Shabbat 16 - Glass vessels, what are they?
In connection with the preceding discussion of the laws of purity, the Talmud mentions the purity of glassware. Since glassware is not mentioned anywhere in the Torah, it should not be susceptible to ritual impurity. However, the Sages did decree such impurity. Why? Rabbi Yochanan quoted his favorite student Resh Lakish: "Since its formation is from sand, the Sages placed it in the same category as earthenware - which is mentioned in the Torah and is susceptible to impurity, though only from the inside."
But if that is true, glassware should not become impure when a source of impurity touches it on the outside, just like earthenware - and we know that it does!" - Glassware differs from earthenware in that if it is broken, it can be melted down and refashioned, which is not true of earthenware. In this it is similar to metalware, which does accept impurity when something touches it from the outside.
If glass is indeed similar to metal, then flat glass utensils should accept impurity just like flat metal utensils do - and we know that this is not true about glass!? - The Sages, when they gave impurity to glassware, wanted to make it different from metal, to remind people that this impurity is only decreed by the Sages, and that when an impure glassware utensil touches ritually pure foods, these foods should not be burned on account of this. The Sages therefore gave glass the laws that are somewhat different from the laws of metal, to serve as such a reminder.
Art: Osias, the Elder Beert - A Still Life Of Three Wine Glasses