FAILURE
First off I pretty much only buy HSS taps. They tend (generally) to be tougher than carbon steel taps.
Today I broke a bottoming tap off in a mold. DANG-IT! At first I just said one of the synonyms of poop over and over a few times. Maybe emphatically with other expletives for emphasis.
Then I figured, "What the heck. The mold is ruined (well not saleable) anyway. I'll try welding a nut to the tap. I thought MIG might be the trick since the wire would poke down inside the nut, but it broke off at the first since of pressure. Then I tried TIG which after I finally quit sticking the nut to the tungsten and regrinding it kinda work, but it also popped off pretty easy. Then I tried sticking little dabs of stainless wire to the top of the tap until there was a little column of material I could slide the nut down over. Let me say this is way beyond my skill level. Then I slid the nut on, and started an arc, added a little material, and let it cool.
It worked... sorta. The column of material stuck to the tap. The nut stuck to the column of filler. The tap just broke off a little deeper.
I drilled it in a different place for handles and added it to my personal collection of molds after stamping an X on all sides to remind me its now a reject. Its replacement is machining right now.
Maybe (but I doubt it) off I had cooled it off slowly in vermiculite overnight and then heated the aluminum mold I might have had a chance, but my gut reaction is the result would have been the same. The tap was just to stuck and it would still just break off deeper.
However, if I had done that with a carbon steel tap I think it might have worked. The tap would have been annealed partially and no longer been as fragile.
I don't know. I just thought the anecdote might be useful to somebody else. It didn't really cost me anything except a few cents worth of filler and a few cents worth of gas to give it a try.