The Mishnah discusses a wooden table whose surface was covered with stone – a substance that is not susceptible to tumah (22:1). The Mishnah discussed the debate regarding how much of the originally wooden surface must be showing for the table to remain susceptible to tumah. The implication of the Mishnah is that if the table was completely covered than it would no longer be susceptible to tumah. The explanation of the Bartenura that “we go according to the [susceptibility of the material of the] cover” therefore seems appropriate.
The difficulty is that in earlier cases the Bartenura appeared to explain the opposite. For example, the Mishnah (11:6) discussed the susceptibility to tumah of a metallic pika (spinner’s coil). The Chachamim ruled that it was indeed susceptible to tumah. However if it was made of wood, which in that form would not be susceptible to tumah, and then coated with metal, it would not be susceptible to tumah. There the Bartenura explains that we are only interested in the ikar (essence) of the utensil and not the coating. How can the apparent discrepancies between the Mishnayot be explained? How can the apparent contradiction in the opinion of the Bartenura be resolved?
The Mishnah Achrona differentiates between these two cases. In the earlier Mishnayot the coating only serves a decorative value. Consequently the coating is insignificant compared to the utensil itself. However in our case of the table, the stone serves a functional purpose – it cools and protects the food.
The Gra differentiate between classes of coatings in a similar way. The reason why in the earlier cases the metallic coating was not significant is because the coating alone cannot turn a wooden utensil into a metallic one (Eliyahu Raba 11:4-6). However if the prime use is through the material of the coating, then significance is given to the coating (Biur HaGra Shulchan Aruch, Yoreh Deah 120:16). The Chazon Ish (Keilim 14:8) explains that when the prime use is made from the coating, for example in the case of the table, then the coating is important. However if the prime use is made with the utensil itself and the coating simple serves as a protective layer, as in the case of the pika, then the substance of the coating is of no importance.
The Rambam (Hilchot Keilim 4:4) however explains that any utensil, even if it has a receptacle, even with a metallic coating, is not susceptible to tumah. He bases the rule on the pasuk that discuss the susceptibility to tumah of utensils as those “that one performs work inside them.” The Rambam understands that this excludes coated vessels whose work is not performed inside them, but inside its coating. Consequently in both our Mishnah and the earlier ones, the object in question is not susceptible to tumah due having a covering.24
24 See the Kesef Mishnah for an explanation of the position of the Rambam.
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