of the food they store — unless it’s green glaze, which contains alum. Cracks in the glaze are an obvious problem because they give the liquid access to the porous interior of the vessel.
Kashrut is as much art as science. A lot of today’s daf is not directly applicable to us today — for few among us regularly use leather wineskins, unglazed pottery or vessels made of cow dung. However, the daf offers a window into a primary concern of kashrut: the absorption and rerelease of flavors. This depends on the material of the vessel, the food contained within (and, often, its temperature), how long it was stored, and the procedure subsequently used to clean the vessel. Today’s daf does not — indeed, cannot — offer a practical guide for kashrut, but gives us a window onto a key principle on which it rests.
Read all of Avodah Zarah 33 on Sefaria.
This piece originally appeared in a My Jewish Learning Daf Yomi email newsletter sent on July 21, 2025. If you are interested in receiving the newsletter, sign up here.
With your help, My Jewish Learning can provide endless opportunities for learning, connection and discovery.
