Question: Can the same set of glass dishes be used for both meat and milk foods?
Answer: The prohibition of mixing meat and milk is based on a verse from the Torah: You shall not boil a kid in its mother’s milk. [Ex 23:19; Lev. 34:26; Deut. 14:21] This verse is repeated three times – from which the Rabbis derived that there are three different prohibitions against mixing meat and milk: it is forbidden to eat meat and milk together; it is forbidden to cook meat and milk together even if you don’t eat it; and it is forbidden to profit from cooking meat and milk together, even if you are not the one doing the cooking or consuming the meal. This is one of the strictest prohibitions in the Torah.
The Rabbis understood that cooking utensils absorb the flavour of the food that is cooked in them. The reason for maintaining separate dishes and utensils for meat and for milk is to prevent any possible mixing of the flavours of meat and milk. If the same utensil was used for both meat and milk, it would inevitably lead to a transgression.
The Rabbis also understood that materials absorb and release flavours differently. This idea is deduced from a passage in the Torah: when the Israelites captured the Land of Midyan, they were commanded to purify the utensils they had taken. Moses commands the Children of Israel: Any article that can withstand fire – these you shall pass through fire and they shall be clean … and anything that cannot withstand fire you must pass through water. [Num 31:23] Based on this statement, intricate procedures were formulated to ‘kosher’ dishes that have become forbidden.
The status of glass is unique. Avot D’ Rabbi Natan, an early Tana’itic source, states that a glass vessel doesn’t absorb and doesn’t release. [Version A, Chapter 41] The majority of the Poskim consider glass to be completely non-absorbent. If we follow this reasoning, a glass dish can be used for both milk and meat because no flavour can be transferred. It is sufficient to give it a good wash between uses. This is the opinion of Joseph Karo in the Shulchan Aruch [OC 451:26], and is the standard practice of Sefardi Jews.
However, Moses Isserlis, [Poland, 16th century] when recording the Ashkenazi custom wrote: there are those who say that glass cannot even be [koshered by] immersion in boiling water, and this is the custom in Ashkenaz and in these lands. The Ashkenazi custom equates glass to earthenware, because glass is made from sand. Earthenware can never be koshered because it is fragile and would break if placed in boiling water. This is also a verse in the Torah the specifically says earthenware can’t be koshered. An earthen vessel in which it was boiled shall be broken … [Lev 6:21]. Therefore, Ashkenazi communities do not Kosher glass, and insist on separate dishes for meat and milk.
Finally, there is an Aggadic Source [Yalkut Shimoni, Esther 3] which mentions washing a vessel three times as the appropriate way to Kosher glass. Therefore, some authorities allow glass to be koshered if it is left to soak for three days, and washed each day.
When there are multiple established customs we follow the principle of Nahara Nahara u’Pashtei – literally, ‘each river flows down its own course’. Therefore, we do not affirm either practice, and the members of each community should follow their communal custom. On a practical note, when catering for a mixed public from different communities, one should have separate dishes for meat and milk. However, if glass dishes are inadvertently confused, one can take a lenient view when koshering them for reuse.
Rabbi Chaim Weiner
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