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Casting with an acetylene/air torch


#1

Hi All,

I’m a jeweler and metalsmith working in Brooklyn, NY. I’ve been
teaching a casting class for some time, using an oxy/acetylene torch,
pretty sure it’s a smith. Generally, we use a bronze alloy for vacuum
casting, which I know has to cast at around 1900 degrees F (with a
lower melting temperature at around 1600 F). It works well with our
largescale torch, melting within about 5 to 8 minutes, depending on
scale. Recently, the studio hired an outside consultant, who has
insisted that they switch to acetylene and air. I have some qualms
about this, not the least of which is the greater toxicity with which
this combination burns, but more importantly I’m unclear as to
weather or not this torch will be hot enough to melt the bronze I use
in casting class. According to the web, most torches burn at above
this temperature but when I was in school we used specifically and
ONLY oxy-acetylene because it was the only fuel combination, we were
taught, that was really hot enough to cast with. Is this still the
case, and if so, what is the temperature difference? Are there other,
lower-melting-temperature metals I can cast using this new torch? It
sounds like I’m stuck with it, either way. Let me know, thanks orchid
forum!

Max


#2

I know of a major art school whose metals department went from a
major thing to zero in about a year, and the head of it got major
health problems, from consultants. Just like the recent “you can
anneal silver only 5 times” thread, you need to choose who you talk
to carefully. Your school, and your consultant, are trying to save
money. There is no other real explanation for such a bone-headed
thing as you wrote. I’d just shine him on - ignore him completely if
you can, as he knows nothing whatsoever about the subject of casting
bronze. If your front office isn’t smart enough to know better, then
you have a problem and that’s going to be tough. It might be, “Well,
what did we pay him for?” to which you will have to ask, “Yeah, what
DID you pay him for?” It weren’t broke, what are you trying to fix?
Silly, stupid bureaucrats…:slight_smile:


#3

Hi Max.

I have been casting with LPG (Propane Mixture as used in BBQ
Bottles) and Compressed Air (Supplied from an Air compressor rated at
18 CFM at 100 PSI) for the past 15 years. This combination was chosen
due to the volume of metal cast and the high cost of Acetylene and
compressed Oxygen in Australia. I usually cast 4 times a week and
melt between 300 to 500 Grams of Silicon bronze at a time, 300 to 500
Grams Nickel Silver and about the same with Sterling Silver and
Brass. This torch combination is a little slower to melt the volume
of metal but has the added bonus that the actual flame temperature
not the Adiabatic flame temperature as listed below is about 1300 Deg
C while the actual flame temperature of the Oxy/Acetylene temperature
is about 2000 Deg C, I guess you are now thinking but the other setup
is hotter why not use the Oxy/Act combo, Well the main reason is that
with the LPG and Air Combination dramatically reduces the risk of
overheating the metal.

Super heating the metal increases the metals solubility for Hydrogen
and Oxygen which is one of the main causes of Gas porosity in
Castings and also causes Hydrogen Embrittlement, Keeping the metal
temperature as low as possible and not super heating it more than 50
to 150 deg C past it’s melting point is good melting practices for
sound castings. Hydrogen is always present in the Combustion of any
of the fuels listed in the table below it will be present as H2 and
H2O (Water Vapour) and in some instances as monatomic Hydrogen as
well as Oxygen as O2 or also H2O.

Adiabatic flame temperatures

In the above two diagrams we can see the dramatic increase in the
solubility of hydrogen in Nickel, Iron and Aluminium as the
temperature increases, other metals behave in much the same way.

Another Plus for the LPG and Compressed Air is that no soot is
produced as with the Acetylene on ignition plumes of black soot
everywhere.

You stated that your were concerned about the toxicity of Acetylene
and Air, I am a little unclear as to what the concerns are, the
products of combustion is much the same for both Oxy/Act and Air/Act
?

