Ok, so my Luke Wilson 15 Arm Tree seems to actually have a hairline crack forming around the join between the mouthpiece tube and the 'can'. So I was referred by the eminently helpful @Frederick McGuire to an Aussie Glassblower, Veronique Southand - who suggested that for this purpose, it is better to use a product called Hextal. Hextal is used for museum restorations of cracked/broken ceramic/glassware. It is the stuff you can inject into a windscreen crack that will fill it in and dry hard. It is a clear epoxy which dries clear and doesn't yellow. It is apparently safe for being used in glassware (although I would not use it on a GonG or something at the start of a hot vapor path without consulting someone in the know as to whether this is safe). It can be used to stick together a part that has outright broken off the rest of the glass piece, or to fill in a gap so that it is invisible and strongly bonded. See the following for more info: http://hxtal.com.au/hxtal-usage-instructions/ Disclaimer: I have absolutely no financial interest in Hextal.