Keilim 5:7-8

Keilim 5:7

An oven that was rendered ritually unclean is purified by splitting it into three parts (lengthwise) and scraping off the outer layer of clay, starting from the top until he reaches the ground. Rabbi Meir says that scraping off the clay and splitting all the way to the ground are not necessary. Rather, it is sufficient to reduce the oven from the inside, four handbreadths from the bottom (about a foot). Rabbi Shimon says that one must also move the oven. If one split the oven into only two parts, the larger part remains impure but the smaller part is purified. If one split the oven into three parts, one of which is as large as the other two, then the large part remains impure but the small parts are purified.

Keilim 5:8

If one cut the oven into rings crosswise, then if they are smaller than four handbreadths wide, the oven is purified. If he reassembled the oven and coated it with clay, it becomes susceptible to impurity once he heats it sufficiently to bake sponge cake. If the clay coating was separated from the oven’s pieces by a layer of sand or pebbles, then the Sages say that a menstruant woman and a ritually-clean woman can both bake in the oven and it is not susceptible to ritual impurity.