פירוש על כלים 5:5

מוסף התנור (chimney-piece of the oven) – the addition of the oven that they at its top in order to preserve its heat.

The additional piece of a householder's oven is clean, but that of bakers is unclean because he rests the roasting spit on it.
Rabbi Yohanan Hasandlar said: because one bakes on it when pressed [for space].
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.

מפני שהוא סומך עליו את השפוד (because it supports the roasting-spit on it) – of meat, for they regularly roast meat on it, and it is a thing that is necessary for the oven. And the All-Merciful states (Leviticus 11:35): “and unclean they shall remain for you,” to everything that are your needs, as we expounded above [in Mishnah 2 of Chapter 5 of Tractate Kelim].

מפני שהוא אופה בו כשהוא נדחק – when he has many forced laborers and he the need for a great deal of bread. But for Rabbi Yohanan [HaSandlar], the reason of the first Tanna/teacher is not sufficient for him because he rests the roasting-spin upon it, and he holds that we require that the additional usage would like the usage of the body of the oven.

מוסף היורה של שולקי זיתים (rim around a boiler in the ground of olive seethers) – it is the manner of those who seethe/thoroughly boil olives and dyers that have large boilers and they make for them a rim of plaster on the border that water will rise upon it at the time of their foaming. But the seething of olives is unclean/impure, because addition/rim Is needed for the vessel and we use it.

ושל צבעים טהור – for they don’t use it in order that they don’t use its dye. But the Halakha is not according to Rabbi Yohanan HaSandlar.