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Zinc Oxide to absorb O2 in melt operations


#1

has anyone tried using a small piece of zinc or zinc oxide in the
crucible to absorb gas when covering with gases are not an option?
Charles Lewton-Brain discusses this in one of his papers and I was
wondering if anyone had tried it. Yes, I realize the implications of
adding any metal to any alloy and the subsequent results- what I am
concerned with is the trials others may have done using this method
to reduce O2 in the melt.

regards,rer


#2
has anyone tried using a small piece of zinc or zinc oxide in the
crucible to absorb gas when covering with gases are not an option?
Charles Lewton-Brain discusses this in one of his papers and I was
wondering if anyone had tried it. Yes, I realize the implications
of adding any metal to any alloy and the subsequent results- what I
am concerned with is the trials others may have done using this
method to reduce O2 in the melt. 

Far out… vaporised zinc is really “bad” for you.

Solder is bad enough, but to add zinc to a melt is a) adding a heavy
metal that vaporises and caused heavy metal sickness, and b) can
contaminate the melt, this can effectively lower the carat of gold.

If you use a clay graphite crucible that will help, and if you
really want to go the whole hog, stir the melt with a green stick (an
old founders trick). You can also get a lid for your crucible
(assuming you are using a furnace).

Regards Charles A.


#3
Far out... vaporised zinc is really "bad" for you. 

Yes, Charles, but who says it must vaporize. Zinc is one of the very
widely used deoxidizers added to casting gold alloys. (Silicon is
the other commonly used deoxidizer) The zinc oxidizes more actively
than copper, and then the zinc oxide slags off with melting flux. You
don’t get zinc vapors unless you’re seriously overheating the metal.
If you buy casting gold alloys in 14K and lower karat, you likely
will be buying an alloy with either zinc or silicon added already to
the alloy as a deoxidizer. Higher karats generally don’t need it.

Zinc is not commonly used, however, as a deoxidizer for silver,
since remaining zinc in the silver is deleterious to the properties
of the silver. Phosphorous, usually as “phos-copper”, however, can be
used as a deozidizer for silver if desired. Not much is needed.

And to Charles, well, vaporized metal of most sorts if “bad” for
you. Proper melting technique in all cases, whether working with
alloys that have zinc or not, includes proper ventillation for just
that reason.

And if you think a little zinc as a deoxidizer added to precious
metal sounds nasty, just try melting brass sometime…

Peter Rowe


#4

Adding zinc to a melt of silver is used in refineries to degas it
(not absorb), that is to release oxygen from the melt. The amount is
tiny, a gram or so for a large (many ounces) ingot. proportionally
the amount is not very evil. I have used it only occasionally, in
the tiniest snippet, immediately before the pour.

best
Charles


#5

I should steer clear of zinc as such, the fumes are not healthy, and
zinc oxide is already an oxide so cant see the point. I may be wrong.
Lots of flux is the usual way if you are doing a direct torch melt.
If you have an electric furnace with a carbon crucible its the way
to go You can add ‘potassium hydrogen tartarate’, a product of the
wine industry. This powder releases hydrogen slowly.You get a lovely
blue flame on top of your metal, but your problem may also be in the
pouring, you need to pour through a soft flame, that is if you are
casting ingots. Stirring with a carbon rod can help. If you are
centrifuge casting you will probably not be able to do that.

Best of luck
David Cruickshank. Australia
jewellerydavidcruickshank.com.au


#6
has anyone tried using a small piece of zinc or zinc oxide in the
crucible to absorb gas when covering with gases are not an option?
Charles Lewton-Brain discusses this in one of his papers and I was
wondering if anyone had tried it Solder is bad enough, but to add
zinc to a melt 

I didn’t reply earlier, because I haven’t read CL-B’s paper. And he’s
a smart man, so there must be something else about it written there.
The title of this thread says it all: "Zinc oxide to absorb O2…"
Unless there’s something else going on, that’s a false idea. Zinc
might be helpful because it will pick up the oxygen, becoming zinc
oxide. Carbon can be helpful because it picks up the oxygen, becoming
carbon dioxide (that’s Charles’ green stick, plus the water vapor
adds something, too). Zinc oxide won’t pick up oxygen because
it’s already oxidized~. First day chemistry class, unless
there’s something really unusual happening. Other gases, maybe -
oxygen, no.


