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Fire scale Prevention During Vacuum Casting


#1

Juan,

A few things you should know. Silver has a high affinity for oxygen
when it is molten. Oxygen is soluble up to 30% which is amazing.
However in its solid state the solubility is very low. From your
e-mail I couldn’t tell if you method was to put the flask in the
bell jar the metal was poured. If so it is probably solidified by
the time you reach an adequate vacuum and the damage has already
been done. You have to think from the point of view of where can
the oxygen is coming from. From the air around the melt.

This is the worst case as the metal will readily absorb oxygen
through the surface. This is why you cast in a vacuum or under a
cover gas or some other cover like charcoal if possible. Experience
has shown me that melting the metal under high vacuum actually has
caused severe fire scale on certain geometries that went away as
soon as I switched to melting under partial pressure of argon. I
can’t really explain it other than theorizing that the high vacuum
placed the metal in a reducing state, allowing it to reduce the
silica in the investment upon casting, sucking up all the oxygen.
That’s the best I can come up with. Speaking of investment, the
molten metal can absorb oxygen from the air in the investment as it
rushes in. Modern vacuum casting machine flush the investment with
inert gas to get rid of this air. If you are casting using a vacuum
table that is just sucking air in, this can be a source of oxygen.
The investment itself is made up of ceramic oxides. You will always
get a metal mold reaction where some of it breaks down releasing
oxygen into the very surface layer of your casting. How deep this
layer goes depends on how hot the metal and mold are as this is the
energy that drives the reaction, and how long the metal stays
molten. For this reason you want to fill it and freeze it as quickly
as possible. Last but not least, you starting material can be
loaded with oxygen to begin with. If this is the case then the inert
gases in modern casting machines won’t help since they will protect
the melt from further oxygen entrapment but do nothing for what’s
already there. At least with an open melt use can use graphite
stirrers to agitate the melt to force the oxygen to react with the
carbon and be released as CO.

Summary: Start with low oxygen metal, protect the melt from air,
flush the flask out with inert gas, and most important keep those
system temperatures as low as you can go. The better you gate the
pieces, the lower a temperature you can get away with.

Tino Volpe Metallurgist,
Technical Manager
Tiffany & Co.
300 Maple Ridge Drive Cumberland,
RI 02864-8707
401-288-0124
@Volpe_Constantino1


#2

Tino,

Is it the silver that has a high affinity for oxygen or the copper
part of a sterling alloy? I have never seen firestains on fine silver
castings.

Regards,
James McMurray
@James_McMurray


#3
Is it the silver that has a high affinity for oxygen or the copper
part of a sterling alloy? I have never seen firestains on fine silver
castings.

Molten silver dissolves up to about 30 times its volume of oxygen.
as it solidifies the oxygen comes out and can cause a lot of
porosity. Fine silver does not get “firescale” but it can get
porosity.

The copper in sterling can react with oxygen and trap it at and
below the surface causing “firescale”.

jesse


#4

James,

Firescale is oxygen combined with copper to form cuprous oxide. Fine
silver, which does not contain copper, will not have any firescale.

Lee Epperson


#5
    Is it the silver that has a high affinity for oxygen or the
copper part of a sterling alloy? 

Silver doesn’t oxidize, but oxygen will dissolve into it as CO2 will
dissolve into water. You may note sometime in the future find that
your fine silver is bubbling when it is molten. My understanding is
that this is dissolved gasses escaping, particularly oxygen.

Many thanks to Tino, as I got a whole new perspective from your
explanation in regard to casting.


#6

James, It is the silver that has the affinity for oxygen. The reason
you don’t see fire stain in fine silver is that the fire stain is
oxidized copper. The oxygen in the silver will find the copper and
oxidize it.

Tino Volpe Metallurgist, Technical Manager
Tiffany & Co. 300 Maple Ridge Drive
Cumberland, RI 02864-8707 401-288-0124
@Volpe_Constantino1