I would be much more concerned with fumes given of the metal such as
Zinc (Turns into a Gas at around 900 Deg C) and Cadmium (700 Deg C)
and other low melting point additives to the Bronze than the
combustion products either way you should have extractor fans and
extractor hood when melting metals.

Acetylene C2H2
Methane CH4
Ethane C2H6
Propane C3H8
Butane C4H10
Pentane C5H12
Hexane C6H14

PS Purchase a Larger Torch than you need as it is easier to hit
lightly with a big hammer than to hit Heavy with a light hammer.

Regards Michael W Kohlleppel


#4

Hello Max,

I use an air acetylene prestolite torch for casting silver and gold
with a vacuum. I find that a couple things help speed up the
process… casting smaller pieces goes faster and heating the
crucible before heating the metal also speeds up the melting time. I
think it will take longer than oxy acetylene, but probably work ok.
Hopefully, you have good ventilation for casting because I think
that is important when working with brass/bronze- well all metals. Is
there any way you can test the equipment before you have to use it
for teaching?

I have a smith little torch oxy propane that has a casting tip and it
can not keep up with my prestolite. I run out of oxygen after a
couple of casts and it take a lot longer. Sometimes, though, I have
used both torches when I am low on acetylene. I think full(er) tanks
heat much better than tanks close to E. Hope you have casting
success,

Melissa


#5

Hi Max,

Several questions:

(A) why’d they go hire an ‘outside consultant’?
(B) what are the consultant’s credentials?
© What field are they most experienced in?
(D) What reasons did the consultant give for insisting on switching
to an Acetylene/Air torch?

The reason I ask these things is because I’m suspicious about how
much they know about jewelry casting processes.

The short form is: No, your typical acetylene/air torch setup
(Prest-o-lite or similar) won’t work for casting anything serious.
They burn at about 2600F, and just don’t supply enough volume of
heat to

get a larger sized melt properly molten.

You can cast an ounce or two of silver with them in a pinch, but it
won’t be fun. It’s easier if you’ve got two going at once. (The
issue isn’t really the ultimate temperature of the flame, but the
sheer quantity of BTU’s the torch can put out isn’t high enough for
larger melts.) For reference, I’ve seen temp values for
Acetylene/Oxygen torches at anywhere from 3300F, to 6300F. Much,
much higher. How big are your normal bronze melts? Depending on torch
tip, you should be able to get 6-8 ounces of bronze molten in 2-3
minutes. Sometimes much less. The 5-8 you mentioned seems a bit
long.

If you really want to get their hair standing on end, recommend an
oxy/hydrogen torch. Works great, and does a wonderful job on the
metal, but the words ‘hydrogen torch’ do tend to make the lawyers
sit up and pay attention.

Actually, they really do do a great job of casting. They’re what a
lot of the professional casting houses use. Ask the consultant why
they didn’t recommend switching over to a hydrogen/oxy torch. The
answer may be enlightening.

Hope you’ve got some way to keep your normal torch. You’re going to
need it.

Regards,
Brian


#6
The short form is: No, your typical acetylene/air torch setup
(Prest-o-lite or similar) won't work for casting anything serious.
They burn at about 2600F, and just don't supply enough volume of
heat to 

You are off by close to 1600 degrees.

Here are flame temperatures for many common fuel gases the first
number is with oxygen the second number is with air.

Hydrogen - H2 5,082 F 3,831 F
Acetylene - C2H2 5,678 F 4,220 F
Methane - CH4 5,037 F 3,548 F
Propane - C3H8 5,111 F 3,622 F
Butane - C4H10 - 3,578 F

While O2/fuel gas would be better a prestolite style torch while not
ideal will work fine for some casting jobs.

Jim

James Binnion
James Binnion Metal Arts


#7

Hi Max,

A couple of further thoughts. My last reply was at 2AM after a long
day, so it missed a couple of things.

