silver wire for interconnect leads?

I found a great thread on this group which produced an informative
article about the use of 20 gsm silver wire for making your own high-
quality audio interconnects.
Without getting into the debate over coathangers versus high-end audio
leads, can anyone tell me why this article stresses using soft silver
wire and says don't use Sterling?
Here in Australia, Sterling wire is at very least a guarantee of
reasonable quality -- how can one be sure that plain 'silver' wire
really is silver without too many impurities? Would anyone know of a
source here for very pure silver wire which would meet this audio
requirement?
Thanks