Thick soot on brass when soldering with oxy acetylene

Hey Everyone!

I’m trying to sweat solder jewelers brass to sterling silver
(although I haven’t got passed getting the solder to flow on brass
yet!)

I am using the smith little torch (oxy acetylene) and soldering on
one of those lazy suzan type bords, using boric acid and alcohol and
rio’s my-T-flux. But after heating the piece for a short time (less
than a minute) it gets covered in a thick layer of what can only be
described as soot - so of course the solder will not flow. Please
tell me what the heck is going on here!

Thanks very much
Yours very frustrated
Lucy