Keilim, Chapter 13, Mishnah 6. The next several mishnayot take up a practical problem: a keli that is put together out of two different materials, part of it metal and part of it wood. Why does the combination matter? Because the two materials do not have the same rules of tumah. A metal keli is susceptible to tumah even when it is completely flat, with no space that holds anything. A wooden keli, by contrast, becomes tamei only if it has a receptacle, some hollow that contains. So when a single utensil is built from both, how do we classify it?
The Mishnah gives the governing principle: "Eitz hameshamesh et hamatechet, tamei." When the wooden portion of a flat keli exists in order to serve the metal portion, the metal is the essence of the utensil and the wood is only auxiliary to it. In that case the whole keli can become tamei, because a keli's status follows its primary part. Here the primary part is metal, so the entire utensil is treated as a metal keli, and it is susceptible to tumah even though it is flat and holds nothing.
Then comes the opposite arrangement: "Vehamatechet hameshameshet et ha'eitz, tehorah." Where the metal component was added in order to assist the wooden component, the wood is the ikkar of the utensil and the metal is merely subordinate to it. The object as a whole therefore remains pure, since we classify it as a wooden keli, and wood carries no susceptibility so long as there is no hollow in it that holds something.
The example of a key
The Mishnah illustrates these rules with an object everyone knew. A key in those days was shaped like a rod: at one end sat a loop by which the key was gripped, and from the opposite end projected prongs. One pushed those prongs into the opening of the lock and rotated the rod, and in that way the lock was opened or closed.
"Keitzad?" How do the rules play out? "Potachat shel eitz vehapin shelah shel matechet, afilu achat, temei'ah." If the body of the key is wood and its teeth are of metal, and even if only a single tooth is metal while the rest are wood, the whole key is susceptible to tumah. The reason is that the working part of a key is its teeth, since they are what engage the lock; the handle merely holds the teeth in place. The metal is therefore the primary component, and the key can contract tumah even though it is flat and has no receptacle.
"Potachat shel matechet vehapin shelah shel eitz, tehorah." If the body of the key is metal and its teeth are of wood, the key is tahor. The essential part of the key, the teeth, is wood, and a wooden keli is not susceptible unless it has a receptacle.
The example of a signet ring
The Mishnah moves on to a ring set with a seal, the sort used for stamping documents. "Taba'at shel matechet vechotam shelah shel almog, temei'ah." Where the circle of the ring is metal while the seal fixed into it is almog, coral, counted here among the woods, the ring is susceptible to tumah. A ring is first and foremost jewelry worn on the finger, and stamping with it is only a secondary use. The metal circle is thus the ikkar and the coral seal is subordinate to it, so the object is judged a metal keli and takes tumah even with no receptacle at all.
"Taba'at shel almog vechotam shelah shel matechet, tehorah." Where the circle is coral and the seal fixed in it is metal, the ring stays pure. Wood is the dominant material in this arrangement, and wood carries no susceptibility so long as there is no hollow in it that holds something.
A tooth on its own
The Mishnah ends with an independent ruling about the prongs themselves. "Hashein shebatass shebapotachat veshebamaftei'ach, temei'ah bifnei atzmah." A metal prong belonging to a tass, a thin sheet of metal worked into a keli, and equally a metal prong of a potachat, a large key, or of a maftei'ach, an ordinary one, carries susceptibility on its own account, even once it has been separated from the object in which it was set. Since a prong of this kind can work the lock unaided, apart from the utensil to which it normally belongs, it counts as a keli standing by itself.
The thread running through the entire mishnah is a single idea: when two materials are joined in one utensil, we ask which one does the essential work. That component determines the law for the whole, whether it renders the keli susceptible to tumah as metal or leaves it tahor as wood.