The advantage is, according to my reading, that platinum is not
in danger of embrittlement. Acetylene is carbon-rich, and
apparently platinum soaks it up to its detriment.
so far as I know, platinum does not actually soak up carbon. The
problem is that carbon, at platinum soldering or melting
temepratures, is an exceptionally effective reducing agent. And in
our workshops, another material thats very commonly around are
various forms of silicates. That includes the silicon dioxide (fused
quartz) we might use for soldering blocks for platinum, or various
silicates in fluxes (borosilicates), or any of a number of other
possible sources, including even just common house dust. Carbon, at
platinum soldering temps, is able to reduce silcates to silicon
metal. And silicon metal DOES alloy with platinum. Even small amounts
quickly cause brittleness and cracking. This is the reason why some
people report being able to routinely do platinum work with
acetyelene, withough having problems, despite the carbon rich flame.
First, they're generally being careful to use a fully oxidizing
flame, which gets rid of the free carbon. And, usually they're
working in a manner that eliminates the contact of platinum
soldering temps with both platinum and silicates, such as holding the
work in tweezers, instead of on a block, and not using any flux.
You'll find, if you try actually melting platinum in a fused silica
crucible, with acetylene, a situation that really is tempting the
fates, the incidence of contaminating the platinum will go WAY up.
Even that, may be possible if the flame is quite oxidizing.
The same mechanism is part of the reason why it can be quite useful
to get in the habit of cleaning your platinum, via steam, or
ultrasonic, etc, before soldering or melting. It removes things like
fingerprints, oil residues, or other surface contaminants that might,
upon heating, carbonize and give you a source of both silicates and
carbon on the platinum surface.