Kelim, Chapter Five, Mishnah Five
1) The additional piece of a householder’s oven is clean, but that of bakers is unclean because he rests the roasting spit on it.
2) Rabbi Yohanan Hasandlar said: because one bakes on it when pressed [for space].
3) Similarly the additional part of the boiler used by olive cookers is susceptible to impurity, but that of one used by dyers is not susceptible.
Section one: This additional piece was put around the hole at the top but since it serves no special function, it is not susceptible to impurity. However, the same piece when part of a baker’s oven is susceptible because the baker may occasionally use this piece to roast some meat.
Rabbi Yohanan Hasandlar agrees that this piece is susceptible but he holds that it serves a different function. Sometimes the baker will use it to bake bread when the oven is full.
Section two: The same principle is employed here. Since the additional part of the boiler is used by olive cookers it is impure, but the dyers did not use this additional part and therefore it is pure.