#7
Adding zinc to a melt of silver is used in refineries to degas it
(not absorb), that is to release oxygen from the melt. The amount
is tiny, a gram or so for a large (many ounces) ingot.
proportionally the amount is not very evil. I have used it only
occasionally, in the tiniest snippet, immediately before the pour. 

Phosphorous is more commonly used, in the form of a copper
phosphorus alloy added to the melt before pouring. Lithium has also
occasionally been used.

James Binnion
James Binnion Metal Arts


#8

It’s my understanding that most sterling casting grain contain a
small amount of zinc as a deoxidizer and to improve fluidity…this
is one of the reasons that they suggest using no more than 50% old
silver when casting. When making sterling silver for casting I will
often use a pre 1983 penny as my copper source as they contain
approximately 2.5% zinc, thus bringing the total zinc to around.2%.
Pure silver absorbs tremendous amounts of oxygen but is not reactive
and therefore is not effected by it’s presence, it is only when
copper is added that the oxygen becomes detrimental. I drop the
penny in after the silver has been fully melted so that the zinc has
less chance of vaporizing and instead dissolves into the silver along
with the copper. There is none of the tell tale "green flash"
indicative of a zinc flare up…none the less I wear a welding mask
as I prefer not to get heavy metal poisoning.

A graphite crucible or a reducing flame that completely covers the
silver is usually sufficient means of reducing the absorbed oxygen.
If you find that your sterling silver castings have a lot of gas
porosity and you are mixing your own sterling you can assure yourself
it it not from absorbed oxygen by bringing the zinc content up to
0.5% or above

http://www.sternleach.com/images/Silver_casting.pdf


#9
I have used it only occasionally, in the tiniest snippet,
immediately before the pour. 

Same here, only that I use a brass of a known alloy composition.
Makes it easier to weight what you add (if your balance doesn’t have
the precision for 4 decimal places).


#10

David,

Lots of flux is the usual way if you are doing a direct torch
melt. If you have an electric furnace with a carbon crucible its
the way to go You can add 'potassium hydrogen tartarate', a product
of the wine industry. 

Do you use the tartar with an electro furnace? Or together with
borax on the open flame melt? Or on both?

Thanks!


#11

Thanks Charles,

just whom I wanted to hear from.

Actually a friend was discussing your shareware article and that he
tried putting a smear of zinc oxide paste around the rise on a
graphite lined crucible he has (somewhat like a burno style with a
top lid like riser, but in one piece) .It works beautifully: - I
watched the paste turn reddish brown before the glass on the
crucible got red.It didn’t contact the metal at all ( I’d say there
was perhaps only 15 dwt. in the crucible) just bubbled in place then
it seemed to have dried I wonder about using a fused silica crucible:
if the reaction would be the same - bubble absorb then dry out…that
way the alloy or casting grain isn’t contaminated- though
contaminated isn’t quite the right word as in such minute quantities
it is found in many alloys- Nonetheless, thanks for the opinion
Charles, et al… oh by the way are there still copies of "Cheap
Thrills in the Tool Shop available? I have never replaced mine since
it was ruined by the flood following hurricane Katrina

rer


#12

I love PHT and have used tit along with sal ammonia c and powdered
charcoal. (though not combined) The point is however, that I was
curious about Charles Lewton-Brain’s discussion of adding a* very
small* amount of Zn to the direct melt crucible…For soemthing
completely different for a change!

and yes, the fumes from melting Zn can be hazardous but that’s why
god invented respirators and fans, no !..rer


#13

Hi

I use potassium hydrogen tartarate in my electromelt, flux should
not be necessary. It has been suggested to me to let the molten metal
sit in the carbon crucible for a short time before pouring.

I think it is difficult to use PHT in a direct flame melt but can be
used to advantage in graphite crucible for melts in a flame furnace
with lots of flux of course.

David
jewellerydavidcruickshank.com.au