By Rabbi Berach Steinfeld
In Devarim 27:5 the posuk says one should not raise a knife on the stones in the Bais Hamikdash. Shulchan Aruch OrachChaim 180 says that our minhag is to cover the knife while bentching. The Bais Yosef brings down two reasons for this. The first reason is that a table is domeh to a mizbeach and since regarding the mizbeach we are told do not raise a knife, we remove it during bentching. The Torah Temima asks the following question: why are we allowed to have honey or yeast on the table, since they are two components that were forbidden on the mizbeach? In addition, why davka does the knife have to be removed during bentching? The second reason is related to astory that took place while someone was bentching. When he got to the brocha of Bonei Yerushalayim he remembered the churban and in his anguish he stabbed himself with the knife that was on the table. We therefore remove the knife during bentching.
The question arises; does a plastic knife have to be removed? The Shailos U’Teshuvos Bais Tzvi Vol 3 145:5 asked this question and answered that it would depend on the two reasons. If the reason is that a knife was forbidden in the BaisHamikdash, it would only apply to a metal knife so a plastic one would be allowed. However, if the reason is to protect a person from anguish, that would disallow a plastic knife as well. The Chashukei Chemed in Sanhedrin 76b says that even if the plastic knife is sharp it does not need to be removed since the gezeirahonly applies to what happened in the story, and in the story it was a metal knife, so the gezeira is only on metal not plastic.
What about during the brocha of Al Hamichya? It would seem that both reasons of the Bais Yosef would apply to Al Hamichyaas well. The first reason, that a table is like a mizbeach, applies to Al Hamichya too. The second reason of speaking about building the Bais Hamikdosh is also mentioned in Al Hamichya.
The Shailos U’Tshuvos Salmas Chaim 184 discusses this question but says that since the story took place during BirchasHamazon, the gezeira only applies to Birchas Hamazon and not Al Hamichya. He brings a proof from the Taz in Orach Chaim 180 that discusses why we do not remove the knife on Shabbos,only during the week, and he answers that since the story that precipitated the gezeira took place during the week, it won’t apply on Shabbos.
The Yeshuos Malko in Hilchos Bais HaBechira 1:16 asks the following question. Is the Halacha of a knife applicable only to a specific type of metal knife, or to any metal, like gold or copper too? He says that it would apply to all knives made out of all types of metals, since the Gemara in Avoda Zara 52b says that the stones of the mizbeach that were defiled by the Greeks became chulin since they were designated for Avoda Zora, so they were not salvageable. The Gemara asks, why could they not be mevatel the Avoda Zora by breaking it? The Gemaraanswers that since one was not allowed to raise a knife on them,they were not breakable and therefore not able to become batel. This would be a proof that one could not use any type of knife. The Ramban at the end of Yisro says that only barzel was assur,but not other types.
May we all be zocheh to get the Bais Hamikdash back and experience no more anguish!