I recommend, for those who have the choice, using vacumn assist
and an electro-melt. Less porosity due to contamination of the
melting metal with gasses from the torch. Why vacumn over
centrifical? You will want a vacumn to remove air bubbles from
your investment after you pour it in around your wax model. The
other alternative is to always have to be strategic about
sprueing the piece so air can’t be trapped under an area.
(Sometimes this is detrimental to the design.) The trapped air
would then be replaced by unwanted metal. Since you need a
vacumn for investment, why not use it to cast with and avoid the
cost of the centrifuge? Then you can spend that money on the
electro-melt. Simply take your flask out of the oven when the
electro-melt has your metal at the optimum temperature, put it on
the vacumn pad, start the machine. When full vacumn is reached,
pour your metal into the cavity. I, however, am poorer than
some, so I have an old centrifuge I found thrown out at the
dental college (I straightened out the casting arm). I have to
sprue thoughtfully, invest carefully, melt with a torch, juggle
the flask into the centrifuge, and re-melt, but it’s a
little more exiting watching it spin like that. 
David, Using a torch to melt when casting does not automatically
mean you will have gas porosity. It is over heating the metal
that causes This kind of porosity. It can look like a foggy area
or a field of very small pits. You can over heat metal with any
melting equipment. I have seen bad castings from a $40000.00
casting machine. There are three types of porosity. The most
common is sometimes called “Hot Tear”. Hot tear is simply, metal
cooling before there is enough feed of molten metal to complete
the proper re-crystalization of the alloy. the second type is the
aforementioned “Gas Porosity”. The third kind is called
"Inclusion orosity". It consists of small broken bits of
investment or other junk like carbon. Good design is still the
best way to prevent porosity. Special attention to spruing of a
model with several “Hot-Spots” is the next best thing. If a model
maker uses the princple of progressive solidification and
understands how metal melts, flows and hardens, they can then
make a model with one “Hot-Spot” where the sprue goes. If a piece
is not going to be molded for future replication you can spend a
little more time with multiple sprues. J.A., J.A. Henkel Co.,Inc.
MOldmaking Casting Finishing
John, Your post caught my eye, could you elaborate on…
Good design is still the best way to prevent porosity. Special attention to spruing of a model with several "Hot-Spots" is the next best thing. If a model maker uses the principle of progressive solidification and understands how metal melts, flows and hardens, they can then make a model with one "Hot-Spot" where the sprue goes. If a piece is not going to be molded for future replication you can spend a little more time with multiple sprues. "
Thank you. Bob Staley, a getting-older dog who can ALWAYS learn
new tricks.
Using a torch to melt when casting does not automatically mean you will have gas porosity. It is over heating the metal that causes This kind of porosity.
Right you are. It is primarily overheating. I’m not an expert
caster, but I think it’s fairly easy to overheat metal,
especially if you’re a beginer. I agree, an electro-melt will
overheat too, I used to do that quite often till I started using
a cooking timer to remember I had metal in the melt. I know,
hard to believe, but in a one-jeweler does it all situation like
mine, it’s easy when you’re doing three things at once (and
you’re 46 years old with the short term memory to match) to
forget. Thanks for your input on this.
David L. Huffman
Hi Bob, Metal expands when it is molten. This expanded metal
enters a confined space, ie: an invested and dewaxed flask. Metal
solidifies from the outside in. If you have an area on your model
that is thicker than the sprue, the sprue will harden first which
will prevent any further metal supply reaching the “Hot-Spot”.
The metal re-crystalize anyway, but not having enough supply will
"Tear" itself apart. This is about 90% of what people call
porosity. It is possible to have many “Hot-Spots” on one piece.
There are many other factors such as flask temperature, alloy,
investment and burnout but understanding the above mentioned
phenomenon will contribute the most to your casting success. If
you know this info before you make a model or carve a wax you can
plan for sprueing instead of it being an afterthought. This
becomes very helpful with production work. J.A., J.A.Henkel Co.,
Moldmaking Casting Finishing
Molten silver has then property of absorbing over 20 times it
own volume of oxygen (really). As the metal cools and solidifies
this oxygen can not be retained and it bubbles out. This is
prevented with an adequate flux cover and continuos cover with a
reducing flame during melting and pouring. This property also
makes silver welding (not brazing or soldering) very difficult. An
additional potential source of gas is from overheated investment
which forms sulfide gases which can form, a heavy black tarnish.
Jesse
I completely forgot to post this…I tried the double melting
with United Precious Metals de-ox sterling alloy. NO difference.
This makes me wonder that if this technique works for some that
in a single melt incident they aren’t stirring with a rod to make
certain the metal is completely melted. When I torch melt the
metal will look shiny and mirror like on the top but will be
slushy below the surface and if cast like that would probably
cause problems. Some have said stirring with a rod (I use the
carbon ones) will introduce oxygen, but I don’t see how that
would be if you’re keeping the flame on the metal and carbon
supposedly removes oxygen. The only advantage I see to double
melting is that on the second melt you put the flask in the
cradle and the metal melts quicker since its already half way
there, so you lose no time melting while the flask is cooling. So
this double melt thing doesn’t do much for me…oh well…Dave
I have spent many years exploring this issue. GIA (Casting),Rio
workshops, Swest workshops, ect. I used to have problems with my
castings retaining flux type excess, and the more than occasional
poriosity, 1/2 casts, and sometimes no casts. Most of my problems
did not come about through the absence of double melting, but
rather such things as guestimating investment to water ratios,
improper burnout cycles, and other brainfart type incedences. A
bench jeweler friend of mine who only does two or three castings
a week first told me about double melt. He told me that if you
allow the freshly melted metals a moment to cogil and then
remelt, the castings will come out beautiful. I tried it once
and so much for my casting problems. Now my castings come out of
the quencher just as beautiful as if
I soaked them in acid for an hour.