Could a person melt brass or bronze with a oxy welding torch in a
cruxible and pour it in a shallow, crude mold of plaster paris or some
other material? I've been reading about sand cast molds but it looks
like too much of a skill to learn for a small project. Thank you.
Watch out with plaster for casting brass and bronze. You have to "calcine"
the mold or you can have an explosion.
Calcining means heating it in a controlled way until some of the
chemically-combined water is driven off. I'm not talking about free water in
the pores of the plaster; this is about the water that combines chemically
to turn plaster into reconstituted gypsum.
When you calcine plaster it can crack. They (US Gypsum, etc.) make plasters
especially for metal molding. IIRC, they don't recommend them for brass or
bronze, which melt at relatively high temps.
Aside from that, I hope you're really good with a torch. Silicon bronze
is the easiest of the brasses/bronzes to melt and cast. Yellow brass
separates, and it requires some knowledge and technique.
Plaster is a bad idea for casting metal unless you've kilned it
*TOTALLY* dry - Otherwise, the explosions tend to turn the project into
a whole bunch of broken plaster and spilled molten metal running down
the stand/your leg/across the floor/etc. Problem is, once you bake it
hot enough to be safe to pour metal into, plaster goes all brittle and
crumbly, and doesn't hold together worth diddly.
Plaster works the same way as cement or concrete or mortar - It doesn't
actually "dry", it "sets". Most of the water you mix it with is trapped
in the finished item, held captive in the crystal structure, with only a
small part of it evaporating. Depending on the thickness of the piece,
the "set" process can literally go on for years - They say that the
interior of the Hoover Dam is *STILL* setting, and will likely continue
to do so for another hundred years or longer before the process
completes and the concrete reaches its full strength. Ditto plaster
pieces - A large enough piece will take years to *FULLY* set, even
though it's usually solid enough to unmold and handle in 20 minutes. But
When you dump 700+ degree metal onto unbaked plaster, the trapped water
explodes into steam almost instantly, usually tearing the mold apart,
and possibly flinging molten metal at you in the process. (Concrete
floors usually spall instantly if molten metal is spilled on them, too -
Exactly the same process is in action for both materials)
Nobody has ever given me a satisfactory explanation for the "why" of it,
but in my experience, baking plaster to get all the water out of it
almost always results in a piece that has lost all strength, and is
likely to crumble at the slightest touch.
The "damned if you do, damned if you don't" of plaster (and its "kissin'
cousins", cement, mortar, and concrete) can be truly frustrating
I hear what you are saying but what, then, is the appropriate material
for making a mold for something like lost-wax casting of metal thingies?
I could probably research it, but it is easier to just ask here.
You should look for investment plaster. This should be available from
jewellers supply places and possible dental supply also. It still needs
to be baked to drive off the chemical water but is formulated to
withstand the high temperatures of the molten metals.
You can make your own casting sand for almost nothing. Then use a
plaster, wax, or wooden master to cast as many pieces as you want.
Sieve some regular plaster sand through a fine screen and mix it with a
little flour, water and glycerin to make a packable mixture. for an
open pour, just one mold is used. Place the master, face up on a sturdy
table and ram the sand into a box form around the master. When the sand
is solid to the top of the mold, turn it over and carefully lift the
Heat the mold in a hot oven just before pouring to drive off any water.
The sand can be re-used and reconditioned. Three dimensional castings
are made with a split molding box called a cope and drag.
Get a book on casting to learn the details.
Go with bronze ("copper" loose change is good, but sort it with a magnet
first) Brass has the problem of zinc boil-off, which is significant for
Oxy-propane maybe, oxy-acetylene is too much temperature and not enough
heat. Best of all is a well-insulated box kiln around the crucible and
something like a Ron Reil propane-air injector burner.
Crucibles are good, but you can get away with steel pipe and a welded
end cap for your first attempts. Don't melt too little at a time,
because you need to maintain temperature from the kiln to the pour.
Don't think crude, you don't have to. Lost foam in sand works
(builder's insulation foam, builder's sand) So does cuttlefish bone.
Plaster is mainly used as an investment around lost wax, and you might
want to buy specific investment plasters for that, then follow the
bake-out rules properly.
Greensand moulding around a woodern pattern is worth trying, but it's
easier to mould a large piece than a small piece, and easier to pour a
small piece than a large piece. For one-offs, I'd use lost foam.
Jarkman and I have been doing some of this lately. We're still pretty
rubbish at it, but it's fun.
Get investment from a jeweller's supply place. It can retain very
fine detail, and it can handle metals as hot as platinum. You need
an oven to bake it out at 1200 to 1400F (follow the directions on the
bag) before you pour hot metal in it.
Not with US coinage. Early pennies are copper/zinc alloy, and newer
pennies are copper plated zinc. The balance of our coins are either
nickel/copper alloy, or nickel copper sandwiched. There are no bronze coins
made in the US.
[I believe the new Sacajawea dollars are manganese bronze, but it would be
rather uneconomical to melt them down. Send them to me, and I'll give you an
equal weight in new silicon bronze ingot, the American foundryman's
As for the Original Poster's question, yes - you can pour brass in a mold.
No, straight plaster of Paris isn't a good idea. Yes, you need to heat any
plaster-based investment to drive off the chemically-entrained water; 1250F
is a good temperature for that. No, you don't have to buy jeweler's
investment; you can add 2 parts sand by volume to 1 part plaster of Paris to
give it sufficient strength to withstand burnout. Yes, you can melt bronze
in a crucible with an oxy-acetylene torch, but no, that won't give you
enough to pour with straight gravity. For small amounts of metal, you need
some way to overcome surface tension, like vacuum from the bottom of the
flask, or centrifugal force, or even steam pressure. If you want to melt
enough to pour by gravity, then you need a furnace to melt in. Yes, you can
improvise a crucible with a steel pipe or whatever, but no, this isn't a
good idea - the temperature of molten bronze is too high, and the
consequences of a crucible failure are too dire. Get a real clay-graphite or
silicon carbide crucible; they aren't that expensive compared to
hospitalization. (Of course this doesn't apply to the British, who get
hospitalized for free...)]
I've been hearing this for years from knowledgeable folk, so it's
probably correct. I'm just glad I didn't know it in 9th grade when I
made a cast brass fireplace tool bracket. I just got some scrap brass,
melted it in the Jr. High foundry & poured it into a sand mold. It
polished up nicely, and is still bolted to the side of my Mom's fireplace
40 years later. I have no idea how I got away with it, but it worked
great. The brass was mixed chunks from the Los Alamos surplus yard, and
could have been all sorts of alloys.
As long as you melted with a crucible in a furnace, that would work fine,
and is commonly done. I mean, that's *how* it's done (unless the metal is
melted by cupola or induction furnace). This guy is asking about torch
melting by applying the torch directly to the metal. That is a recipe for
failure with brass.
Congrats on the nice fireplace tool bracket. Such projects are a pleasure to
have in our later years. I lost all my high school machine shop projects
when I was burglarized many years ago. Must have been one damned desperate