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Casting silver ingot problems


#1

hi there,

In last few days I got problems with making a good sheet of Sterling
silver. I am using Hoke torch with oxygen and propane. Casting into
4" wire/sheet combination steel mould.

Ok, so I bought my new Hoke torch, cast few ingots and they were OK.
I cast another ingot to make a sheet and it looked good. After
rolling it down to about 1mm of thickness and annealing it "bubbles"
appeared on the both sides of sheet. So I thought I have trapped some
air in silver, So I melted it and poured again. Rolling it down,
annealing and air pockets are there again! So I remelted it again -
but same thing happend. It was my scrap silver- clippings of wire and
sheet, few things with a little solder with it. So I thought that it
could be solder that doing this, because I have got some small crack
at the edges of sheet while rolling it - but they were small… So I
took some fine silver and fine copper, and made some new "clean"
alloy of sterling silver. Poured the ingot and rolled it down to 1mm
and annealed for futher rolling. And guess what? Bubbles shoed up
again! Remelted it but the same thing happend. So I went to the
computer, searched through Ganoksin forums, found few posts there but
there was nothing i already known about this poblem. So I looked up
itno the goldsmithing books that I have but again there is nothing
about how to solve my problem.

I am thinking that the torch could be the problem. Befor I got hoke
I used large air/propane torch to melt silver and never experienced
problem of air pockets. So I hooked up that torch and cast ingot of
"clean" alloy and scraps. Rolled them down but id didnt solved my
problem.

So what could it be?

When I am casting ingots I am covering entire mould with soot of
candle or from forch (setting small amount of oxygen). I also
preheat the mould with my torch befor casting - It is hot so you
would not want to touch it but they are not too hot (soot is
disappearing is mould is too hot). I also preheat my crucible before
I add my metal. Then I melt it with some borax, stirring with
graphite stick and then I pour with the edge of crucible as colse as
possible to the mouth of mould.When I pour metal is passing through
the flame so it wont soldify befor fillng the mould.

I also thought that my silver could be oxidised, so I used agrofulx
for reducing copper oxide and other importunities that could be in
my alloy. It also makes molten alloy more fluid. It also SHOULD
reduce chances of trapping air in silver.

As I already said, I amd using wire/sheet combination steel mould
and crucible made of fused coal and ceramics.

So what could couse air trapping in my silver? As I said before I
have never experienced this before getting my new torch, and when I
get it I have cast few good ingots. Could it be foo fast/slow
pouring? pouring from wrong side? I think not because I always have
good pouring and I dont think it have changed in one day.

Pleas give me some adivce.
Cheers, mark


#2

Silver has a nasty property of trapping oxygen. Once temperature of
metal start dropping, oxygen escapes. If temperature drops too
rapidly some of it gets trapped in metal.

There are 2 ways to deal with that. Melt silver with cover flux, but
for this furnace is required. Or, bring it to higher temperature
than required ( at least couple of hundreds degrees ) before pouring
and pour thought the flame, if casting with torch. The later is not
without a price, because metal crystalline structure will be very
coarse and ingot must be restored to workable state via forging.

Leonid Surpin
www.studioarete.com


#3
So what could couse air trapping in my silver? 

If I read correctly; with all new metal this was happening. If that
is the case it would probably be the metal cooling too quickly after
having been poured too quickly. When you first poured with your new
setup you probably used slower, more deliberate actions, resulting
in good pours. Also, make sure your mold is hot enough to give the
metal time to release trapped air before starting to solidify. Stir
the melt before pouring, but not too vigorously and let it settle a
little. Pour slowly and deliberately, minimizing splashing and
swirling. Keep the heat on the melt as your pour. If you have the
dexterity, after the last melt goes in the ingot, reduce your oxygen
and play the flame over the top of the ingot for a few seconds.

Dan Culver


#4

You could try opening up the gas a bit more, you could be using a too
oxygen rich flame. Just as you are about to pour turn the oxygen down
slightly or increase the gas to give a flame that is hungry for
oxygen. Hopefully that will help with the bubbles. But if you are
getting cracking when rolling you may have trouble, too much solder
in the scrap or some iron? I never melt silver filings they are too
difficult to clean even with a magnet.


#5

Dear Marek,

I use a Hoke torch regularly. I think it is too small to pour much
of an ingot with. A small ingot of 10 to 20 dwt perhaps, but not much
more than that. How much are you making?

You can drill out the torch tip to allow a bigger flame as one
suggestion. Torch tips on the Hoke are easily modified and done by
most jewelers. New they are all too small. I use a lager casting
torch for most ingot making.

The silver should have a clean mirror look and roll with the torch
before pouring. Mack sure you stir the metal before the pour. The
torch flame acts like a wind on the metal and you can even push the
metal around with it. Pour through the flame into an oiled ingot
mold. The ingot mold should be preheated until it smokes. Don’t burn
off all the oil before pouring. Pour quickly and wait about ten
minutes until you quench, pickle and it will be ready to mill.

Finding the proper flame to melt is a skill unto itself. Consistency
is the key here.

When milling, measure the starting thickness. A good rule is to
reduce it to half its thickness, stop and anneal. The cracking can
happen if you mill it too much before a second annealing.

