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Can glass be koshered for Passover?

Can glass be koshered for Passover?

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Whether glass can be koshered – not only for Passover, but also whether non-kosher glassware can be koshered for year-round use – is the subject of debate among the early halachic authorities.

Many authorities1 maintain that glass is to be treated like earthenware, a substance which is completely un-kosherable. According to them, there is no way to kosher glass.2

On the other extreme, there are those who opine3 that glass is a completely non-porous and non-absorbent substance. As such, it never absorbs any traces of chametz or non-kosher foods, and a thorough washing of its surfaces is sufficient to make it kosher.4

Taking the "middle ground," there are those5 who see glass as a material similar to metal,6 which can be koshered through exposure to high levels of heat.7

Ashkenazi Jewry generally follow the most stringent view, and do not use chametz glassware on Passover, nor do they kosher non-kosher glassware. However, there are exceptions, so consult with your rabbi regarding accepted protocol in your community.

Yours truly,

Rabbi Menachem Posner

Footnotes
1.

Smag, Rama and others.

2.

They offer alternative interpretations for the passage from Avot D'Rabbi Nathan that seems to indicate othewise.

3.

Ran, Rashba, Yosef Karo and others.

4.

This opinion is based on a passage found in Avot D'Rabbi Nathan (41:6) which states that "glass neither absorbs nor exudes."

5.

Ohr Zarua and others.

6.

They deduce this from the fact that the two materials are treated with equal stringency in the Talmud, tractate Avodah Zarah 75b.

7.

For more on the koshering procedure for metal – and other similar substances – see Koshering Appliances and Utensils.

Rabbi Menachem Posner serves as staff editor for Chabad.org.
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Albert April 8, 2015

According to Sephardim, even glass used for cooking, does not absorb the taste of hametz and need only be washed. Reply

Gitta Zarum London, UK April 8, 2015

Isn't there a difference between ordinary glassware and special glass that can be used for cooking? Reply

Gergely Budapest, Hungary August 28, 2013

reply to Spector Shalom!

Indeed, organic molecules could be entrapped between the atoms of glass materials. This is a physical (i.e. revesible) process which does not invole the creation of new chemical bonds. Protein adsorption onto the surface of glasses is well known phenomena e.g. in pharmaceutical technology. The only problem is that proteins exposed to heat change their structure and will no longer able to leave their place that might be one of the reasons for the "non kosherable" status of glass materials. Reply

Spektor August 22, 2013

reply to Gergley If you could have food molecules get in between the very atoms of metal or crystal, wouldn't you be creating a fusion reactor on your stove?

Food could get trapped in cuts or pits in the metal of a pan or glass, but not on the atomic level.... Submolecular comparisons or concerns is taking things way too far... Reply

Gergely Budapest, Hungary August 22, 2013

Structure of glassy materials The Rabbi wrote: "There are those who opine that glass is a completely non-porous and non-absorbent substance"

According to scientific viewpoint glasses are dissimilar to metals. The atoms of a metallic material are tightly arranged into one type of the metallic lattices. On the other hand, molecules of a glass are not arranged into a tight crystalline lattice and they are structurally in the amorphous (or glassy) state and they are certainly able to absorb materials, due to their loose structure. My question is wheter it is possible to clean these materials from contaminants and verify the procedure by modern methods in a laboratory? Is this a possible way of halacha?

Thank you! Reply

Spektor Chicago, IL April 6, 2012

We need a modern, scientific interpretation, pleas Well some materials that appear to be "glass" are actually far more complex materials, and demand a modern update on these old rules. For example, if you can kasher a "glass" electric stovetop (which is really a form or fully crystalized glass or "glass-ceramic") using high heat, then what about other items of like materials that can withstand high heat? The special "glass" windows in self-cleaning ovens for example -- any chametz on them gets incinerated. Likewise, "pyroceram" items like Corningware or Visions cookware. Corningware CAN be placed into a self-clean oven and run through the 1,000 degree cycle without a problem (in my experience). I kasher my Corningware this way for Passover and l feel totally comfortable doing so. It's time for rabbis to really reexamine the issue. It would be a great help to many households! In +50 of Corningware being sold and self-clean ovens, no one has thought of this? I bet a lot of busy Jewish housewives have! Reply

Albert plainview, New York April 5, 2012

glass Acoording to Ramah and to all Sephardim, glass does not absorb and Only needs to washed. That is the Sephardic Minhag. Reply

Lyone Newark, OH April 17, 2011

Glass and Enamel I don't understand why enamel ware is considered non-kasherable? Because enamel *IS* glass that has been high-fired onto a surface (usually metal), and thus is non-porous. Can anyone explain the reasoning? Reply

Moshe April 21, 2009

its a little disengenguous to say Ashkenaz Jewry doesnt kosher glassware, maye not for Pesach, but for the rest of the year, many Ashkenaz Ravs would tell you glass doesnt absorb, including those of the major Kashrus organizations. It would be more accurate to say that some Chassidim dont use "nonkosher" glass. I remember reading Rav Steinsaltz in his book about teshuva says you can just scrub your glassware clean and its Kosher although possibly he wasnt addressing Chabad Chassidim. I personally at home dont use "non kosher" glassware but that said I dont consider it halachich by Ashkenaz standards, and (based on Rav Steinsaltz) maybe not even by Chabad Reply

David Goorskey Fayetteville, AR/USA April 2, 2008

Glass is sometimes porous Glass is often porous near the surface depending on what materials come into contact with the glass. For example, certain acids can render the surface porous. Even water can slowly eat holes into the surface of glass. Bases, however, eat up the whole surface of glass exposing a clean new surface layer underneath. Perhaps glass that has been in contact with chametz could be koshered by washing the surface with a base (such as lye) solution? Visit the Corning Museum of Glass online for more information. Reply

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