There are other alloys of platinum that are more likely the culprit.
Most likely you are running into pt/cobalt alloys. Some companies
have totally switched to pt/cobalt; Stuller is an example. Another
difficulty is palladium, though this is pretty rare. My
recommendation is to change from welding to the plumb platinum
solders put out by Precious Metals West or Hoover and Strong (I like
pmwest's product a little better myself, but they are both good)
The easy solders are great on the pt/cobalt alloys (and lower
melting temp 900 pt alloys) and even the medium solders work OK, but
if you don't have a lot of experience with using the plumb solders
you may have more problems in regards to melting so take a little
more care. The hard solders are great for the other platinum group
alloys that have higher melting temps like 950pt/iridium,
950pt/ruthenium and 999pt.
One trick with the plumb pt solders is to not use a solder pick in
the traditional manner. Instead, place a little spittle on the seam
with a clean pick then, with some saliva still on the pick touch the
pallion of solder and place it on the seam. Heat the seam very
slowly until the saliva has evaporated and then solder as usual.
The plumb solders just don't work well with a solder pick.
Filing and sawing, or other techniques that require a sense of how
the different alloys feel are helpful ways of determining the type
of alloy, but they require a lot of practice and discrimination, but
this will probably work most of the time: First heat the shank to a
high red heat and let it air cool (protect any diamonds with boric
acid, don't get it on the area where you are going to solder). If
it is pt/cobalt it will oxidize, if it is palladium it will oxidize
too so you still need to take some care, though palladium has a much
lower temp and is likely to melt. The other, nonoxidizing alloys
are a little more difficult to determine so If I wasn't sure I would
use the easy plumb platinum solder and not risk it. After your
soldering is done, recoat the entire piece with boric acid and heat
to a very light red. After cooling, pickle the item and the
oxidation will be removed.
The S+ alloys from Hoover are also tricky to determine if they are
not stamped S+, but you can detect them by the way they file; they
are not as "sticky" as other platinum alloys and in fact feel much
more like gold alloys except in it's heft. S+ alloys also have a
lower melting temperature and require the use of the easy pt solders
and the refluxing/pickling steps. These alloys may also have been
heat treated so care must be taken to note if the item seems to have
softened significantly after heating. Heat treating is a whole
other subject that I won't go into, but be aware that it can be a
I've not worked with the newest pt/au alloy, but it sounds fun.
The bottom line is that the use of platinum is prolific and has
never been more rewarding or difficult to work with. Those who
choose to learn the ins and outs of this metal, can really demand
good pay; there's always someone out there who will butcher it so
specialists will be more and more in demand.