Shabbat 16A

לא ליטמו מגבן. אלמה תנן: כלי חרס וכלי נתר טומאתן שוה: מיטמאין ומטמאין מאויריהן, ומיטמאין מאחוריהן ואין מיטמאין מגביהן, ושבירתן מטהרתן. כלי נתר וכלי חרס הוא דטומאתן שוה אבל מידי אחרינא ⁠— לא! אמרי, כיון דכי נשתברו יש להם תקנה, שוינהו ככלי מתכות.

they should not become impure from their outer side. Why, then, did we learn this in a Mishnah? With regard to earthenware vessels and vessels made from natron [neter], the halakhot of their impurity are equal in that they become impure if a primary source of impurity enters their airspace, and, once impure, they render food that enters their airspace impure from their air space. And they become impure from behind, i.e., if a primary source of impurity enters into the bottom of the vessel, where there is an empty space and a receptacle, the vessel becomes impure. However, earthenware vessels do not become impure from their outer side, i.e., if a primary source of impurity came into contact with the outer side of the vessel, the inside of the vessel does not become impure. And the breaking of earthenware vessels renders them pure. By inference, specifically natron vessels and earthenware vessels are those whose halakhot of impurity are equal, as is their status. However, with regard to other matters that is not the case. Why, then, were glass vessels not listed together with those vessels? The Talmud answers: Since if the glass vessels broke they have the capacity to be repaired, as the glass can be liquefied and recast into a new vessel, the rabbis equated them to metal vessels that can also be liquefied and recast.

אלא מעתה, יחזרו לטומאתן ישנה ככלי מתכות? דתנן: כלי מתכות, פשוטיהן ומקבליהן טמאין. נשברו ⁠— טהרו. חזר ועשה מהן כלים ⁠— חזרו לטומאתן ישנה. ואילו גבי כלי זכוכית תנן: כלי עץ וכלי עור וכלי עצם וכלי זכוכית, פשוטיהן טהורין ומקבליהן טמאין. נשברו ⁠— טהרו. חזר ועשה מהן כלים ⁠— מקבלין טומאה מכאן ולהבא. מכאן ולהבא אין, למפרע ⁠— לא!

The Talmud asks: But if so, if glass vessels were truly equated with metal vessels, then broken glass vessels that were liquefied and recast should reassume their previous impurity, like metal vessels. As we learned in a Mishnah: Metal vessels, both their flat vessels, which have no airspace, and their receptacles, which have airspace, are all impure if they came into contact with a primary source of ritual impurity. If they broke, they thereby became purified. However, if one remade the broken vessels into new vessels, they reassume their previous impurity. While, with regard to glass vessels, we learned in a Mishnah: Wooden vessels and leather vessels and bone vessels and glass vessels, their flat vessels are pure when they come into contact with impurity, and only their receptacles are impure. If they broke, they thereby became purified. However, if he remade the broken vessels into new vessels, they can become impure from that point, when they were recast, forward. By inference: From that point forward, yes, they become impure; retroactively, no, they do not reassume their previous impurity. Apparently, there is no halakha of previous impurity as far as glass vessels are concerned.