Can Glass Be Kashered?

Glass is one of the sharper disagreements in kashering, and the reason is that opinions differ about something physical: how much glass actually absorbs.

Two starting positions

One view, following the Ran, holds that glass absorbs very little — less than metal, and certainly far less than earthenware. On that basis glass would be easy to deal with.

Ashkenazi practice, however, treats glass like earthenware, which cannot be kashered at all.

Between those, the Magen Avraham and Shach rule that where there would be real loss, glass may be kashered and used — relying on the view that it absorbs minimally.

So the position is not "glass cannot be kashered". It is closer to "glass is treated strictly, with room where the cost of strictness is genuine".

Glass used on the fire

The harder case, and the one that affects most kitchens: Pyrex and similar, used directly in heat.

One position holds that all opinions agree glass used on the fire absorbs like earthenware and cannot be kashered by any view. Rav Yaakov Kamenetsky held this way — that glass utensils used on the fire cannot be kashered and used even where there would be real loss.

Rav Schachter recorded a memorable dissent, and it is worth repeating because of how it was arrived at. As a college student he asked his chemistry professor whether Pyrex absorbs. The professor's reaction was that the suggestion was impossible: laboratories run experiment after experiment in Pyrex test tubes, and if residue from one experiment remained in the walls, every subsequent experiment would be contaminated. From which he concluded that Pyrex should have the status of ordinary glass in the Shulchan Aruch — and possibly be treated more leniently still.

Two serious positions, reached by different routes. This is a live dispute, not a settled question with an outlier.

Glass stovetops

A practical problem that follows from all this, and where the ruling is fairly clear.

The difficulty with a glass hob is the area between the burners. It gets hot, but not hot enough to be kashered. On a conventional hob that area can be left, because absorbed taste cannot travel from there into the pots. On a glass surface it is continuous with the cooking area.

You cannot simply cover the glass to raise its temperature, because that risks cracking it. Most rabbonim therefore tell people that a glass hob cannot be kashered and the glass should be replaced for Pesach.

An elaborate method was proposed — a foil tent on rocks, all burners at maximum, testing with rolled newspaper until it browns or smokes. It was examined carefully and rejected, on a ground worth understanding: the manufacturer warns against covering the hob because it may crack, and someone worried about destroying their cooker will not persist long enough to kasher properly. That is a recognised reason not to permit a method.

Suggested alternatives — a perforated metal sheet, or a blech raised on trivets to let heat escape — were floated, but the same concern about confidence and safety was not resolved.

One argument worth knowing

There is a striking point in this discussion. Oven walls are porcelain enamel, which is glass-coated steel. If the reasoning that permits treating glass leniently for libun kal were wrong, it would follow that no oven could be kashered with libun kal at all.

That consequence is part of why the lenient reasoning is taken seriously.

What your own glassware and hob need is a question for your rav. See also the three methods of kashering and kashering an oven.