Articles

By Rabbi Dovid Cohen, Administrative Rabbinic Coordinator

Q. Can you give me a basic overview of why and how we kasher?

A. If a utensil was used for non-kosher (or meat, dairy, chametz, etc.), then the first step needed to use it for kosher food is to remove all non-kosher residue. In addition, if the utensil and/or food was hot (i.e. hotter than 120 degrees F), ta’am/taste of that non-kosher food was absorbed into the metal, plastic, or other material the utensil is made of. That ta’am must also be purged before the utensil is used for kosher food, and the process used for that is known as kashering. [Ta’am can also be absorbed without heat if liquid remains in a container for 24 consecutive hours (kovush). Details of kovush and how to kasher such a container are beyond the scope of this article.] 

The most common form of kashering is called hag’alah and is used when the utensil either absorbed ta’am off the fire (e.g., a fork used to eat meat) or through a liquid medium (e.g. a pot used to cook non-kosher soup). The hag’alah process involves bringing a pot of water to a boil, and then submerging the non-kosher utensils in the water one at a time. [For surfaces like counters and sinks, it is sufficient to carefully pour the boiling water over all areas.]

There are two prerequisites to hag’alah. Firstly, the utensil must be perfectly clean. Secondly, the utensil cannot have been used for 24 consecutive hours before hag’alah begins. Additionally, while hag’alah is effective for items made of metal, stone, and many other materials, it cannot be used for ceramic, China, porcelain, brick, or glass. [There is a difference of opinion if it can be used for plastic and other synthetic materials.]

Another method of kashering is libun kal, and it is used for the interior of an oven. After ensuring the oven is clean and has not been used for 24 hours, one turns on the oven to 550 degrees F for an hour, and the oven is now kosher. Libun kal is appropriate for standard home ovens where food is always in a pan and never put directly onto the surface or grates.  It cannot be used for a barbecue grill or a pizza oven where the (dry) food is placed onto the cooking surface without a pan.

If a person runs a full high-temperature self-clean cycle on the oven, that qualifies as libun gamur and is effective even if the oven is not perfectly clean, was used within the past 24 hours, and/or the food had direct contact with the grates or surface.

This is just a brief overview of the halachos of kashering, and you should consult with your Rabbi for other details.

This article first appeared in the Let’s Talk Kashrus column, Yated Ne’eman, March 28, 2025.

For additional kashering guidelines for a wide variety of utensils, appliances, and materials, please see https://consumer.crckosher.org/consumer/kashering/.