(A) If they’ve suddenly discovered that oxygen breathing torches are
the root of all evil, you can still melt without using a torch at
all. Get them to spring for an electro-melt furnace. (Induction
furnace) They look like overgrown thermos jugs, and can pour molten
metal directly into a vacuum casting flask. Work great. Slower than a
torch, but if all you’ve got is a B-tank torch, it’ll work a lot
better, and you could argue that it’s safer. (That’s debatable, but
admin types will go for it.)

(B) A lot of the bigger casting houses used to use H2 torches. Now
the serious players have switched to induction melt in inert
atmosphere, which is a whole different ballgame.

For whatever that was worth.
Brian


#8

Hi Michael,

What are you melting? I melt gold silver, copper, bronze etc with
straight propane, and naturally aspirated air…

I don’t melt hideously large quantities of metal, a kilo of bronze
in 10-15 minutes from a cold start. That’s with the mid sized
furnace. The micro furnace only takes a thimble size crucible, but
will melt any metal in 3-5 minutes, also straight propane with
naturally aspirated air.

I’m currently making a burner for a Mario di Mayo I picked up cheap,
so may be melting much larger quantities of bronze soon :slight_smile:

Do you find that the compressed air helps significantly?

Regards Charles A.


#9
You can cast an ounce or two of silver with them in a pinch, but
it won't be fun. 

Interesting this negative statement about Prestolite and other
acetylene air torches. I have used nothing but a Prestolite for over
35 years and routinely cast and pour ingots of 3to 5 oz of sterling,
18kt and 14kt gold with no problem. I just wonder what I have been
doing wrong all these years. Admittedly I would not want to melt a
10 oz or kilo size or plat. casting, but for a small shop that is
working in 3-5 oz lots the Prestolite and other air acetylene
torches are quite satisfactory and very versatile, with the various
tips available. They are also a relatively inexpensive torch for the
small shop or beginner and with the appropriate tip can be used for
soldering and annealing not only small items but larger vessels as
well. My from my 35 years of experience and use.

Frank Goss


#10

Hi Frank,

This is going to be shorter than normal, I’m at a show, tapping this
out on an iPad.

Prestolites are fine torches, I’ve had one for nearly 30 years, and
its still my “go to” torch for generic heating and larger scale
soldering. Mine’s a Goss, weirdly enough.

But I don’t use it for casting. It just can’t put out enough heat,
fast enough for larger (300-400 Gm ) melts, especially in bronze.
Yeah, it may eventually get them molten, after 10-20 minutes of
slowly oxidizing the melt to bits, but that’s only something I’d try
if I had no other option. It’d be sort of like trying to tow a boat
with my Civic. Yeah, it’ll do it, but I’d much rather use my truck.

When I was teaching an adult-Ed casting class, and had 10-14 flasks
to get through in an hour, I use an oxy/hydrogen torch. Could get
300 Gm of bronze molten in about 2-3 minutes. At home I use
oxy/acetylene, same sorts of speeds. Faster on silver. I’ve watched
people trying to cast midsized silver flasks with B-tank torches,
and it takes FOREVER. Meanwhile the melt just gets oxidized as hell
from being “not-quite-hot-enough” for so long. (Believe it or not,
both the oxy/hydrogen and oxy/acetylene torches were mixed on the
reducing side, and still got the melt going that fast. ) (My
emphasis on speed isn’t for speed’s sake, it’s to reduce the time
the melt is exposed to oxygen while molten. I mix the torches on the
reducing side specifically to give me a gas cover to use to shield
the melt from atmosphere while hot. B-tank torches can’t do that,
and they compound the problem by taking so long to melt midsized and
larger loads. )

As far as temperature goes, everything I can remember reading over
the years put them at about 2500f, and that certainly accords with
my experience of them: nice soft flame, but not very hot. I double
checked that by way of a quick google search last time, and found a
temp chart that listed them at 2660 degrees. What I missed was the
little “c” at the bottom of the chart. Oops. So they are hotter than
I thought the were, but still not nearly hot enough for anything
beyond smallish melts, at least for me. Absolutely great torch
otherwise though. Never meant to imply otherwise.

