Mastering the behavior of your solder comes down to understanding how to combat oxidation under the flame. The moment precious metals are heated, oxygen goes to work creating firescale, which acts as a barrier that blocks your solder from running. To beat this, always apply a crisp layer of professional liquid or paste flux .

Applying your flux evenly over a clean seam serves as a protective shield against oxygen, dropping the surface tension so your silver or gold solder can pull neatly through the joint.
Pair this with a fine-tipped soldering pick to move your solder pieces precisely into position under the flame, and you will achieve perfectly invisible seams with half the cleanup time.