Another tirck to aid a wide welding seam in platinum is:
As has been stated already, prepare the seam with "V" notches on
either side of the split and have some fill wire (in the case of a 5
x 3 mm band-about 20 gauge round would be appropiate) at the ready.
Now here's the trick: Roll out very thin (about 30 gauge) a piece of
pure platinum, spread open your prepared seam and slip this piece
into the seam. The piece should be sized, in this case, to be about
1mm larger than the metal remaing after preparing the seam with the
notching. As the weld begins, the pure platinum will melt at a very
slightly lower temp than the platinum alloy of the ring allowing a
weld that easily penaetrates to the center of the seam without
notching deeply. Use a slight swirling motion of the flame's cone to
stir the now melting metal for the weld to mix both sides of the seam
and the pure platinum together and continue the weld uising the fill
wire until the notches have been filled all the way around the shank;
I always over fill the joint slightly. If you find any voids in the
weld, fill them before finishing the surface, then use a tungsten
carbide burnisher on the weld to comress the joint and any unseen
voids prior to filing and sanding. Repeat the fill welding as
necessary to eliminate and pits, low spots, etc, then go on to the
usual finishing steps.
Credit for this trick goes to "Doc", United PMR's metalurgist.
Thanks Doc, I've been using it for the past 6 or so years since you
told me about it.
Paul D. Reilly