Keilim, Chapter 3, Mishnah 7. This chapter has been occupied with the question of when a clay utensil is still a utensil and when it has been ruined past the point of receiving tumah. Our Mishnah turns to a related question: when a keli is made to be used together with a coating, is that coating treated as part of the keli itself? Everything will depend on the material of the coating and on the purpose for which the keli is used.
A kettle with a coating
The Mishnah begins: "meicham shetipelo bechomer uvecharsit", a kettle that was plastered over with thick clay and with powdered earthenware. Two different materials were applied to the outside of this kettle. Thick clay adheres firmly and stays in place. Powdered earthenware does not bond in the same way; it sits loosely and in the end it flakes away.
Now the kettle itself contracted tumah. The Mishnah rules: "hanogea bachomer, tamei", food or liquid that comes into contact with the clay plaster becomes tamei, because a coating that holds fast is regarded as part of the kettle. Touching the coating is touching the keli. But "uvecharsit, tahor", whatever touches the powdered earthenware remains tahor, since a coating that does not hold and is destined to fall off never became part of the keli at all.
The principle is straightforward. Chomer, the thick clay that stays put, joins the utensil and shares its status. Charsit, the powdered earthenware that will not stay put, remains a separate substance, and so it stays tahor even when the kettle beneath it is tamei.
Repairs and utensils made of materials that do not suit their use
The Mishnah now moves to utensils that were patched with a material unfit for their function, and to utensils made entirely out of such a material.
Next the Mishnah takes up a repair made with an unsuitable substance. It speaks of a "kumkum", an urn in which water is heated, that became perforated: "shenikav", and the breach was wide enough for water to drain out, which by itself leaves the urn unfit for service and therefore tahor. Its owner then stopped up the opening with pitch, "ve'asa'o bazeft". The ruling follows: "Rabbi Yose metaher". And the reason he keeps it tahor is "she'eino yachol lekabel dechamin", it has no capacity to hold hot liquid, "ketzonin", the way it holds cold liquid. Pitch gives way and runs once heat reaches it, so in a vessel whose entire purpose is boiling water such a seal accomplishes nothing. The urn remains damaged and incomplete, and a keli in that condition is not susceptible to tumah.
Rabbi Yose extends the same reasoning: "vechein haya omeir", and this was his ruling as well concerning "kelei zefet", vessels formed entirely out of pitch. Such vessels have no ability to hold anything hot, and that ability belongs to the very concept of what a vessel of this sort exists to do. Even when the one before us was manufactured for cold liquids alone, Rabbi Yose refuses to grant it the standing of a keli, and he maintains this ruling for pitch vessels of every kind.
Copper utensils lined with tar
"Kelei nechoshet shezifatan", copper vessels that were given a protective inner coating of pitch, and which afterwards became tamei: "tehorin", foods and liquids that meet the coating stay tahor. The ordinary service of a copper vessel is above a flame, and pitch cannot endure a flame. A layer that is worthless for the work the vessel exists to perform is not reckoned as part of it, and it therefore does not absorb the vessel's tumah.
The Mishnah then draws a distinction: "ve'im leyayin", should the copper vessel have been set aside for holding wine and coated on the inside with pitch, then "temei'in": contact with that coating conveys tumah. Here the pitch was applied intentionally and fills a real need, keeping the wine from taking on the flavor of the copper. A coating that exists in order to guard and improve the taste of what the vessel holds counts as one with the keli, and it receives tumah together with it.
So a single substance, tar, can be nothing at all or can be part of the keli. Placed where heat will destroy it, it is disregarded. Placed where it genuinely serves the utensil's purpose, it becomes one with the utensil.