Keilim, Chapter 15, Mishnah 2. The mishnah before this one set down a sweeping rule: objects fashioned of wood or of leather that are entirely flat, having nothing that serves as a container, stand outside the laws of tumah altogether. Our mishnah now gathers the cases that depart from that rule, flat wooden items that are susceptible after all, and it ends with a disagreement over a metal baking sheet embedded in a wall.
"Arubot shel nachtomim temeiot" Bakers use flat wooden boards on which they arrange the loaves before sliding them into the oven. Although they are flat, they can become tamei, but only on a derabanan level. Why? Because these boards have a feature that lends them the look of a genuine keli rather than a plain slab of wood. That is the guiding idea of this whole mishnah: flat wooden items are exempt by Torah law, yet when they carry the appearance of a real vessel, the Chachamim declared them capable of receiving tumah.
"Veshel ba'alei batim tehorot" The same sort of baking boards belonging to ordinary householders remain tahor, even on the rabbinic level. Theirs simply look flat. Nothing about them projects the impression of a keli, so there is no reason to extend tumah to them at all.
"Serakan o kirkeman temeiot" If the owner painted his boards with a red dye, or coated them with saffron, which gives a yellow color, they do become tamei on the rabbinic level. The coloring is an investment of care that changes how the board presents itself: it now has the look of a proper keli, and the rabbinic decree applies to it.
A baking shelf fixed into the wall
The mishnah now turns to a fresh case: "Daf shel nachtomin", a baker's sheet, here one made of metal, "shekeva'o bakotel", which its owner set firmly into the wall. On this the Tannaim divide. As a rule, metal utensils are susceptible whether or not they hold anything inside them, so flatness by itself would be no protection here. Even so, Ribbi Eliezer declares the sheet tahor: from the moment it was planted in the wall it is reckoned as part of the structure, mechubar lakarka, joined to the ground, and whatever is joined to the ground does not receive tumah.
"Vachachamim metame'in" The Chachamim hold the reverse, that the sheet is susceptible even while it sits embedded. Their argument: it was already a full keli while it was still loose, and being planted in the wall does not strip that standing from it. It remains a keli, and metal utensils need no cavity at all in order to be susceptible, so tumah can reach it even in its place in the wall.
The serod
"Serod shel nachtomin tamei" The word refers to a wooden board upon which the baker slices his dough into portions before taking them to the oven. When it belongs to a professional, it is susceptible. He handles it throughout the day, and it is the surface on which he readies his dough, so it presents itself as a real keli, and the Chachamim ruled that it receives tumah.
"Veshel ba'alei batim tahor" The serod of a householder is tahor. In a private home it is not used frequently enough for preparing dough, and the owner therefore never designates it as a keli for that purpose.
"Gipefo be'arba ruchotav tamei" Should the owner raise a border around the board on every side, it becomes susceptible even by Torah law, for it has now turned into an actual keli with a cavity that holds.
"Nifratz meruach achat tahor" Should that border be broken open along any single side, the board ceases to serve as a container. With its cavity lost, it is tahor. And a board that never had a border in the first place is not reckoned a keli whatsoever.
So according to the Tanna Kamma, the serod of a ba'al habayit, of an ordinary person in his home, becomes susceptible only when it has a rim running along four sides. Ribbi Shimon adds a second path.
"Ribbi Shimon omeir" Ribbi Shimon teaches that if the owner readied his board for the task, "hitkino lihyot koretz alav", reworking it so that it would serve him for slicing dough, it is susceptible. It has become the equivalent of the baker's board, which the Chachamim declared capable of receiving tumah. What renders the baker's board susceptible is that it stands set aside for slicing portions of dough over and over, and such designation confers upon it the standing of a keli. The same applies in a private house, says Ribbi Shimon: once the owner adjusts the board and reserves it for that one purpose, it ranks as a keli and can contract tumah.
"Vachein hama'aroch tamei" So too the rolling pin, with which dough is flattened into sheets, is susceptible. No Tanna argues here; on this item the Tannaim are of one mind, since its very form is already the form of a keli.
The thread running through the mishnah is worth holding onto: flat wood is exempt from tumah by Torah law, but the Chachamim looked at how an object presents itself and how its owner treats it. A board a baker leans on all day, a board he colors, a board he reworks for cutting dough, all of these have crossed the line from raw plank into keli, and tumah follows.