Using MAPP gas for silver work?

At one extreme is platinum. We play our torch on the joint, the
heat stays near the joint but we need to get it really hot, so we
use a very hot but rather small flame. At the other extreme is fine
silver. The heat from our torch is rapidly conducted away to heat
up the entire work piece. The whole piece gets hot but our solder
flow point is much lower than the platinum example. For the silver
piece we need a fairly large QUANTITY of heat but not a really high
TEMPERATURE; with these requirements a tiny, super-hot flame can
get you in trouble much faster the a larger, cooler flame.
Think separately of TEMPERATURE and QUANTITY, it may help you (I
hope so). 

This is why for any soldering operation on anything short of
platinum or palladium I use a propane-compressed air torch with the
appropriate sized tip (http://nationaltorch.com/3ab.html). Lots of
heat at a manageable temperature. Little chance of firescale, or
burning up all the flux or melting the work. Works from tiny tiny to
teapot sized work. For casting I definitely use propane-oxygen but
you have different requirements in needing to get the metal molten
and cast as soon as possible.

James Binnion
@James_Binnion
James Binnion Metal Arts

360-756-6550

Hi, Jim–

I use a propane-compressed air torch with the appropriate sized tip
(http://nationaltorch.com/3ab.html). Lots of heat at a manageable
temperature. Little chance of firescale, or burning up all the flux
or melting the work. 

This is the first I’ve heard of this torch. Why does it mean there
is little chance of firescale or using up flux?

What is the source of the compressed air? Do you run a compressor?
They are very noisy!

I have three or four torches already, but anything that minimizes
the chance of firescale gets my attention!

Noel

Mr. Binnion, Could you please give us more on use of the
National Torch which You mentioned in your last letter. Specifically
Airpressure and volume required as well as gas types, volumes, and
pressures.

Please include recomendations for torch tip sizes for particular
type jobs. I have a small medical vaccum pump which will put out 10
to 12 psi. at moderate volume. I do not have compressed air piped
into my shop area. I had purchased the torch years ago and never
used it for lack of and the need for a torch of that
size. These torches are ver= y well made and look like they would be
a usefull addition my torch choices. Help me put it to work.

Respectfully Yours,
Robb

This is the first I've heard of this torch. Why does it mean there
is little chance of firescale or using up flux? 

This is the torch I learned to solder with back in high school in
1970. It has been around for a lot longer than that but I have used
it on and off since then.

Flux is acting as a barrier coating to the oxygen as well as a
reducing agent. As a barrier it prevents oxygen from reaching the
metal and as a reducing agent it combines with oxygen in the air and
will also strip oxygen from the oxides on the metal surface leaving
behind pure metals. The problems in using flux come from not getting
a good complete coating to act as a barrier, and in overloading it
with oxygen by having it at heat too long, leaving too much oxide on
the metal surface before applying the flux and having too much
heated oxygen around it. The gas air torch addresses the time at heat
and too much oxygen problems. The flame size from these torches are
much larger in diameter than the oxy-fuel torches used for the same
work. This allows you to bathe the whole piece or at least a sizable
chunk of it in the flame. This heats the entire piece faster so there
is less time at temperature, the large flame if properly adjusted
consumes all the oxygen in its confines so the area inside the flame
is not getting exposed to excess oxygen while soldering. Oxy-fuel
torches are almost always producing an oxidizing environment except
at the very tip of the inner cone on neutral or even moderately
reducing flames. This is because the pure oxygen mixed with the fuel
does not completely combust due to less than perfect mixing and it
therefore will not consume as much of the oxygen from the
surrounding air in the secondary flame as a gas air torch. With gas
air there is just less oxygen present. It is also an improvement over
a naturally aspirated torch (prest-o-lite or bernz-o-matic or little
hand held butane models) because you have total control over the mix
of fuel and air and do not rely on the venturi used by those types of
torch. So if you want a more reducing flame you just turn up the gas
or turn down the air.

What is the source of the compressed air? Do you run a compressor?
They are very noisy! 

Yes you need a compressor, and most compressors are noisy. I have
mine outside the studio for this reason. This is the big drawback of
this type of torch.

I have three or four torches already, but anything that minimizes
the chance of firescale gets my attention! 

Most times I solder with this torch the flux is left as a clear
glassy coating with little if any black or green flux glass. Such
dark coloration is an indication of the flux being overloaded with
oxygen. This clear coating comes off easily and leaves behind a
metal surface that is little affected by the soldering process.

Jim

James Binnion
@James_Binnion
James Binnion Metal Arts

360-756-6550

Noel,

Jim made my point on TEMPERATURE vs QUANTITY in different words,
pointing out that overheating leads to burning up flux, more
firescale and melting the work-piece.

It doesn’t take a lot of air volume to supply our tiny torches and,
in my case a Graver-Max. My compressor is in the garage, about 60
feet of 1/4 inch air hose from the shop. I have a relay switched
outlet for it with the switch in the wall beside my bench. But there
are lots of ways to skin this cat (God help be if Sparkie hears me
say that), such as a portable air tank, a thickly insulated box for
the compressor, etc.

Dr. Mac

It doesn't take a lot of air volume to supply our tiny torches
and, in my case a Graver-Max. 

I bought an air brush for $5 at a garage sale… would the little
compressor that came with that be enough? (never actually tried to
use the air brush… no instructions.)

Noel