Commentary for Avodah Zarah 66:6
א"ר יוסנא א"ר אמי כלי נתר אין לו טהרה עולמית מאי כלי נתר א"ר יוסי בר אבין כלי מחפורת של צריף
R. Yosna said in the name of R. Ammi: A vessel of natron can never be purified (i.e. koshered). What is a vessel of natron? R. Yose b. Abin: A vessel made of crystals coming from a mine of alum.
Daf Shevui to Avodah Zarah
Natron seems to be sodium carbonate. It was mined in places that also had alum. Vessels made of this material cannot be kashered for they would dissipate in water.
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