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#1

George Fox gave an interesting explaination of why zinc is
necessary in the alloy when casting gold. I had never heard
that.

Gold casting is not my speciality, but this interests me.(Most
casting I have done in the past has been in sterling) I was
always told that one wanted to cast with at least 1/2
new(uncast)metal, that the silver reacted with the investiment
and was likely to cause pitting in a subsequent cast. Using an
amount of new metal would cut down on this.

Now, if zinc is necessary for a good casting, why are we told to
be so careful and remove all solder from scrap before casting it?
Or is it only necessary in casting gold?

Mary


#2

Mary-

I don’t have an answer about your casting question, but I’m
assuming that you are Mary Lee Hu, and I want to tell you that I
thoroughly enjoyed the 3 day workshop on textile techniques with
metal, and chainmaking, you taught at MASSC here in southern
California. Thank you! Ruth

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#3
Now, if zinc is necessary for a good casting, why are we told
to be so careful and remove all solder from scrap before
casting it? Or is it only necessary in casting gold? Mary >>

Hi Mary, One one the reasons to remove all solder from scrap
before casting is the difference in melting temperatures between
the solder and the gold (or silver). The solder will melt at a
lower temperature and actually boil by the time the rest of the
metal melts. This will cause pits and porosity in your cast. One
should avoid mixing any scrap into their melt due to the fact
that most alloys are composed of different percentages of base
metals and you wont get consistent results. If you must mix
alloys or use “old” metal, you may want to consider using an
additive that will replenish the metal. These additives usually
consist of silicon copper and will scavenge most of the oxygen
from your melt; giving you an oxide and porosity free cast. Most
alloy suppliers furnish their grain with these silicon additives
already added and when I was in a bind for metal, I have cast
three times with the same button (sprue) and had good results. The
only drawback with these alloys is that you cant use them for
rolling or drawing. Hope this helps. Ken

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#4
Now, if zinc is necessary for a good casting, why are we told
to be so careful and remove all solder from scrap before
casting it? Or is it only necessary in casting gold? Mary >>

I thought if you are casting gold and have scraps with plumb
gold solder you don’t have to worry about using those scraps as
well. Is this incorrect?


#5

I don’t think many of you know that zinc is also an oxygen
scavenger. I can’t give away the technique I use but it involves
zinc removing the oxygen in casting and I get bright silver
casts. If anyone has any metallurgy background maybe they could
illucidate on this point for the rest of us. I do know zinc is
used in the industry but most jewelerys aren’t aware of that
use…Dave
http://www.opendoor.com/stephensdesign/crystalguy.html Crystalguy
Art Jewelry, Magical Art Jewelry for the Enlightened Mind

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#6

rw> I thought if you are casting gold and have scraps with plumb
rw> gold solder…

G’day; Have you thought that the word “plumb” is derived from
the latin word for LEAD; Plumbum? (the Bee is silent as in some
folk’s bonnet) As in plumber, plumb-bob, plumb-line, etc. So
shouldn’t plumb gold solder therefore have a high proportion of
lead in it? The Stirrer.

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#7

Plumb refers to exactly 14kt gold .585 not .583 as defined by
US law

Kerry


#8
G'day; Have you thought that the word "plumb" is derived from
the latin word for  LEAD; Plumbum? (the Bee is silent as in some
folk's bonnet) As in plumber, plumb-bob, plumb-line, etc. So
shouldn't plumb gold solder therefore have a high proportion of
lead in it? The Stirrer.

Hey, here I thought people were talking about someone who was so
good about being a bum that they were referred to as a plumb
bum.

Marilyn Smith


#9
Plumb refers to  exactly 14kt gold .585 not .583 as defined by
US law

Methinx 14/24= .58333…, not .585. Does U.S. law supercede some
laws of mathematics?

Bruce D. Holmgrain
e-mail: @Bruce_Holmgrain
http:\www.knight-hub.com\manmtndense\bhh3.htm
snail mail: pob 7972, McLean, VA 22106-7972
phone:: 703-593-4652


#10

I don’t think many of you know that zinc is also an oxygen
scavenger. I can’t give away the technique I use but it involves
zinc removing the oxygen in casting and I get bright silver
casts. If anyone has any metallurgy background maybe they could
illucidate on this point for the rest of us. I do know zinc is
used in the industry but most jewelerys aren’t aware of that
use.

Hi Dave,

In silver alloys, zinc is used for removing oxygen, as you said,
and to reduce the melting range, increase ductility and
resistance against tarnishing, if alloyed to fine silver max.
14%.

In gold alloys, zinc gives a better colour in 14kt and reduces
segregation of copper at the grain borders, this way the alloys
get softer and more ductile, you can’t temper them like the
usual ternary alloys. In 8kt alloys, it increases the resistance
against sulfur (fine, isn’t it - wrong, as the sensitivity to
ammonia is increased, and ammonia is contained in sweat. Try
exposing brass to ammonia fumes and look what happens: it will
crack at points of stress.).

For casting zinc is added in very small quantities as reducing
medium.

However, I’m not likely to experiment with adding other metals
to the usual gold-silver-copper alloys, as I don’t have any
exact data of how those alloys behave (and believe me, I’ve got
2 yards of bookshelf length with books about jewellery), and for
trial and error,I don’t have the money.

regards, Markus