There are different materials that a utensil can be made out of. Some materials can absorb taste – beliot, and some cannot. If a utensil has beliot that one wishes to remove, the utensil needs to be koshered. The Torah writes (במדבר לא:כב-כג): “אך את הזהב ואת הכסף את הנחשת את הברזל את הבדיל ואת העפרת כל דבר אשר יבא באש תעבירו באש וטהר אך במי נדה יתחטא וכל אשר לא יבא באש תעבירו במים.” “Gold and silver, copper, iron, tin, and lead – any article that can withstand fire – these you shall pass through fire and they shall be clean, except that they must be cleansed with water of lustration; and anything that cannot withstand fire you must pass through water.” From here we learn that a utensil which has beliot that were absorbed through fire, require fire to remove them. Similarly, a utensil which absorbed through cooking, needs cooking to remove those beliot. The Torah writes elsewhere (ויקרא ו:כא): וכלי חרש אשר תבשל בו ישבר An earthen vessel in which it [meat of an offering which has been invalidated and forbidden to eat] was boiled shall be broken; if it was boiled in a copper vessel, [the vessel] shall be scoured and rinsed with water. From this verse we learn that a utensil made of pottery can absorb beliot but there is no way to remove those beliot from the utensil. Pottery absorbs so many beliot, that if you cook in it, even if you boil it after and do hagala, the beliot do not come out.