Hi Todd the teacher, May I comment on this as well as Palonso? If I
understand you correctly, are you fluxing before plastering? If so,
this is the source of your difficulty with getting the solder to run.
The wet plaster will always contaminate the flux coating.
I use this method of holding components frequently when soldering and
all I do is roll a thin “snake” of plasticine and lay it along the
joint before setting it up in plaster. When the plaster (I use casting
investment) is dry, I simply peel away the plasticine, clean any
residue with an old toothbrush and a little lighter fluid, and my join
is ready for soldering without tears. I use this method, where
appropriate, on silver to platinum without problems.
Of course I clean and prepare the joints before setting up in
plaster. One other tip, I found that a lightly filed surface soldered
more reliably than a surface that had been buffed with emery then
soldered. I always seemed to have pit-holed solder joins on emeried
surfaces and suspected that tiny grains of emery were the culprits.