How do you prepare your kitchen for Pesaḥ? Do you “kasher” your silverware or do you have a separate set for the holiday?
Ravina: “What do you do with knives in preparation for Pesaḥ?”
Rav Ashi: “I craft new ones.”
Ravina: “How about people who cannot afford to purchase new utensils? What should they do?”
Rav Ashi: “I didn’t mean that I actually make new ones. What I meant was that I refurbish my knives every year. I put mud around the wooden handle so that it should not be ruined, and I put the metal part of the knife into fire. Then I remove the clay and put the handle into boiling water.”
The Gemara concludes that utensils can be made kosher for Passover by means of putting them in boiling water of a keli rishon – a pot that is directly on a flame – based on the principle ke-bolo kakh polto – something that absorbs taste will expel it when subjected to the same situation. Since kitchen utensils are often used in a boiling pot over a flame, that is the level of heat necessary to remove whatever had been absorbed.
One type of material from which hametz cannot be removed is earthenware. According to the Gemara, once an earthenware pot is used for cooking, what had been absorbed can never be fully removed.
One of the issues raised by the Gemara is whether glazed earthenware pots will have the same rule as simple ones. In this photograph of an earthenware jar that was found in Dura Europos in Babylonia, the difference between glazed and simple earthenware finishes is apparent. From the discussion in the Gemara it is clear that glazing was done with different materials that gave different color finishes, each of which allowed for a different amount of absorption by the pot.
All of this is only if the utensil had been used for hametz in a situation where heat was applied. If, however, something was only used for cold food, then we do not assume that anything had been absorbed and it can be used on Pesaḥ.