I am not the most authoritative person out there to answer this
question. (Any takers?)
From what I have learned so far, it is powered solder applied in a
mix of boric acid sprinkled over a dampened assembly. The tops of the
filigree is dried off by blowing on it before the solder is sprinkled
on. This keeps the surface free of obvious solder build up.
I don't know about making filigree in a mold. Everything I have
learned so far suggests filigree is assembled flat then formed into
different shapes. It is amazing how these delicate wires stand up to
the forming process. But they don't stand up unless you have done a
good job with your soldering.
I hope this helps.