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Unresolved issues with casting


#1

Dear Orchidians, I have been casting rings, brooches and a couple of
meshes lately. Everything has gone very well so far. I paint my
pieces with a debubblizer (I use Griffith) and let them dry, while
preparing the investment (I use Satin Cast). I mix the investment and
the (distilled) water first with my hands and then with a kitchen
mixer, making sure that the blade of the mixer stays below the
surface at all times. After that, I vibrate the investment for about
one minute. After that, I pour the investment into a 4"x4" flask and
use my vibrator again for approximately two minutes. I do not use a
vacuum machine. I was really afraid that I would have to reckon with
a lot of air bubbles, but I have no problems with them at all.
However, there are still some unresolved issues: 1) I worry about my
crucible, which has been turning red / deep brown inside. Till now,
I have been casting Sterling. I do not know the origin of the red
substance. Could someone please explain what it is and whether it is
supposed to be there or not? Would it (still) be safe to use the same
crucible for gold?; 2) Working with a Little Torch, I use a tip with
six orifices (for propane/oxygen) to melt the metal (Dave, Tim, John
and others here, thank you again for your . This is
certainly a big improvement, but it remains problematic to melt 45 -
50 gr. of Sterling. It takes a long time before the metal begins to
melt and, also, I have difficulties to obtain a reducing flame -
whatever I do, the flame doesn’t seem to be a really reducing one. I
have been working in the garage and it was really cold there. Could
that have something to do with it? Thank you for reading and with
very best wishes for the new year to everyone, Will
(@W_Denayer).


#2
            I worry about my crucible, which has been turning red
/ deep brown inside. Till now, I have been casting Sterling. I do
not know the origin of the red substance. Could someone please
explain what it is and whether it is supposed to be there or not? 

G’day. The red-brown colour is a copper/oxy/boron compound which
is derived from the copper in sterling and low carat gold alloys.
There are two oxides of copper; one is red-brown cuprous oxide and
the other is black cupric oxide, and that is the one which produces
the black, dreaded firestain on sterling. These oxides form when
sterling or just plain copper is strongly heated in the presence of
air, and the purpose of borax type fluxes is to dissolve these
oxides, thus keeping clean the surface of the metals be soldered or
otherwise worked when heated. The presence of this coloured borax
related compound is quite normal, but if the build up of flux in the
crucible becomes too great the excess may be removed by heating the
crucible very strongly indeed and allowing the flux to slowly run
out - it has the consistency of molasses on a cold day.

Would it (still) be safe to use the same crucible for gold?; 

The answer is, “probably” , but I think that one should keep a
crucible for gold and another for silver to be sure; they aren’t all
that expensive.

    Working with a Little Torch, I use a tip with six orifices
(for propane/oxygen) This is certainly a big improvement, but it
remains problematic to melt 45 - 50 gr. of Sterling. It takes a
long time before the metal begins to melt 

It occurs to me that you may be using the form of crucible which is
the tall, hollow form. These are really better for use in a kiln, but
for torch heating small amounts up to about 100 grams of metal, the
type of crucible to use is a flat slab of ceramic which has a shallow
hollow to contain the metal. Thus the flame may be played directly
on to the metal, and a sprinkling of powdered borax on the pool of
liquid metal will instantly remove any scum of oxides which may have
formed, and allow it to run freely like mercury. This heating
method is far faster and will result in a lower formation of oxides
because you aren’t heating it for so long.

and, also, I have difficulties to obtain a reducing flame -
whatever I do, the flame doesn't seem to be a really reducing one. 

If you could cut off a very small amount of air entering the torch,
using say, copper or aluminium foil, you can control the
oxidation/reduction properties of the flame, or in the case of an
oxy/fuel gas torch, simply cut the amount of oxygen fed in.

   I have been working in the garage and it was really cold there.
Could that have something to do with it? 

No; the ambient temperature of the working area has little to do
with melting temperature. – Cheers for now, and may 2003 be extra
kind
to you John Burgess; @John_Burgess2 of Mapua, Nelson NZ


#3
    I worry about my crucible, which has been turning red / deep
brown inside. Till now, I have been casting Sterling. 

The red stuff is oxidized copper I believe, nothing to worry about
unless you use the same crucible for gold.

    Working with a Little Torch, I use a tip with six orifices (for
propane/oxygen) to melt the metal (Dave, Tim, John and others here,
thank you again for your . This is certainly a big
improvement, but it remains problematic to melt 45 - 50 gr. of
Sterling. 

Funny I was told that the melting tips was to be used for
acetylene/oxy only. Could someone confirm this?

Jonathan Brunet


#4
    I do not know the origin of the red substance. Could someone
please explain what it is and whether it is supposed to be there or
not? 

Copper oxide, formed during melting of your sterling, being
dissolved into the flux coating on the crucible. It’s normal. You
can, if you wish, use the same crucible with gold, at least as far
as that color is concerned. If you’ve got bits of sterling remaining
in the crucible, it can throw your karat off on the gold.

I have been working in the garage and it was really cold there.
Could that have something to do with it? 

some, perhaps, but not likely a significant factor. The heat loss
from the crucible and melting metal will be greater in a frigid
environment. But considering that you want to raise the temp to 1700
F, the difference between starting at 80 F vs 60 F isn’t that much,
and probably isn’t a noticable difference. Mostly, it’s just that
even with the multi orifice tip, the little torch can only put a
limited amount of heat into the metal. What you really need is
simply a bigger torch, with bigger hoses, so more fuel and oxygen can
go into a larger flame with more BTUs being generated. At 40 to 50
grams, almost 2 ounces of silver, you’re melting quite a lot of
metal. More, really, than that torch can comfortably handle.

Peter


#5

Hello Jonathan, Thank you for your answer. Actually, there are two
’rosebud’ tips for the Little Torch: the one with four orifices is to
be used with acetylene/oxy; the other one, with six
orifices, which is for propane / oxy. Best regards, Will


#6

Working with a Little Torch, I use a tip with six orifices (for
propane/oxygen) to melt the metal (Dave, Tim, John and others here,
thank you again for your . This is certainly a big
improvement, but it remains problematic to melt 45 - 50 gr. of
Sterling. Funny I was told that the melting tips was to be used
acetylene/oxy only. Could someone confirm this? Dear Jon, I have used
propane for anything that acetylene will do (for 30 years)! To work
on platinum ( or melt large quantities of Sterling), I boost the
poundage on my tanks - especially the propane (20-25).
Good luck, georockman


#7

The red in the crucible is from the copper in sterling as has been
stated before. It may also be from other materials that settle out
when the silver is melted. I was told, long ago, that when the
silver is shot into the flask, the liquid from the center enters
first and the top and bottom arrive last. This would leave a lot of
the junk behind. I would not use the same crucible for gold. It may
have some residue that you don’t see and that can change the karat
value of the gold. It is best to use one crucible for each metal
cast. The torch problem is a money problem. You need more push than
the little torch can give. Look into an acetyline air torch that
can be found at plumber supply stores. Some were called
Presto-lite, there are other brands. If you want to use the little
torch, heat the metal from a back corner and keep the flame steady
until that area melts. As it melts move the flame forward towards
the flask. The flask will warm the metal at that
end to help start it. Good luck. Steve Ramsdell