If a person who has not kept a kosher kitchen decides to make his kitchen kosher, it is essential to go through a process to render the used kitchenware kosher. This applies even to a situation in which only kosher-certified foods had always been used in the house. The requirement to kasher the items in such a case is due to the likelihood that meat and milk were mixed together.

The principles of the laws of kashering of kitchenware are straightforward, but their practical implementation can be quite complicated. Therefore, it is best to seek the advice and practical assistance of someone who is knowledgeable in these halakhot. Not every person, even if he is mitzva-observant and a scholar, is an expert in all the details of these laws. Only a rabbi who is thoroughly familiar with the halakhot involved can address the various problems that arise during the process. Nevertheless, it is important to be familiar with the main difficulties and their solutions in the field of keeping kosher, as this will help one avoid mishaps, and even could solve the simple errors that can occur in any home.

For the purposes of the laws of kashering kitchenware, there are three factors that must be examined in order to categorize the various types of equipment:

(1) the material from which the kitchenware is made;

(2) the temperature at which one uses it;

(3) the manner of its usage, whether it is with dry or liquid substances.

Other relevant factors are: whether it is used with an implement that interposes between it and the food, or if it comes into direct contact with the food; whether it is used for cooking, frying, or baking.

It is generally not recommended to rely on memory regarding the usage of an item of kitchenware. Sometimes people will say, “We never used this utensil for non-kosher food,” but they might be mistaken. Those who are insufficiently aware of the many halakhot of kashrut often do not realize what kind of problems can arise with regard to different foods. With that said, if one is absolutely certain that a specific pot was never used for cooking non-kosher food, for example, or if it was not used at all, this certainty can help him decide what should be done with it.

Before any kitchenware item can be kashered, it must be put aside and not used for twenty-four hours.

The question of the temperature of the item during its usage is a basic factor in many regards. Kitchenware that is used only to hold cold or room-temperature foods, and is never heated, does not require special kashering. Rather, it is enough to clean it thoroughly. This is the case, for example, for a container or bin in which one keeps bread or cakes, or a refrigerator or similar storage facilities.

By contrast, an item of cookware that is used with hot food does require kashering.

In the case of knives, including bread knives, even if they have not come in contact with hot foods, they require kashering. The same applies to containers in which one stores liquids such as wine and alcohol.

Regarding this matter, there is an important fundamental rule: Any vessel or utensil whose material and shape enable it to be kashered must be kashered by immersing it (or in some cases by rinsing it) in clean hot water, as explained below.

Before kashering a kitchenware item, one must clean it and make sure that it contains no food residue, stickers, rubber linings, and the like.

Next, one takes a large (kosher) pot and boils water in it. Once the water is boiling, one places the items that require kashering into this water. One dips it for a few seconds into the boiling water and then takes it out. After this action, which is called hagala, “purging,” in the halakhic literature, the item is fit for kosher use.

In this fashion, one can kasher metal kitchenware, including many cooking implements and silverware.

With regard to kitchenware that is used at a high temperature with direct contact with food (without water), such as cookware used for roasting and frying, their kashering is similar to the manner of their use: Metal utensils which can be heated to become glowing hot should be put into a fire until they glow from the heat. For items for which this is impractical, such as many frying pans, they cannot be kashered. Ovens must be cleaned thoroughly, turned on or lit, and left burning at their highest temperature. After this process, they are fit for any use.

Earthenware dishes generally cannot be kashered, although in rare cases it might be possible to kasher them (one should consult a rabbi about this matter). It is generally advisable to discard them and buy new ones.

Wooden kitchenware items may be kashered by hagala in hot water.

Regular glass items, since they are very smooth and are not used at high temperatures (even in the case of drinking glasses, one does not pour boiling water into them), do not require hagala. One should rinse them thoroughly and then leave them to soak in water for a period of three days, changing the water each day.

Pyrex and the like, in which food is cooked directly, can be kashered by hagala (although there are many Ashkenazic halakhic authorities who are stringent and prohibit their further use).

It is customary to kasher enamel items by means of hagala.

In the case of products made of plastic materials that are used to hold only cold foods, it is enough merely to wash them. But if they are used to heat foods, one should not kasher them, but rather discard them.

A kitchenware item made of two different materials attached together requires thorough examination. One should be particularly careful to check pots or the like that are comprised of different parts connected by screws or gluing. In all such cases, it is appropriate to clarify each case individually by consulting a rabbi, as sometimes the item cannot be properly cleaned or kashered by hagala.

Most kitchenware items that have an electrical mechanism in some part of them require careful examination. Generally, they can be kashered by heating at their highest temperature, when they are completely empty, or by placing only water inside them. One must pay special attention to items that are used by means of pressure or which are used for spicy substances. A mixer requires a special inquiry in each individual case, although it can usually be dismantled, and each part dealt with separately.

The supplementary equipment that one uses when preparing or eating food also requires kashering by one method or the other. Following are a few examples:

Wooden or metal boards on which one customarily cuts food require thorough cleaning, and if possible, hagala as well.

Tables, on which food sometimes spills, should be cleaned well.

Dishwashers require cleaning and heating to the highest level, and some maintain that they cannot be kashered at all. It is therefore advisable to consult a knowledgeable rabbi regarding their kashering.

If one has dentures that can be removed from his mouth, they also require thorough cleaning and soaking in hot water.

The cooking surfaces in the kitchen, as well as washbasins for dishes, involve unique problems. If the kitchen countertops are made of metal or stone, they should be cleaned as thoroughly as possible to remove any dirt and food residue. Then one should pour on them water that continues to boil while it is being poured, e.g., from an electric kettle and the like. Here too, it is advisable to consult a rabbi who has expertise in the field.

Kitchen sinks present a separate problem. Today, many observant Jews install two separate sinks in the kitchen, one for meat use and one for dairy use, and thereby they avoid many problems. If this is not possible, the sink itself should be regarded as a surface that is not kosher, and one should make sure that no hot food comes into contact with it. As mentioned earlier, one can put in the sink a rack upon which to place utensils.

Usually when things are done in the right way, especially with the guidance of a qualified and experienced rabbi, the process of kashering a whole kitchen takes no longer than a few hours. This preliminary stage is essential to maintaining the halakhot of kashrut.

After kashering the kitchenware, it is important to separate the items that are used for meat from those used for dairy, and to mark them in such a way as to prevent them from becoming mixed up. In any case of a mix-up between them, one must re-examine their status, and until the halakhic clarification has been completed, they must be set aside and not used.

As an aside, it is worth pointing out that there is a fundamental difference between the kashering of kitchenware all year long and the kashering of such items from hametz use prior to Passover. The process of preparing kitchenware items before Passover is more severe.

Further reading: For more on preparations for Passover, see p. 274.