In article ,
mark wrote:

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.

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
I digress...

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
sometimes.

--
Don Bruder - - If your "From:" address isn't on my whitelist,
or the subject of the message doesn't contain the exact text "PopperAndShadow"
somewhere, any message sent to this address will go in the garbage without my
ever knowing it arrived. Sorry... http://www.sonic.net/~dakidd for more info

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.

xray wrote:

On Wed, 09 Nov 2005 00:27:25 -0800, Don Bruder wrote:

The "damned if you do, damned if you don't" of plaster (and its "kissin'
cousins", cement, mortar, and concrete) can be truly frustrating
sometimes.


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.

"xray" wrote in message
...
On Wed, 09 Nov 2005 00:27:25 -0800, Don Bruder wrote:

The "damned if you do, damned if you don't" of plaster (and its "kissin'
cousins", cement, mortar, and concrete) can be truly frustrating
sometimes.

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.

Investment ceramic. An important part of how an investment mold works is
that the material is *porous* -- and the layer is thin enough to let steam
escape.

--
Ed Huntress

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
master out.
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.

On Wed, 09 Nov 2005 10:39:38 GMT, xray
wrote:

On Wed, 09 Nov 2005 00:27:25 -0800, Don Bruder wrote:

The "damned if you do, damned if you don't" of plaster (and its "kissin'
cousins", cement, mortar, and concrete) can be truly frustrating
sometimes.

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.

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.