The extension to a tanur (this was a round earthenware attachment at the top of the stove or just inside its top, meant to conserve its heat)

of householders

is tahor,

and of professional bakers

is tamei

because

he will support on it

the spit;

Rabbi Yochanan Hasandlar says

because

he will bake with it

when he is pressed [for space].

Similarly,

the extension of the kettle

of olive boilers (an earthenware rim added to the top of their vessels to allow them to voil greater amounts by prevent boilovers)

is tamei,

and of dyers

is tahor (as they did not want the dye to come into contact with the earthenware extension, as it would ruin the dye).

פרק ה משנה ו

A tanur

that had dirt placed in it

up to its midpoint

from the dirt on down

becomes tamei by contact [of the upper portion of the walls of the oven with a tamei object],

from the dirt and up

becomes tamei by [entry of a tamei object into its] airspace (but below that does not become tamei).

If one placed [an oven] over the mouth of a pit (which is created underground by digging)

or on the mouth of a cistern (which is made with masonry, some say above ground, some say below ground)

and placed a stone there (some say it is wedged between the pit or cistern and the oven, to keep the oven from falling in, other say that the stone was larger, and placed on the mouth, and the oven on the stone; there are yet other opinions as to what this arrangement is),

Rabbi Yehuda says

if he heated from below

and [the oven] is heated above (by its walls)

it is tamei,

and the Sages say

since it was heated from anywhere

it is tamei.