A Crystal-Clear Halachic Approach to Glass

Published Spring 2016

Glass is one of nature’s most versatile products created from some of nature’s most prevalent raw materials: sand, soda and lime. In the food industry, glass applications are extremely diverse. Glass can be made into delicate drinking glasses, as well as tough heat resistant ceramic cooktops tops withstanding temperatures over 1000°F.

How is glass made? Basically, the raw ingredients are heated and melted in a large furnace. The molten glass is shaped, blown, or pressed into its desired shape. The finished product is then annealed in an annealing oven and tempered to give the newly formed glass strength and durability.

Although glass can be made to be stronger and less porous than steel, the halachic status of glass remains enigmatic. On the one hand, Chazal recognized the fact that glass is tough, resistant and non-porous. On the other hand, glass raw materials are the same as earthenware which is very porous, extremely absorbent and unable to be kashered in a normal kashering fashion if the vessel was used in a hot non-kosher food application.

How does Halacha treat glass dishes and utensils? Below are a series of questions and answers about glass posed to Rabbi Moshe Heinemann, shlit”a, Rabbinic Administrator of STAR-K.


Q: There are many varieties of glass on the market. Do Arcoroc, Duralex, Pyrex, Corelle and crystal have the halachic status of glass?
A: Yes, they do. However, please note that Corelle mugs and some Arcoroc dinnerware are ceramic.
Q: Is Melmac like glass?
A: No, Melmac is plastic.
Q: Is china like glass?
A: It is customary to consider it as earthenware.
Q: Is Corningware like glass?
A: No, it is like china.
Q: Do display mirrors have the halachic status of
glass when used for food?
A: Yes.


Q: Can one use the same glass beverage glasses for both dairy and meat meals?
A: Yes.
Q: Can these glasses be used for both hot and cold beverages?
A: Yes.


Q: Can the same whiskey glasses be used for both dairy and meat meals?
A: B’dieved


Q: Can these beverage glasses be washed together with the regular dishes after the conclusion of the meal?
A: If the dishes are hand washed in a dish basin, they may be washed together. If the dishes are washed in a dishwasher, the glasses should not be washed together.


Q: What constitutes Kli Rishon & Kli Sheini?
A: A utensil that is used for cooking, baking or roasting on the fire or in the oven is called a Kli Rishon (i.e., literally the first utensil). This would not be permitted for both dairy and meat. The next utensil into which hot food is transferred is called a Kli Sheini (i.e., literally the second utensil), may be permitted under certain circumstances.
Q: Can one use the same glass utensil as a kli rishon, to cook and bake both dairy and meat dishes?
A: No.
Q: Can one kasher a glass utensil that was used as a kli rishon for both meat and dairy dishes?
A: No.
Q: Can other glass dishes, such as salad bowls or casseroles, be used for both dairy and meat meals?
A: If the food is cold, or the glass dish is used as a kli sheini, it may be used for both dairy and meat meals. Unless it is used in the oven or on the range, a kli sheini is okay.


Q: Can one purchase and utilize used or antique crystal bowls or glasses?
A: Yes.
Q: Do antique glasses require kashering?
A: In pre-war Europe, where glass was expensive and hard to obtain, it was customary to kasher drinking glasses by immersing them in cold water for three periods of 24 hours. This is accomplished by submerging the glasses for one 24 hour period. The water should then be emptied, refilled, and allowed to sit for another 24 hours. This kashering method is known as miluy v’irui. In general, kashering glasses using the miluy v’irui method is recommended only in cases of great expense.
However, nowadays the custom is not to kasher antique glasses if they are to be used only with cold liquids.1
Q: Can one purchase used or antique crystal whiskey or wine bottles?
A: No, it should be avoided.


Q: Can one use the same glass turntable in a microwave oven for microwaving both meat and dairy dishes?
A: No, it should be avoided. The turntable can be covered with styrofoam – one for meat and one for dairy.
Q: If one used the glass turntable to microwave for both meat and dairy dishes, can the turntable be kashered?
A: Not recommended.


Q: If someone poured hot milk on a cold meat glass/Pyrex utensil or hot meat on a cold dairy glass/Pyrex utensil what should be done?
A: The dishes should be washed off with soap and water and not used for 24 hours. The glass/Pyrex utensil can then be used as it was originally designated.
Q: If someone poured hot milk on a hot meat glass/pyrex utensil or hot meat on a hot dairy glass/Pyrex utensil, what should be done?
A: Ask a shaila from your rav.


Q: Can the same Corelle glass counter saver be used for meat and then for dairy hot utensils?
A: Yes, but the glass has to be cleaned of any residual spills. A Corning glass counter saver should not be used for both meat and dairy hot utensils.


Q: How do you kasher a glass stovetop?
A: The elements of the stove should be turned on until they come to a glow. The glass burner areas are now considered kosher. However, the remaining areas that do not get hot are not kashered. Real kosherization can be accomplished by holding a blowtorch over the glass until it is hot enough to singe a piece of newspaper upon contact with the glass. However, this may cause the glass to shatter and is not recommended.

The only practical method of kashering a glasstop that requires kosherization (i.e., when one moves into an apartment with a glasstop) is to keep the non-kashered areas clean. If there is a spill, the burner areas will burn off the spill and the burners will automatically be re-kashered. The non-burner areas should be cleaned from any residual spill so that it will not ‘assur’ a utensil that was placed on the surface in error.

1. See M.B. OC 451:26 and M.B. OC 151-157.