Soldering Platinum or Gold in belt furnace

I have read numerous posts here and find them very helpful as well as
interesting. The reason I write is to ask for any advice on soldering
platinum/gold mountings in a mesh belt SM furnace. I now use it just
for all gold rings, but want to run my pt/gold through it also. The
problem is what type of gas to use as to not embrittle the
platinum.At this time I use a cracked ammonia mix. I tried pure
nitrogen but this did not seem to protect the gold. Any help would be
appreciated.

Thank You
Michael Merritt