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.
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