Using a Microwave to Heat Water for Kashering

Some hot milk spilled on my fleishig counter and I would like to kasher it. Can I boil the kashering water in my microwave and then pour it over that spot?

The general rule that governs kashering is k’bolo kach polto (the method that was utilized to absorb is the same one that is needed to purge). Since the counter became non-kosher by having hot milk spill on it, one may kasher by pouring boiling water over that area. However, Shulchan Aruch (Orach Chaim 452:5) writes that if the non-kosher item (or, in this case, the milk) was heated by fire, the kashering water must also be heated by fire. Therefore, one may not kasher utensils that became non-kosher through fire in a hot spring. This is true even if the water in the spring is equally hot (boiling). Since a microwave oven heats water without fire, it has the same limitation as water from a hot spring, and cannot be used in place of water heated by a fire. If however, the hot milk that spilled on the counter was also heated in a microwave, then the microwave can be used to heat the water for kashering.

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.