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 cook 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, that is very porous, extremely absorbent and unable to be Kashered, if used in a 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, Rabbinic Administrator of the 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.
Q: Is melmac glass?
A: No, melmac is plastic.
Q: Is china glass?
A: It is customary to consider it as earthenware.
Q: Is corningware glass?
A: No, it is like china.
Q: Can one use the same glass beverage glasses for both dairy and meat meals?
Q: Can these glasses be used for both hot and cold beverages?
Q: Can the same whiskey glasses be used for both dairy and meat meals?
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 the sink, they may be washed together. If the dishes are washed in a dishwasher, the glasses should not be washed together.
KLI RISHON & KLI SHEINI USAGE
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), which is permitted.
Q: Can one use the same glass utensil as a Kli Rishon, to cook and bake both dairy and meat dishes?
Q: Can one Kasher a glass utensil that was used as a Kli Rishon for both meat and dairy dishes?
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 on the oven or range, a Kli Sheini is okay.
Q: Can one purchase and utilize used or antique crystal bowls or glasses?
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. If one places meat dishes directly on the glass turntable, then the turntable can be covered with styrofoam for use with dairy, or vice versa.
Q: If one used the turntable to microwave for both meat and dairy dishes, can the turntable be Kashered?
A: Wash the turntable, and do not use it for 24 hours so it is an Aino Ben Yomo. Then the turntable can be used as it was originally designated.
MEAT / DAIRY MIX-UPS
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 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 Sheilah from your Rav.
Q: Can the same corelle glass counter saver be used for meat and then for dairy hot utensils?
A: Yes, but not a corning glass counter saver.
GLASS TOP STOVES
Q: How do you Kasher a glass top stove?
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 rest of the cooktop presents a serious kashering problem. Since glasstops are made of tempered glass that are not meant to be heated, there is a risk that the cooktop will shatter if it is heated. Therefore, the areas not kashered should be kept clean. One should not put food directly onto this surface. For Passover use, see glasstop kashering section of “The Star-K Pesach Kitchen” article.