The Temperature of Libun

Q. As explained previously, libun is a method of kashering with dry heat. How hot must the vessel be for libun to be effective?

A. The Talmud Yerushalmi (Avoda Zara 5:15) describes libun as heating with fire until sparks emanate from the utensil. The Talmud Bavli (Avoda Zara 76a) defines libun as heating a utensil with fire until a layer of the metal burns off. How does fire remove a layer of metal? The Nimukei Yosef (AZ 76a) explains that this refers to superheated metal which is then dropped into cold water. This will cause the top layer of the metal to peel off. The Shulchan Aruch (OC 451:4) cites the ruling of the Yerushalmi, however poskim write that either of these are acceptable signs that the utensil has been kashered.

Today’s metal utensils do not spark or peel, so it hard to determine what temperature Chazal would require for our metals. However, Rishonim give another definition for libun. The Nimukei Yosef (AZ 75b) writes that it is considered libun once the metal begins to glow like a coal. Rav Belsky zt”l determined that at 900º F, metal will glow and emanate visible light. (It should be noted that metal begins to emit light at about 750º F, but the light will hardly be noticeable until it reaches about 900º F.)

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The Gerald & Karin Feldhamer OU Kosher Halacha Yomis is dedicated to the memory of Rav Yisroel Belsky, zt"l, who served as halachic consultant for OU Kosher for more than 28 years; many of the responses in Halacha Yomis are based on the rulings of Rabbi Belsky. Subscribe to the Halacha Yomis daily email here.