Glass, Pyrex and Corning ware

Q: What is the status of Pyrex and Corning ware in regard to kashrut?  Are they like glass?  Or are they more similar to other materials that cannot be “koshered?”

A:  Earthenware is considered so porous that once it is rendered as non-kosher, it cannot be undone.  Earthenware is made of clay.  Yet, that is the same substance that is used in the manufacture of glass and similar substances.  And glass, as most authorities hold, is practically non-porous.

Generally speaking, glass may be koshered and can be used for meat or milk but must be thoroughly scoured beforehand.  Some authorities do not allow for this as it may lead to inadvertent errors.

Pyrex and corning ware are similar to glass in that they are strong, resist absorption and cracking.  Yet, they are somewhat more porous than glass as can be demonstrated by their color deterioration through time.  For this reason these cooking materials should not be used interchangeably for meat and milk  yet can be koshered by haagalah, boiling