...on the cheap.

Grooming blocks - they're normally used for removing loose hair from (horse) numnahs and the like; I decided they look a lot like pumice.

The one I bought is. Sat pointing a MAPP gas torch at it for a few minutes; the spot I torched reflected the heat extremely well. There's some heat transmission across to the other side, so I wouldn't use this directly on a vulnerable surface; however, they are something I'd have no problem digging into to provide support for soldering or the likes.

Or you could break it into lumps for an annealing pan, I guess.