The reason you don’t see a sooty flame with your air/acetylene torch
is that there are the little holes at the base of the torch tip that
correctly mix ambient air into the acetylene pre-ignition. If you
cover those holes with your fingers with the torch lit, you’ll see a
"pure" acetylene flame, which is a white/yellow in color and very
sooty due to incomplete ignition. This technique is actually quite
handy if you want to soot up some steel nails to use as positioning
jigs, or to soot up an ingot mold to prevent the precious metal
sticking to it.
When you first light an oxy-acetylene rig, you’re lighting the
acetylene only, and that torch doesn’t contain the ambient-air holes
that pre-mix the acetylene. Therefore, you get those little sooty
"floaters" that fly around the studio.
That being said, I use an air-acetylene rig for 90% of my
fabrication/soldering (on silver), oxy-acetylene for most of my
casting. Oxy-propane is my choice for reticulation and fine detailed
fabrication work, but I have yet to get someone to come out and
properly install lines from an outside propane tank into my “new”
(remodeled) studio. Soon, I hope!
Hope this helps!
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