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By Rabbi Dovid Cohen, Administrative Rabbinic Coordinator
Q. Do ceramic knives require tevillas keilim? Can they be kashered?
A. Ceramic knives are made from a mineral called “zircon”, which is converted into zirconium dioxide, pressed into the shape of a knife, baked in a kiln at very high temperatures for an extended period of time, and then sharpened with a diamond-tipped blade to create a knife which is amazingly sharp and strong.
Utensils used for eating or cooking food require tevillah if they are made of metal or glass, and, therefore, it seems clear that ceramic knives do not require tevillas keilim. The only reason to possibly think otherwise is that if the knives were made of pure zirconium dioxide, they would pose a security risk, since they would not be noticed in a metal detector, and to avoid this concern, the manufacturers add a certain amount of metal to each knife. Does the presence of that metal mean that the knives need tevillah? The answer to this can be found in Rema YD 120:7 who says that if the only metal in a utensil is merely decorative but not functional, the utensil does not require tevillah, even if the metal has food contact. The ceramic knife is similar – the metal plays no role in the use of the utensil but is rather there just for security purposes, and, in that way, is like metal that is decorative, and its presence does not demand tevillah.
The criteria for which items may be kashered are different than those noted above. One of the primary rules of kashering is that utensils made of cheress – loosely translated as ceramic – cannot be kashered (and, therefore, cannot be used if they become non-kosher). Does a knife made of zirconium dioxide qualify as cheress? The answer can be found in Magen Avraham (451:4) who poses the following question: Why is it that utensils made from minerals mined from the earth (klei adamah) can be kashered, but cheress which is made from clay cannot? Clay is also taken from the earth, so why can’t we kasher a dish made of cheress? He answers that when a mineral’s physical properties are dramatically affected by being fired in a kiln, that is what the Torah describes as cheress, and what cannot be kashered. [In that way, glass is similar to cheress, since it is made from sand that takes on new physical properties when it is fired in a kiln.] Based on this definition, zirconium dioxide knives should be treated as cheress. Firing them in a kiln converts them from a brittle material into a strong metal-like substance, and that change is dramatic enough for us to treat them as cheress, which cannot be kashered.
As a side note, theoretically, ceramic knives would be a wonderful choice to be used as a chalif for shechitah, since they maintain their sharpness even under adverse conditions (and there is no specific requirement that a chalif be made of metal, see Shulchan Aruch YD 6:1-3). However, in practice they can get nicked (i.e., pegimos), and once that happens there is no way for the shochet to sharpen them (without a diamond tipped tool!) which means that they would be useless for shechitah.
This article first appeared in the Let’s Talk Kashrus column, Yated Ne’eman, August 4, 2023.