Hagalah, Libun and Irui: The Three Methods

There is one principle underneath all of kashering, and once you have it the three methods stop needing to be memorised: a utensil is purged the way it absorbed. How the taste got in determines how it comes out.

Libun — burning it out

The strictest method, for absorption that happened against direct heat with nothing between. Libun gamur means heating until the metal glows or throws sparks; the absorbed taste is not purged so much as destroyed.

Libun kal is a lighter heating, short of glowing. The important rule is that wherever hagalah would be enough, libun kal is enough too — so libun kal is not a weaker substitute for libun gamur, it is an alternative to hagalah. Knowing that saves a great deal of unnecessary worry, because most absorption is not the direct-fire kind.

Hagalah — boiling it out

The common case. The utensil is fully immersed in water at a rolling boil, so that boiling water reaches every absorbed surface. A vessel too large to immerse is filled with water and brought to the boil in itself, with the boil made to overflow the rim so the top edge is reached too.

Two details matter and are frequently skipped:

• The water must actually be at kashering heat — not merely hot. Water that has gone off the boil is not doing the job.
• It is the heat that matters, not the bubbling. Flavour is drawn out of the utensil by the temperature of the water; the fact that it is boiling plays no independent role. That is why kashering with a liquid that boils below water's boiling point — alcohol, or water under reduced pressure — is not appropriate. It looks like a boil and is not hot enough.
• Every absorbed surface must be reached. A handle, a seam, a rivet, a join where two pieces meet — anything the water cannot get to is not kashered.

Irui — pouring it out

For surfaces that absorbed only from hot liquid poured onto them rather than from cooking in them. Boiling water is poured directly from the vessel it boiled in, over the whole surface, so that it runs across every part.

This is the method for things that cannot be immersed or heated — a counter, a large fixed surface — and it is correspondingly limited in what it can address.

Two further methods

Beyond the three above, the poskim bring two more, each for a narrow case.

Milui v'irui — filling and emptying over three days. Used for glass vessels that absorbed cold. Fill the vessel completely with water, up to the brim and over all its walls, and leave it for twenty-four hours. Change the water, and repeat — three days in all.

Note what this is for: absorption that happened cold. It is not a gentler substitute for hagalah on something that was heated.

Ne'itzah — driving into the ground. Used for knives that absorbed cold. The knife is driven into hard ground ten times, each time in a different spot. After that it may be used even to cut a sharp food such as a radish.

A knife used to cut hot forbidden food is a different case and requires libun or hagalah. On that, there is a view that hagalah alone suffices for knives taken from a non-Jew without libun, and it is suggested that the requirement of libun for a small knife is a stringency rather than strict law.

Ben yomo: the twenty-four hour rule

A utensil used within the previous twenty-four hours is ben yomo, and the taste absorbed in it is still considered good. After twenty-four hours it is aino ben yomo, and the absorbed taste has deteriorated.

The practice is not to kasher a utensil while it is still ben yomo — so the ordinary procedure is to leave it unused for a full twenty-four hours first. This is a custom rather than an absolute bar: after the fact the kashering is effective either way, and in genuine pressing circumstances there is room to rely on that.

Making the water unfit

Where waiting twenty-four hours is genuinely impossible, one recognised approach is to render the kashering water unfit — adding something that spoils it — so that whatever is purged into it cannot render anything else forbidden.

The careful practice here is specific and worth stating accurately. We do not consider ourselves able to spoil taste that is still inside the walls of a vessel. What is relied on is a full, proper hagalah in which the water itself has been spoiled, so that taste emerging into the water meets something already unfit. Because a thorough purge wants clean water rather than water with something added to it, the practice is then to repeat the hagalah a second time.

This is not a shortcut to be improvised. It is a recognised route under pressure, and it needs a rav to direct it.

What kashering cannot do

• It does not replace cleaning. Kashering removes absorbed taste, not food. Clean first, properly.
• It does not work on earthenware. Unglazed earthenware is not kashered by these methods at all.
• It is not tevilah. Immersing a utensil in a mikveh is an entirely separate obligation with a different reason, and doing one does not discharge the other.

Which method your particular utensil needs turns on what it is, what it was used for and how — which is a question for your rav. See also kashering an oven.