Keilim 5:9-10

Keilim 5:9

Let’s say that an oven came from the craftsman in vertical pieces and he made hoops to hold those pieces together. Once one puts those hoops on (i.e., assembles the oven), it remains insusceptible to impurity. If it (was coated with clay and) was rendered unclean, then removing the hoops (i.e., disassembling the oven) purifies it. When he returns the hoops (i.e., reassembles it), it remains insusceptible to impurity. If he coats it with clay, it becomes susceptible to ritual impurity without being heated because it had already been heated.

Keilim 5:10

If one cut an oven into rings (i.e., horizontally) and he reassembled it putting sand between the layers, Rabbi Eliezer says that it is insusceptible to ritual impurity but the Sages say that it is susceptible. This is the oven of Achnai (the subject of a famous Talmudic debate). Regarding the vats of Arabs, which are dug into the ground and coated with clay, if the coating can stand on its own, the vat is susceptible to ritual impurity; if it cannot, it is insusceptible. This is the type of oven used by Ben Dinai (the name of a particular highwayman).