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Casting metal purity


#1

i just want to know that whether the purity of metal decreses or
increases if v use used metal which has been previously used for
casting twice or thrice

thanks and regards
hardik shah(bonny)


#2
i just want to know that whether the purity of metal decreses or
increases if v use used metal which has been previously used for
casting twice or thrice 

If you are casting into investment then the metal gets more
contaminated every time you use it. The molten metal is above the
breakdown temperature of the investment. When investment is heated
above the breakdown point it produces sulfur dioxide, This gas is
absorbed into the metal and cannot be removed short of refining the
metal. This gas is one of the contributors to gas porosity in
castings. So each time you reuse the metal the more gas it contains.

If your alloy has low melting components in it like zinc then you
will lose some zinc to boiling off so the alloy ratio will change,
but in standard sterling silver this is not an issue as silver and
copper have very high boiling points so unless you drastically
overheat the metal neither will evaporate.

James Binnion
James Binnion Metal Arts


#3

Jim,

If you are casting into investment then the metal gets more
contaminated every time you use it. 

Thanks for this Can you comment on the effect of
melting the same metal multiple times and whether that too adds to
the problems, especially for silver.

All the best,
j

J Collier
Metalsmith
http://jlcollier.com


#4
Can you comment on the effect of melting the same metal multiple
times and whether that too adds to the problems, especially for
silver. 

The issue with reusing previously cast silver is that sulfides build
up. The rule is that you use 50% new metal, but I have reused sprues
and cast scrap without adding any new metal. I have done it hundreds
of times. Use flux and mix the metal well, but do not over flux or
the flux will run into the flask and leave voids. There is a telltale
red where the void is after the piece is pickled.

I use a centrifugal casting machine. After the metal is melted, I
add the flux, I release the pin, and I gently shake the crucible and
heat the metal making sure the sides of the crucible get hot enough
to pull the flux off of the silver. If the flame is too close to the
metal you will see black swirling on top of the metal, pull the flame
back until you see the impurities on the top roll off and the metal is
bright and shiny and then let it go.

I had a large stock of parts and pieces I had cast over time
intending to use at some point, when the economy got bad, I started
melting these pieces and I have not had any problems.

The main problem I have had with using all scrap is occasional crack
across a shank. I have not had issues with porosity. I cast hundreds
of pieces at a time and I might have a few pieces that have problems
from one casting.

Richard Hart G.G.
Denver, Co.


#5
Thanks for this Can you comment on the effect of
melting the same metal multiple times and whether that too adds to
the problems, especially for silver. 

Yes, more gas is picked up every time you melt, and pour into
investment. Also metals that from stable oxides (copper, zinc etc)
will not reduce back to metal without some help from fluxes or
hydrogen cover gas over the melt. So during melting and casting some
of those oxides end up in the metal and some are on the surface of
the button and some end up in the flux. Some alloys have components
like silicon that oxidize out and float to the top of the melt and
are lost to the alloy. Some alloys are better at multiple castings
and some really want to be cast once or a very limited number of
times then refined.

The reality is that the alloy changes every time you cast it and
eventually this leads to problems in the casting. WYou cannot afford
to refine after every casting but you should try to limit the amount
of old metal, 50% seems to be the accepted number but I have worked
with some finicky alloys where it seemed like 50% was too much. So
one troubleshooting step when you are having problems with a casting
that has worked in the past is to use fresh metal and see if the
issue goes away.

James Binnion
James Binnion Metal Arts


#6
I have not had issues with porosity. 

All castings have some porosity. The question is where is it and how
big are the pores? Good sprue design and proper casting technique
can go a long way towards keeping it out of sight. The sulfur dioxide
often ends up as a very fine porosity on or near the surface of the
part that can sometimes be removed or burnished over by normal
finishing routines. If it runs deeper it can leave a hazy surface
that just will not polish well. If it gets worse you can begin to see
pores with the naked eye.

James Binnion
James Binnion Metal Arts