A tanur that came in pieces (i.e. in segments split vertically, like wooden barrel staves)

from the craftsman's shop,

and he made hoops for it (to hold them together when assembled)

and them on it

and it is tahor (it is not susceptible to tumah until the hoops are put on it),

it became tamei (after the hoops were put on)

and he removed the hoops (and separated the parts of the oven from each other)

it is tahor (as it is considered 'broken'),

it he put them back it is tahor (and cannot become tamei, as a previously broken vessel cannot become susceptible to tumah unless rebuilt into a new vessel, which is not the case here; another opinion is that once disassembled, the pieces never quite fit together again like new, leaving gaps).

If he coated it in plaster (after replacing the hoops, thus creating a 'new' vessel, or according to others sealing the gaps)

it is susceptible to tumah

and he need not heat it

as it was previously heated (it is assumed that when first used by the householder, it was heated).

פרק ה משנה י

If he cut it into [ring-shaped] pieces

and placed sand

between each piece (when they are put back together, and a coating of clay is put around the assembled pieces),

Rabbi Eliezer rules it is tahor,

and the Sages rule that it is tamei;

this is the tanur of the 'coiled snake' (as the halachic arguments relating to it encircled it like an ouroboros, a mythical serpent endlessly consuming its own tail).

Arabian kettles

that [is created when] one digs in the ground

and coats [the hole] in plaster [which is then heated],

if the plaster is able

to stand on its own [if the surrounding earth were removed]

it is tamei,

and if not,

it is tahor,

this is the tanur of Ben Dinai (an infamous fugitive who had need of such ovens while on the run).