Kelim, Chapter Five, Mishnah Seven
Our mishnah discusses how one can purify an oven. To recall, in the time of the mishnah the oven was like an upside down pot, and it was attached to the ground before it was used. To provide with extra insulation, they would cover the walls with clay.
1) If an oven contracted impurity how is it to be cleansed?
a) He must divide into three parts and scrape off the plastering so that [the oven] touches the ground.
2) Rabbi Meir says: he does not need to scrape off the plastering nor is it necessary for [the oven] to touch the ground. Rather he reduces it within to a height of less than four handbreadths.
3) Rabbi Shimon says: he must move it [from its position].
4) If it was divided into two parts, one large and the other small, the larger remains unclean and the smaller becomes clean.
5) If it was divided into three parts one of which was as big as the other two together, the big one remains unclean and the two small ones become clean.
Section one: According to the first opinion, the first step in purifying an oven is to divide it into three parts, from top to bottom. As we will learn in the final clause, he should divide it so that no part is equal to half of the oven. Then he scrapes off the clay on the outside of the oven. This either refers to the clay that covers the wall of the oven, or the clay which attaches the oven to the ground. In either case, the oven has now ceased to be a functional vessel and is therefore no longer impure.
Section two: Rabbi Meir is a bit less strict. To purify the oven, he can split the oven from inside, without removing the layer of clay on the outside. The split should be less than four handbreadths from the ground. Since Rabbi Meir holds that any oven that is not four handbreadths from the ground is not susceptible to impurity (see 5:1) this oven is now pure.
Section three: Rabbi Shimon rules more strictly than either two previous opinions. Not only must he split the oven and scrape off the coating, but he must also move it from its position.
Section four: If he split it into only two pieces, one large and one small, the small piece is pure because it is less than half the size of the original oven. The large piece remains impure.
Section five: Furthermore, if he split it into three pieces but one piece was still as large as the other two combined, then the large piece is still impure because it is more than half of the size of the oven. In other words, when section one stated that the oven must be split into three, it meant that no piece should be more than half of the size of the original oven.