Lisa, Paste solder is the way to go with PMC.
I think your PMC tutor is right on the money.
By preparing your PMC piece properly, the paste solder is able to be
utilized in the two formulas that have the lowest melt and flow
temperatures. By having two different flow temperatures you can
solder on more than one item to the piece.
The paste allows you to place the solder just where you want the
join. It is precise in application. Sort of a point and shoot
situation (thanks Kodak). The paste contains the flux, there is no
need to more flux. With fired PMC, which is very porous, this is
an advantage; the PMC would need lots and lots and lots of flux to
make another type of solder work as well. As I said before, the
proper preparation of the PMC surface is paramount to getting a
good solder join, as well as using the proper paste solder. There
are two ways to prepare your PMC surface for soldering. The first
is to think ahead when you are making the item from the clay. If
you know where you will be putting your findings, simply imbed a
very small piece of fine silver sheet into the clay where you will
want to place your findings; it should be big enough to hold the
base of what you want to solder on and then just a little bit.
This small piece of fine silver will become the base of the item
being soldered on. It gives the findings a stable foot hold so it
can be soldered easily.
If you change you mind in midstream there is still a way to prepare
the fired PMC for soldering. Burnish the area where you wish to
attach the findings. This tightens the pores. This area will now
absorb less solder and have a more stable base for the findings to
be attached with paste solder. The more you can burnish the area,
the easier it will be to get the solder to work. PMC, once fired,
is a very porous metal. You have taken the clay, burned out the
binder and left spaces where the binder used to be. It is still
metal, you just have to recognize the porosity of the item created.
Anything you can do to tilt the odds in your favor should be done
when working on your project.
Another item to keep in mind is that if you overheat the PMC piece
with a torch, it may collapse. The temperatures needed for the
higher melting temperatures of solder can be very tenuous. You
need to have the lowest melting temperature of solder that is
available in silver. The extra easy (#56) and the easy (#60) are
totally acceptable to be used on PMC. Paste solder will be shown
at the PMC conference next month in New Mexico. There are several
classes that are being offered on soldering and PMC.
New materials offer new challenges and rewards. I believe you have
to reevaluate your ideas about paste solder. Please see web site
for more on PMC and soldering.
Unique Solutions, Inc.
Paste and Powder Solder for Jewelers and Metalsmiths