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Spread Porosity


#1

Dear All, We are getting spread porosity in our castings of 10K
yellow gold, which comes to notice after the piece is polished.
This is not visible to the naked eye but when seen under 10X loupe,
you can see as if something is embedded in the metal(it looks like
white patches). Also in certain places there is pinhole porosity.

Pl suggest remedies and also the causes.

Thanx in advance for all the help!!
Rgds,
Lenoj.


#2

Lenoj, Is the problem a new one that has not occurred in previous
castings? Normally porosity on the surface is caused by residue left
on the mold due to incomplete burnout. Porosity that shows up while
polishing is caused by oxygen absorption.

Porosity may be caused when the combination of the metal/mold
temperature is too high. The metal will absorb oxygen. There is a
rule of thumb when vacuum casting that states if the sprue button is
concave the metal/mold temperature is too cool. If the surface of
the sprue button is flat the temperature is correct. If the surface
is convex the metal/mold temperature was too hot. I remember
reading some place that porosity may be caused when investment is
left on the scrap metal you use in the melt. The porosity with
imbedded white material may be caused by floating flakes of
investment. Is the casting missing part of the design? Investment
can flake off when pouring the metal and become imbedded in the
metal. What mixture of old to new metal are you using? Too much
scrap metal in the melt may cause porosity. I have found porosity on
the sprue end of pieces that I had someone centrifugal cast for me.
The pits makes me thing of stretch marks. I don’t have my pieces
centrifugaly cast any more so the problem for me has gone away. My
2cents Good Luck Lee


#3

Lee The porosity I dealt with today was from a little problem
involving the way the piece is sprued. I tapped a cast ring up half a
size it cracked in about a dozen spots. The shank was thin and the
top half of the ring was very thick. If the person who made the ring
(I did not) had sprued it correctly I would not have the problems I
did. When the metal is heated you change the way the crystals of the
metal are oriented. To put it simply they get more space between the
crystals. Room to move. They want to dance and when the dance is over
and things start to cool down the crystals tighten up into what is
called a crystal lattice structure. It shrinks. Say you are casting a
gents ring that is solid and thick at the top and has a thin shank.
You have sprued it on the shank. You have a modest button and small
sprue bar. The thinnest part of the casting, the shank, will freeze
or solidify first after the metal has been poured. The thick top end
of the ring will cool more slowly. In these different parts (the thin
and the thick) of the ring the space between the metal crystals is
tightening up or shrinking. The thin part is shrinking faster than
the thick causing a tug of war in the structure of the metal. The
results of this struggle can be a bummer. Micro porosity are small
microscopic voids they will not polish up and cause the surface of
the metal to look dull. You can burnish them to some extent. Macro
porosity are voids that you can see with the naked eye. These are
much harder to fix. You can cut out the section they are in and hand
fabricate a new section. If they are localized you can fill them
with gold as a patch. When the metal shrinks it also causes stress to
the piece this can result in tears and cracks in the piece. The
theory and practice of goldsmithing has great info on all of this as
do the Sante Fe symposium books.

Regards J Morley Coyote Ridge Studio