The following principle governs the koshering of utensils: In the same manner that the utensil absorbs—it emits.
Consequently, a utensil is koshered (in other words, purged of its absorbed contents) by putting it through the same process as it went through in absorbing what it absorbed.
The 6 Categories of Utensil Use
1. Items placed on the fire without liquids (such as a barbecue skewer or barbecue racks). These items must be koshered by libbun gamur.
2. Keli Rishon (Primary Vessel). An item placed on the fire with liquids. Example: a pot in which liquids are cooked or in which food is cooked using liquids. These items must be koshered by hagalah. (Libbun kal, light libbun, works as well.)
Note that, according to the above, there is a difference in halacha between two types of items which are used on the fire: those in which liquids are placed and those which are used without liquids.
Note, also, that something that is a keli rishon, such as a pot, can be koshered by making it the keli rishon in hagalah (i.e. by boiling water in the pot itself) or by placing it in a keli rishon in which water is boiling.
Note, also, that, in addition to a keli rishon, which is something (such as a pot) that was used on the fire itself, there is the type of item that is not actually a keli rishon itself, but was used in a keli rishon. An example of this is a ladle that is used to remove food from pots that are on the fire. Such an item also needs hagalah in a keli rishon.
3. Keli Sheni (Secondary Vessel). An item which is not placed on the fire, but which is poured into directly from a vessel that had been on the fire. (An example is a colander, which is poured into from a pot that had been on the fire). This item must be koshered by pouring from a keli rishon.
4. A utensil that is used in a keli sheni (such as a pasta fork that is used in the colander above). These items must be koshered by placing them in a keli sheni.
5. Keli Shelishi (Tertiary Vessel). An item into which food from a keli sheni is poured. An example is a plate, after one pours from the pot (keli rishon) into a colander (keli sheni) and then from the colander into a plate (keli shelishi). Since a keli shelishi does not absorb, it need only be rinsed in water.
6. Items that are used with cold foods or cold liquids. (Examples are some spoons and some forks). Since these items do not absorb, they need only be rinsed in water.
Before going further, a number of principles must be established:
1. According to Sephardic tradition, the method of koshering is based on the normal use of the item. In other words, if an item is usually used only with cold foods, and once or twice was used with hot food, it is koshered as items used with cold foods are koshered.
Example: A regular fork, which was normally used for eating food that was in a plate. Even if this fork was once or twice placed in a pot of food that was on the fire, it still is koshered in the way other forks are koshered.
2. Any item may be koshered by a method of koshering that is stronger than the methods noted above. In other words, items from category 2, above, may be koshered by the same method that is used for items in category 1. A utensil that is normally koshered by hagalah—can all the more so be koshered by libbun gamur.
Below are links to Step-by-Step Guides to how to perform various koshering methods