Kelim, Chapter Eight, Mishnah Ten
1) A pit which has a place on which a pot may be set is unclean.
2) And so also an oven of glass-blowers, if it has a place on which a pot may be set, it is unclean.
3) The furnace of lime-burners, or of glaziers, or of potters is clean.
4) A purna:
a) If it has a frame is unclean.
b) Rabbi Judah says: if it has coverings [for compartments.]
c) Rabban Gamaliel says: if it has edges.
Section one: The pit to which this section refers is an oven placed into the ground and attached to the ground with clay. There was a place on top where they could rest a large pot. Since it is an oven, and it can be used to make food, it can contract impurity.
Section two: Since this oven can be used for cooking food, it too is susceptible to impurity, as long as it has a place on it upon which one could balance a pot.
Section three: The furnace used by lime-burners, glaziers or potters is not made for cooking or baking and therefore it is not susceptible to impurity, even if it has a place on which a pot can be placed. In other words, whereas the oven used by glass-blowers is also made to be used for cooking or baking the oven used by these three professions is not.
Section four: A purna is an oven similar to the ovens that we use today. It has a hole in its side, and one doesn’t stick the bread to the side as was typically done with ovens during the mishnaic period. Rather the bread was put into bottom of the purna-oven as we do today. There are three different opinions as to what the purna needs for it to be susceptible to impurity. The first opinion holds that if it has a frame around it upon which one could balance a pot, it is susceptible. If it has no frame, and only the ground part of the oven is used for cooking, then it is not considered to really be an “oven” and it is not susceptible. The second opinion holds that if it has covered compartments it is susceptible. The third opinion is that it needs edges upon which one could balance a pot. Without at least one of these three things, the purna is simply some clay attached to the ground to keep the heat in, and none of it is used for actual cooking. Such a simple oven is not susceptible to impurity.