I have had trouble with sterling billets (small round bars), but
with 18ct gold I have had no troubles at all. When the metal first
becomes liquid I add a small pinch of borax. Literally a half pinch.
I mix the metal with a carbon rod, and most of the borax sticks to
the rod. I heat until the metal will remain molten for a second after
the heat is removed. I tend to pour the through the torch flame into
the mould. I make round billets (19g, 3 inches long) and then use the
rolling mill to make them square. Now I don't get borax includions
very often, but when I do, they all roll out in the mill, but as I
said I'm going from round to square, so by the time I have the
largest square billet, I have flattened all of the surfaces.
I used to get borax inclusions for 2 reasons. Firstly, I was using
too much borax, and secondly I didn't stir the molten metal with a
carbon rod. Now on the borax note, when I was using too much borax,
most of it would solidify in the crucible, and so I had to chip out
most of it before the amount of borax added to the melt mattered at
all to the inclusions in the cast.
P.S. I offer these sugestions as an amateur and have no professional
training whatsoever in this art....