Yeah, I don't think Pyrex glass is used for bong making so putting it in a freezer then immediately exposing it to heat is not the bast idea. Kinda tuff, because if it's a glass bong, it'll still be subject to cracking and expansion as opposed to plastic which can bend and stretch. If the crack is not close to any where you'd put your heat source (lighter), I'd use rubber cement. Reason being that as you heat the glass, it'll still expand some. Most epoxys will dry to a hard consistency with very little flex. So as the glass expands from heat, it'll crack more and eventually break from the pressure. Rubber cement will expand as the glass expands and should give you a little more life out of your bong. BUT...it CANNOT be anywhere near a part of the bong that is exposed to flame or gets heated.