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Thick soot on brass when soldering

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Hello Lucy,

I’m just back from vacation and still getting caught up on my Orchid
so sorry for the late response.

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! 

This happens all the time in my beginning soldering classes. With a
fuel oxygen torch, such as yours, you have the ability to produce
three types of flames.

Reducing - the coldest flame, more fuel, less oxygen, has a feathery
less defined cone with A LITTLE** yellow at the end of the flame.

Neutral - equal parts fuel and oxygen, a defined flame with defined
inner and outer cones. Most common flame you’ll use.

Oxidizing - the hottest flame and very concentrated heat, especially
with acetylene. More oxygen than fuel, very small defined inner and
outer cones, more bright neon in color than the reducing and neutral
flames, typically has an audible “hissing” sound. You’ll almost
never need this type of flame.

Your problem is you have too much of a reducing flame, not enough
oxygen is being fed into the torch tip. Increase your oxygen to a
neutral flame or a reducing flame with only a HINT** of yellow. If
you still have issues brining the pieces up to temperature then
increase the tip size to compensate for a cooler reducing flame, and
remember, larger tip means a larger flame so don’t freak out.

For your work keep the flame adjusted in the neutral area and for
large items that require more time to heat a reducing flame will
most likely be best, especially for long joins.

**I apologize for the CAP’s but I can’t stress these points enough.

Feel free to call me anytime.


Thackeray Taylor
Rio Grande Technical Support
505-839-3000 ex13903