There was another very interesting comment on this thread about
silicates and carbon, too. The real point, I think, is that we live
in the real world, not a theoretical one. If one could melt platinum
in a vacuum (do-able), not touching anything else, suspended in "the
ether" (not), with electrical heat, it would be perfect every time.
But we, at the bench, need to do it for real, and the metal has to be
in, or on Something, and electrical heat is impractical at the bench.
I use a porcelain "Platinum" crucible - melt in the side-grooves to
get more of an ingot. A charcoal block will trash your metal, and
burn to a crisp, too. Oxy-Acetylene is, above all, the hottest flame
we can get, in practice, and I use all the O2 I can get without
blowing the flame out - hissing, bright blue cone. It's fast
(because slow melting is not good for metal, either), and clean
enough for practical purposes.
A comment to students Re: Which torch? Again, distilled to the
essentials, there are only a few-acetylene/air, oxy/acet.,
air/propane, etc. Just to stress- don't be confused by Brand-Names,
The first choice is what "Type", and then pick a cost/handle/gas/regulator combo.