if the flame is going to be on the "adhesive" (whatever it is you decide to use) there is nothing safe... all of it would burn with flame (everything from elmers glue to 2-part epoxxy) touching it, and all i would imagine wouldn't be good... so there's not much you can do outside of getting a glass blower to fix it for you... look around on the net for glass blowers.. there are alot out there that have decent reputations and fix glass things for people, i'll find a link later if you can't (if you actually try) and i would imagine it might cost 20-30 bucks to get it fixed, but i'm sure someone would be willing to do it out there... there is no home remedy to smoke out of it safely if flame is going to get near this stuff... i understand why you want to fix it, i have a bong me and my bro used to use all the time, an acrylic one kind of like a hookah... he died, and a year later my other bro foun dit in someone's closet so he stole it back and gave it to me, but all that was left was the main body part of it... the tubes, the top, the bowl, everything was gone... i just keep it hanging around now... pretty much unusable.... i'm sorry to say it, but if you can't get a glass blower to do it for you, there is nothing that will get it back... if you do, make sure you clean it good with alcohol to make sure all resin and residue is gone, just to keep everything legal...