Regards,
Brian Meek


#11
Interesting this negative statement about Prestolite and other
acetylene air torches. I have used nothing but a Prestolite for
over 35 years and routinely cast and pour ingots of 3to 5 oz of
sterling, 18kt and 14kt gold with no problem. I just wonder what I
have been 

Frank, I am right with you on the torch and the years of use. I have
used the Prestolite acetylene & air torch all these years for every
imaginable kt of Gold and both Sterling Silver and Fine Silver. We
have cast like that almost daily all of these years, then use the
torch with a smaller tip at the silver bench as well. I think for
the first 10 years I made jewelry, that’s all we had in the shops. I
know many many times I have cast 6 & 8 oz. of Gold & Silver using
the torch. I still have it…still use it! Like you… I wonder what
we are doing wrong!!

Dan.
dearmondtool.com


#12

Dan and Frank, you aren’t doing anything wrong. For 40 years I have
also used the Prestolite acetylene and air torch for everything. It
is a matter of the correct tip. I have a #6 for casting and a #5 for
the channel inlay bracelets 20 ga with #3 triangle wire construction.
Generally use the #3 tip for every thing. Then there are the #2 and
#1 tips - you have to learn which one for a particular small job.
It’s practice and knowing what can be accomplished. Sometimes I use
two torches for the channel inlay bracelets to get the soldering done
before the flux “dies”. I tried the little torch and burned up too
much gold - so that gets soldered using the Prestolite and air torch.
Just reading all my Orchid mail here at 1:44 a.m. Now I will go back
to bed with all these Orchid “Lollipops” in my head! Rose Marie
Christison/Rosie’s 'Riginals!


#13

I have decided to insert myself into this discussion, simply because
I see an opportunity to assert my confrontational posture once again,
and to demonstrate that in goldsmithing as in everything else - the
only thing that matters is understanding of science and it’s
application to the chosen field of endeavor. No matter how much
soul, or good manners, or emotion, and etc. one brings to the task, -
one cannot melt anything with it. On to the demonstration!

Archimedes said “Give me a lever long enough and a fulcrum on which
to place it, and I shall move the Earth” Paraphrasing Archimedes I
can say - give me a candle and a space to concentrate the heat of
it, and I shall melt anything. Of course it may take a while so to
speed things, a torch makes more sense than a candle.

Few months ago, I got a call from one of my apprentices asking to
borrow my big torch. He got himself in trouble. As I have stated many
times, my torch of preference is Little Torch. I even use it for
melting small amounts of metal. My apprentice, let’s call him Bob,
decided to melt 8 ounces of platinized silver using Little Torch.
Predictably, all he could manage is to fuse metal and crucible
together in one huge mess. Been a cantankerous curmudgeon, as many of
you can attest to, I bet him a bottle of fine armenian brandy that I
shall fix his problem using Little Torch and a few bricks.

I alway keep a few K23 bricks around. I arranged them in a box (2
bricks per side) with front open. I took one brick and closed half of
front on left side. To improve on the design, the front brick was
inclined towards the back wall. Crucible was placed in the left half
of the box and flame was directed from the right. Fuel was
propane/oxygen, Little Torch with rosebud tip. Due to
thermodynamics, the left side of the box became virtually sealed heat
envelope with very little heat lost to the right side of the box,
where I placed a mold for preheating, so no heat was wasted. The
whole thing was melted and poured under 7 minutes, and I am still
enjoying a shot of brandy before going to bed. Not much of it left
though, but I am not fretting. Sooner or later the phone will ring
and my brandy cabinet will be restocked. The moral of this story is
that educated goldsmith with small torch can melt more metal than
polite, softly spoken, soulful philistine with a large one.

Leonid Surpin
www.studioarete.com


#14

leonid, i really always appreciate your contribution… clear,
concise, skilled and fun! and no matter if somebody do not
understand your fine humorism under the comments you post. thanks a
lot for allowing me always improve my skills (i’m a beginner, so i
have lot of room for improvement!)

:slight_smile:
ciao
micaela