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I know there's a whole thing about a Jew being allowed to use the kli of a non-Jew without tevilah, but I don't know the extent of that allowance. I was part of a conversation on facebook where someone said that a Jew can use a non-Jew's knife and cutting board to prepare kosher food without a problem, but I was under the impression that knives are in a special category of "DO NOT DO THIS." Anyone know what the source is for either a yes or no?

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The two issues, as I see it, are -- dipping in the mikvah, and any transfer of non-kosher taste.

As for the mikvah -- if it belongs to a non-Jew and the Jew is just borrowing it, there's no obligation to dunk it. The obligation is only on Jewish-owned vessels. If the lender doesn't mind, I suppose you could borrow it and kasher it, which would take care of the kosher concerns.

Now as for if that knife isn't kosher: firstly, the knife may not be quite clean. Traditionally there were many valid concerns that knives retained grease, especially around the serrations. (And especially before stainless steel and modern detergents!) This is why the old-fashioned recommendation for kashering a knife was "first stick it into frozen earth, which will scrape off all the gunk. Then kasher it." (Or today's recommendation -- use steel wool, then kasher.) I've heard differing opinions today how much of a concern this continues to be. (And if the cutting board has lots of little grooves ... good luck getting it clean well.)

Secondly, even if I am sure the non-kosher knife is 100% clean, there is a rule that using it to cut sharp-flavored things like fenugreek, onions, or lemons will still get some of the non-kosher taste transferred from the metal into the food.

Lastly, even when dealing with cold, clean, non-kosher utensils, the rule is to use them "only occasionally"; if you use them regularly, sooner or later someone is going to use something hot (which transfers more taste). Rabbi Hershel Schachter's opinion is that once a month is the maximum that can be called "occasionally."

So theoretically if I can be certain this knife is clean, I can use it to cut a cucumber -- but not an onion! -- once a month. There are enough conditions here that I can see why many would just say "not recommended."

(See Shach, Yoreh Deah 96.21)

  • Thank you for a full and thought-out response! I definitely see what you're saying, though the more I learn about kashrus, the less I feel I know. lol I would assume that a non-Jew who has no reason to be so conscientious about what they've cut and when wouldn't really remember if that knife had been used for sharp foods and flat out treif like pork or basar b'chalav, in which case, it's more of a "taste" transfer than specifically grease and even cold it could be a problem cutting a kosher vegetable like a cucumber. – FrumFromBaaleiTeshuva Dec 10 '18 at 5:04
  • @FrumFromBaaleiTeshuva yes it's complicated. But the rule about onions is if a non-kosher knife cuts an onion, the onion becomes non-kosher. A cucumber cut with a cold, completely clean knife is always kosher. So I don't care if the non-Jew had cut onions with this knife. – Shalom Dec 10 '18 at 21:58
  • Very interesting! Do you have the sources so I can share that with the original asker? They also wanted to know if it was ok for their non-Jewish friend to make them a salad. Most of the responses mentioned the issue of bugs, but if it was just something like cucumbers, tomatoes, and carrots, would that be ok for a non-Jew to prepare without supervision and with their own keilim? – FrumFromBaaleiTeshuva Dec 11 '18 at 21:58
  • @FrumFromBaaleiTeshuva I will get the citation about the clean knife, bli neder -- but you're making a very big jump from "I will borrow a knife, make sure it is absolutely clean, and cut this stuff myself" to "I'm trusting that he got every step of this mystery product exactly right." Even if he's trying to be truthful, mistakes can happen that you don't notice unless you're kosher-attuned. – Shalom Dec 12 '18 at 1:56
  • Oh, I definitely get the difference, and I myself would have a seriously hard time trusting a non-Jew to prepare my food on their own keilim without supervision. But like I said, this started with a conversation on facebook. Most of the responses told the person asking to speak to their rav, but when that other person weighed in with saying it's ok to use the keilim, I was way more taken aback by that even being remotely true. And with so much information flying around, I figured it was worth an ask, especially from people who know more and have sources. – FrumFromBaaleiTeshuva Dec 13 '18 at 10:16

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