A pot can become not Kosher if it used to cook milk and meat within the same 24 hour period, if it is used to cook non-kosher food, or on Passover, if it was used prior to Passover for Chametz (wheat or barley or any of the 3 additional grains). Flavor from the foods cooked can remain in the walls even after being washed, and it can add flavor to food cooked in the pot afterwards.
The only time we are required to apply the Koshering Process is on items that are normally used directly on the fire, or are in contact with hot foods that are directly on the fire. These are called Keli Rishon and Iruy Keli Rishon respectively.
The basic concept of Koshering these pots are to cook with them in the same way to remove any possible edible flavors that may come out of the wall of the pot. Each type of item has a specific process related to its standard use. To determine which process is to be used, we follow its most common use. For example, an item normally used as a Keli Sheni (not directly on the fire),even though a few times you use it as a Keli Rishon (directly on the fire), you treat it as a Keli Sheni and it does not need any Koshering.
For Ashkenazim only, Koshering is only done after the item and the pot used for Koshering were both not used for the non-kosher food or the Chametz for a 24 hour period. Bedi Avad, (if it was already done) it is kosher anyway. Sephardim will require this only on items that were used for Treif (non-kosher food).
Hagalah is used for pots, pans, and utensils that are used normally as Keli Rishon or Iruy Keli Rishon. Even though technically Iruy Keli Rishon is a bit less strict than Keli Rishon, it’s simpler to do the same process for both. You take the item and drop it into boiling water and then remove it and put it under cold water. Make sure the entire item gets to the boiling water, even if you need to turn it around and immerse it again. If the item to bekoshered is a pot, you may put enough water into that pot and just steam it up with the cover on, and then put it under the cold water.
If there was extra caked on food that you cannot remove, which can often build up in cracks by the handle, just put detergent and/or bleach on it to render that food inedible.
The following is a list of items and the specific Koshering Processes required:
Metal Pots, Metal pot covers, soup ladles, utensils used in cooking, silverware (Metal), all can be koshered with Hagalah, described above.
Frying pans, kitchen grills, and barbeque grills can be koshered by putting the fire on it without anything in it, until it gets pretty hot. No need to burn the pan.
Baking pans can be koshered by putting them in the oven for a half hour on the highest flame.
Glass, plastic, glazed china, glazed dishes, Corelle, Pyrex, Corningware, or glazed earthenware does not need any Koshering for Passover, since they do not retain any flavor.
Ovens, toaster ovens, and convection ovens can be koshered by putting them on the highest flame for a half hour. If they have a self-cleaning option, running the self-cleaning process is certainly good.
Warming drawers do not need to be koshered if they do not normally exceed 125 degrees Fahrenheit.
Microwave Ovens can be koshered by putting a bowl of water on the highest setting for twenty minutes.
Keli Cheres (clay type pots, similar to some flower pots) that are not glazed cannot be koshered, because the flavor it retains cannot be removed.
Sinks, refrigerators, trash cans, counters, dishwashers, stovetops, tables, cutting boards, mix-masters, silver kiddush cups, and anything not used with hot foods from the fire do not need any koshering.