So for some reason (idk why, just randomly came off when i took it out of the sleeve) the top part (the “knob” of a BT37 lamp) of my 400 watt mercury vapor blacklight broke off, leaving the top of the arc tube exposed. Since the broken part is all in one piece and the woods glass is very thick, would it be possible to use some heat resistant glue to put the lamp back together? I plan to get a new lamp anyways since the UV output is getting lower but in the meantime.

I don't think glue will work because a 400 watt mercury lamp generates a lot of heat. If the arc tube is still usable, why not try fitting it in a halogen fixture? I made a remotely-ballasted 300w mercury vapor floodlight using an old AEL decorator halogen floodlight. I recommend using ONLY a halogen floodlight designed for the wattage though as it can be a fire hazard if the arc tube is too big. Note: This only works for "type-R" HID lamps because the open-rated ones will self-extinguish. The arc tube came out of a broken HF-300x bt-37 mercury vapor lamp.

See here: https://www.lighting-gallery.net/gallery/displayimage.php?album=7721&pos=2&pid=228705

So for some reason (idk why, just randomly came off when i took it out of the sleeve) the top part (the “knob” of a BT37 lamp) of my 400 watt mercury vapor blacklight broke off, leaving the top of the arc tube exposed. Since the broken part is all in one piece and the woods glass is very thick, would it be possible to use some heat resistant glue to put the lamp back together? I plan to get a new lamp anyways since the UV output is getting lower but in the meantime.

Special sorts of glass are always tricky in the work, sometimes giving nasty surprises, and the inside surface of the top part might get nicked by a support frame. Glass tubing is very much weakened by the scratches on the inside. No way to use any organic glue as the tube will be too hot. Some silicone could probably survive for a limited time, but I am not sure. Also a loop of a gold kapton tape may temporary fix this.

I don't think glue will work because a 400 watt mercury lamp generates a lot of heat. If the arc tube is still usable, why not try fitting it in a halogen fixture? I made a remotely-ballasted 300w mercury vapor floodlight using an old AEL decorator halogen floodlight. I recommend using ONLY a halogen floodlight designed for the wattage though as it can be a fire hazard if the arc tube is too big. Note: This only works for "type-R" HID lamps because the open-rated ones will self-extinguish. The arc tube came out of a broken HF-300x bt-37 mercury vapor lamp.

See here: https://www.lighting-gallery.net/gallery/displayimage.php?album=7721&pos=2&pid=228705

could you stick it in a 400 watt cobrahead? The glass refractor would block the UV in the same way as the floodlight glass.

I think it would work in a cobrahead, but if the cobrahead isn't vapor tight like a floodlight, it will emit a lot of foul-smelling ozone in the air.