Make sure you fire coat before annealing or cover the metal with
white paste flux. An old silversmith (not Ray my teacher), once told
me in the old days, you would heat the metal with the flux spread
over the entire piece, and when the metal was hot enough to turn the
flux clear you were at annealing temperature. It was a visible guide
to annealing sterling silver. He used a larger compressed air and
oxygen torch for annealing, but these were larger pieces also.

God bless the old masters who shared.

Good luck,
Todd Hawkinson
Southeast Technical College


#6

Hi Dan,

This is one of the reasons I like making ingots and sheet in Delft
clay, you don’t have to heat the mould up, just pour quickly and
you’re ready for rolling.

Also with a steel mould you can burn your fingers :wink:

Regards Charles A.


#7

It sounds to me like turbulence during the pour. The metal hits the
bottom of the mold and starts to bounce up and then starts trapping
air in the silver as it cools. I would suggest that you get a carbon
rod and stir your melt slowly before you pour to release those
bubbles then tip the mold as you start the pour so it rolls down a
side and not crash into the bottom of the mold. It sounds like you
are fluxing the melt but are you stirring the flux into the melt or
just surface treating.

Anyway that’s my 2 cents worth. Hope it helps.
John (Jack) Sexton


#8

Hi John,

Stirring does help, especially with a large quantity of metal.

You could use a green stick to do the same, or you could take a
smidge from a pool chlorine tablet (others swear by this, a bit too
industrial for me), although a carbon rod or green stick have served
me well.

Regards Charles A.


#9

Actualy I find clue what is making these air pockets in my ingots.
Silver that I have bought is not FINE silver… It does not have any
copper but there is something that is burning when you melt silver
and it produce white stain on your soldering block, crucible etc.
there is some gas going from my silver. I dont know what it is…
Maybe it is lead? Or not completly reduced silver chloride? Who
knows? Personally I dont think It could be silver chloride because I
haeve tried to refine some of this silver using chemical method and
it didnt helped. In few days Ill try to refine it chemical y but I
will see that there is some lead. If there is I will sperate it from
my silver. If there will be no lead I will refine it by
electrolisys, maybe this will help. BTW someone mentioned about
modifing my Hoke torch tips. Something about drilling bigger hole.
Could someone give me better details about that?

Cheers, Mark


#10

Helo Charles,

You could use a green stick to do the same, or you could take a
smidge from a pool chlorine tablet (others swear by this, a bit
too industrial for me), although a carbon rod or green stick have
served 

Could you elaborate on the green stick (is this a freshly cut
branch?) and the smidge (as in small quantity or do you make a
shape?) of pool chlorine tablet? I find that the 3/8 carbon stick is
big for the small crucible that I use.

Thanks
Christian


#11

HI Christian,

Could you elaborate on the green stick (is this a freshly cut
branch?) and the smidge (as in small quantity or do you make a
shape?) of pool chlorine tablet? I find that the 3/8 carbon stick
is big for the small crucible that I use. 

Well a “smidge” is one of those measurements that people tell you
and isn’t an exact quantity. I’ve never used chlorine tablets at all,
just mentioned it because others have.

The green stick method is basically ripping a live stick off a
living tree. The meaning of green does not mean turgid stick, so
using a succulent would cause a few issues… like a micro volcano.

Just find a tree, and select a sturdy stick. The impurities in your
melt will stick to the stick. The moisture will leave the green
stick leaving a piece of charcoal, which can be used for other
purposes… like stirring other melts or using as fuel.

I use the green stick method for small and large crucibles, just
choose the right size stick.

Regards Charles A.


#12
It does not have any copper but there is something that is burning
when you melt silver and it produce white stain on your soldering
block, crucible etc. there is some gas going from my silver. 

Oh crud, that sounds like zinc! Ventilate.

Regards Charles A.


#13

I have been told that in Thailand and other countries in that part of
the world that silver is alloyed with tin, or (less likely) zinc to
produce a white alloy. I have seen some of what is said to be this
alloy, and it was truly white, but not the same as depletion "gilded"
sterling. (depletion silvered?)

Any thoughts from someone in that area?


#14

I dont think it is zinc. because I have refined this silver and
there is no option that it could contain zinc. One thing that is
possible is it can have LEAD! If you refine using chemical refining,
and you make silver chloride, there is also lead chloride if your
silver contained lead, it looks just like silver chloride so you cant
see it is there. I must refine it by electrolisys to get ride of
lead(if it is there, which I think there is).


#15
If you refine using chemical refining, and you make silver
chloride, there is also lead chloride if your silver contained
lead, it looks just like silver chloride so you cant see it is
there. I must refine it by electrolisys to get ride of lead(if it
is there, which I think there is). 

Once silver chloride is obtained, it must be washed with hot water.
This is very important step in refining and must never be skipped,
precisely for the reasons you stated. LEAD CHLORIDE.

Lead chloride is only slightly soluble in cold water, but when water
at boiling point, the solubility increased to an ounce per liter. If
you suspect that lead is present - dry and weigh your silver
chloride; add a liter of water and bring it to boil; filter, dry, and
weigh again. If weight reduction is less than an ounce, you are done.
If not, then washing cycle must be repeated.

Leonid Surpin
www.studioarete.com