What “Clean” Means Before Kashering

Kashering removes taste absorbed into the walls of a utensil. It does not remove food sitting on the surface — that has to be gone first, or you are simply cooking it in.

The question is how clean is clean enough, and the halacha has a specific answer that is neither "spotless" nor "near enough".

The standard: cleaned the ordinary way

The principle is k'derech hamichabdim — cleaning in the manner people normally clean. You are not required to remove every last particle. A thorough, ordinary cleaning discharges the obligation.

The reasoning matters, because it sets the limit. The Taz explains that this allowance holds where removing the remaining traces is genuinely difficult — difficult enough that leaving them cannot be called deliberate. Where that is so, what remains is disregarded.

Where it stops working

The distinction is between what cannot be removed and what has merely not been bothered with. If someone is genuinely trying to clean and cannot remove everything, that is cleaning in the ordinary manner. If the cleaning is perfunctory because they are content to work with dirty equipment, it is not.

A related implication: the allowance covers residue that is not conspicuous. Material plainly visible on the surface is a harder case than traces that are not.

It is also worth knowing that some are careful not to rely on this allowance at all where a better option exists, preferring the view that it applies only where there is no reasonable alternative.

What this means in a kitchen

• Clean thoroughly, by ordinary means, before kashering anything. Hot water, detergent, a brush where needed.
• Pay attention to the places that hide residue — seams, rivets, handle joins, the rim, the underside of a lid.
• Remove rust and burnt-on deposits. These are not "absorbed taste" that kashering addresses; they are material sitting on the surface.
• If something genuinely will not come off, that is the case the allowance was written for — but ask, rather than deciding for yourself that it qualifies.

How clean your particular item needs to be, and whether it can be kashered at all, is a question for your rav. See also the three methods of kashering.