What Is Kashering?

Kashering is the process of making vessels, utensils, and surfaces kosher for use. Whether preparing a new kitchen, restoring items that absorbed non-kosher food, or switching items between meat and dairy use, understanding kashering principles is essential for kosher living.

The Underlying Principle

כָּל דָּבָר אֲשֶׁר יָבֹא בָאֵשׁ תַּעֲבִירוּ בָאֵשׁ... וְכֹל אֲשֶׁר לֹא יָבֹא בָּאֵשׁ תַּעֲבִירוּ בַמָּיִם

Translation: Whatever can withstand fire, you shall pass through fire... and whatever cannot withstand fire, you shall pass through water.

—in the same manner that something absorbed taste, so it expels taste.

Types of Kashering

Hagalah (Boiling)

Used for items that absorbed taste through liquid (pots, utensils). The item is immersed in boiling water, which extracts the absorbed taste.

Libun Gamur (Heavy Burning)

Used for items used directly on fire (grill grates, baking pans used without liquid). The item is heated until it glows red or until a paper touching it would scorch instantly.

Libun Kal (Light Burning)

A less intense heating, used for items that had only incidental direct fire contact. Heat until a straw touching the surface would scorch.

Irui (Pouring Boiling Water)

Used for surfaces and countertops. Boiling water is poured from a kettle directly onto the cleaned surface.

The 24-Hour Wait

(degraded) and loses its halachic significance. This ensures that the kashering process purges only stale, degraded taste rather than viable taste that could reabsorb.

Kashering Different Materials

Metal

Most metals kasher well through hagalah or libun, depending on use.

Wood

Can be kashered through hagalah if smooth. Cracked or porous wood cannot be kashered.

Stone and Granite

Kashered through irui (pouring boiling water) after thorough cleaning.

Glass

Ashkenazic: Generally cannot be kashered for Passover; year-round, some are lenient after thorough cleaning. Sephardic: Can be kashered, as glass is considered non-absorbent.

Plastic

Debated. Many authorities say plastic cannot be kashered due to how it reacts to heat. Some permit kashering hard plastics through hagalah.

Ceramics and Earthenware

Cannot be kashered. These must be replaced.

Enamel

Cannot be kashered according to most authorities.

Common Kashering Scenarios

New Kitchen Setup

Some items (especially used or of unknown history) may need kashering before first kosher use.

After Non-Kosher Use

Accidentally cooking non-kosher food in a pot requires kashering before continued use.

Switching Meat/Dairy

While generally avoided, an item can sometimes be switched from meat to dairy or vice versa through proper kashering—though not repeatedly.

Before Passover

The most common kashering scenario. Items used for chametz (leavened foods) must be kashered before Passover use.

Practical Steps for Hagalah

list:1. Clean thoroughly - Remove all food residue, rust, or deposits|2. Wait 24 hours - Don't use the item at all during this period|3. Prepare a kashering pot - A large pot of water at a rolling boil|4. Immerse completely - The item must be fully submerged in the boiling water|5. Rinse in cold water - Immediately after removal, rinse the item|6. The item is now kosher - Ready for use in its new capacity

When to Consult a Rabbi

Many kashering situations have nuances that require rabbinic guidance: unusual materials, complex scenarios, multiple issues at once, or items of high value. When in doubt, ask before proceeding.

Understanding kashering transforms mistakes from catastrophes into correctable situations and enables the kosher kitchen to adapt and grow over time.