Never having worked with platinum before, my question is can it
be silver soldered at all? Will the silver solder adhere?
Yes. It will work just fine.
I'm told that the acetylene flame is rich in unburnt
carbon; and I do remember reading that carbon in platinum is harmful
in some way. What does it do to the platinum?
You won't have to worry about it at silver soldering temperatures.
The problem occurs at platinum melting and soldering temperatures.
As I understand it, the problem is that at thise high temperatures,
carbon is a very good reducing flux, and is able to reduce silica or
silicates in various forms to metallic silicon, which then alloys
with the platinum and causes brittleness. silica or silicates, etc,
are common a jewelers workshop, being in fluxes, soldering boards,
various other items. hard to avoid, and normally harmless. But
with carbon present, it causes contamination of the platinum.
Hope that helps.