פירוש על בבא מציעא 117:12

If one cut [an earthenware oven widthwise] into segments: An oven made out of segments. One fires [the segments] in a kiln in the way of all earthenware pottery, and then combines the segments and places sand between all of the segments.

If one cut [an earthenware oven widthwise] into segments and placed sand between each and every segment: If there was no sand between them, Rabbi Eliezer would have conceded that it is considered a vessel (and so susceptible to impurity), since the plaster around the oven connects the segments and structures them one next to the other. So it is because it was learned [in a mishnah, that] sand between them is to separate the segments from one another that Rabbi Eliezer renders it pure. And it is learned thus in Tractate Kelim (5:8), "If an oven was cut up by its width into segments that are each less than four handbreadths in height, it is ritually pure. If he plastered it over with clay, it becomes susceptible to ritual impurity when it is heated to a degree that suffices for the baking of spongy cakes. If he distanced the plastering and put sand or gravel in between, of such an oven it has been said, 'A menstruant as well as (a ritually impure woman and) a ritually pure woman may bake in it and it remains ritually pure.'" It taught [here that] sand is similar to gravel, as it is dry.

Rabbi Eliezer deems it ritually pure: As this is not an earthenware vessel. Rather it is a structure like a dung vessel or a mud vessel, such that it is not susceptible to ritual impurity.

And the Rabbis deem it ritually impure: As they go after (base their decision upon) the segments. Their other ovens were made like big jugs with an opening above. And one fires it in the urn like other jugs and portables; except that he situates it on the earth or on a board. He then attaches plaster of clay all around it to make it thick, such that it can insulate and strengthen its heat.