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Silver casting porosity


#1

Hi

I Have started to make silver rings but I get porosity.

I use pure silver, copper and aluminum but even though there is a
big reduction I still find porosity in my castings.

Can anybody lead me a good source for some answers?

Thanks
Canan


#2

Dear Canan, Why are you putting aluminum in silver? Sterling is
silver and copper.
Pauline


#3

Canan I am sure you will get lots of responses. I bet most of them
will say “ditch the aluminum. bad metal.” or words to that effect.

good luck gregor


#4

Hi Canan:

1 - You should not use aluminum in silver alloy to do castings.

Just copper is enought.

2 - After using the right alloy(just copper or any standart alloy

for silver) you must take care with the right temperature(Flask
temperature and metal temperature) the right temperature is the most
important to reduce porosity. I suggest 480� C for the flask(sterling
silver) but for the metal temperature, if you don’t have any device to
"read" the temperature(like optic pyrometer) you must pay attention on
the infra-red emition. In general the rule is: You get the better
castings when the temperature(flask and metal) is as low as possible.

3 - If you don’t have large sprues you could also have problems.

The tree must be very well mounted.

4 - You should use high quality investment(Kerr, Hobben,

Ransom&Randolph…) and have a right burn cicle. Well, after all you
must take care about the temperature!

Good luck!

Christo Kiffer from Brazil
christokiffer@christokiffer.com
P.S.: Forgive the poor english.


#5

Some things that I have tried that gave me good results were to use
more metal.

I think that most books tell you to use 10 x wax weight plus 10%
more, or 110% more than the wax weight.

If you use 125% of the wax weight, you’ll have more metal forcing
itself into the hole, which should compress out some of those holes.
For really large castings (multiple rings on 1 sprew), I’ve even gone
up to 150%. This is easier with silver, as it is so cheap. But too
much metal and it overflows the sprew hole and flings out to the
sides of the casting well. Ugh!

Also, don’t blow a big gassy torch over the melt for any longer than
you need to. Once it’s melted, let it go into the mold.


#6

Check the orchid archives for on this problem, as it has
come up before and there were some excellent responses. Perhaps the
greatest contribution to porosity in casting in general comes from
casting in a mold that is too hot (this is my observation, others
may disagree). Secondly, silver absorbs a lot of oxygen during
melting. Of course, you will want to melt with a reducing flame, but
in my experience, it helps to flux the silver with borax as soon as
it’s hot enough for the borax to melt a bit. And it is helpful if
you can minimize the time you spend with the torch on the molten
silver. In other words, set yourself up so that you can quickly pour
(or release your centrical caster) as soon as possible after melting
your metal. You also might try the new “no-ox” or “de-ox” silver
casting alloys available. I use one from United Metals called #57
sterling.


#7

Hi, It sounds like you are ‘casting’, not ‘making’ rings! If this
is the case, then it sounds like your metal is not the problem, but
your casting process (though I think adding aluminum is weird). If
your wax is not clean, has bubbles in it, if your investment is too
wet or dry, If your temperature is too high or low, you burn out too
long or short, you will have problems. (Oh and maybe your metal sprue
is too big or small, or your wax sprue too long or short…) But lets
face it, what is done is done, the ring must go out, the wedding is
tomorrow!! You can deal with porosity by fusing silver into the
spots, soldering balls into it or burnishing. Use the fusing and
soldering on the big stuff, and pound the little stuff out. I have 2
favorite burnishers: free - take 1.5 inches of coat hanger wire, bend
the bottom 1/4 inch of one end to a 45 degree angle. Stuff the long
end into your foredom, and watch the cool little hammer rotate! This
will just rip your metal though, it must be polished by ‘whirling’ it
on a piece of sandpaper (on a phone book) until smooth. The other
burnisher is store bought, go see what your supplier has in stock,
personally I like the soft square stone-on-a-mandrel thingy. But
everyone will tell you about their favorite. Good luck and try
constructing! karen in vancouver


#8

Dear Pauline

I added 1/1000 aluminum in order to prevent porosity. I knew copper
and silver absorb oxygen up to 20% of their volume aluminum is a big
help for the release of that absorbed oxygen to prevent porosity.

That helped to get rid of %50 of the problems but I still getting
some. I am planning to change my electrical furnace to gas or some
other which I can get a uniform heat.

Regards
Canan


#9

Hi Christo

I live in Turkey and sometimes its hard to find the brand names you
mentioned.

I used 1/1000 pure aluminum and it really helped to remove half of
the problem but as you mentioned temperature control is very
important too. I have to buy a better controlled furnace.

I have been casting “graduation-class rings” and after the furnace
change If I still get some prosity I am planning to change the shape
and thickness f the sprues.

I will let you know about the improvements.

Thanks a lot
Canan


#10

Hi Canan

I live in Brasil(not Brazil) and sometimes is hard to find many thing
here too, form books to machines and tools. I understand well your
problem. I’ve been casting silver and gold for the last 10 years, in
many diferent conditions. From high technology machines to my actual
single machines: A torch(Oxigen and gas), eletric furnace with
electronic kiln controller and a manual centrifugal casting machine.
All my casting laboratory cost to me less than 2500 US$( with the wax
injector, steam dewaxer and investment includer machine). Today I get
better results than I usually got with medium and high frequency
induction centrifugal casting machines. A good investment as well a
correct burn cicle(15 hours) is important. The right alloy is
important. The right metal temperature is important. But after all I
believe that 75% of a good casting you will get before the casting.
The quantity, the thickness and the position of the sprues as well how
the tree is mounted is the most important to ensure a good casting.
Look for a book of ancient Greek and Roman bronze/brass sculptures and
try to imagine how they did those castings thousands years ago. Think
about it. The most important technology is in your mind and your eyes,
not in machines.

Christo
christokiffer@christokiffer.com


#11

Hello Christo:

Think about it. The most important technology is in your mind and
your eyes, not in machines. 

I do not like to be polemic but I live in Argentina, so we have a
likeness situation. I suggest that always put one eye in some new
developed machines. Here it goes an example. Five years ago, we bought
a continuous casting furnace. I heats under medium frequency power,
that is induction. It melts under argon atmosphere and can produce up
to 80 kilogrames of gold in 8 hours. Red gold (more copper alloy) is so
soft as the yellow and the green gold (more silver alloy). Metal is
easy to roll as if it was fine gold. Another nice machine is the
magnetic polisher. It saves a lot of time. Laser spot welding machines
are too, dryers, and infinite others tools and machines. Of course,
more time for our minds and products will be best finished and more
production will be possible.

Best regards from
Daniel Mischelejis


#12

Hi Daniel and Christo

you are both have a good point…I learn from your experiences a lot.
I have Bs in metallurgical engineering but just started a totally new
business which had begun as a hobby!

Real life is “a kind of different” than books and “hands on
experience” has a big impact for success as well as good machines and
materials…

Thanks
Canan


#13

Hello, I also live in Argentina and I never found a casting company
that would make pieces without porosity (silver & gold) AND silver
stains…(another very important problem for me), at least in little
quantities! I’m very interessed in this discussion, and it’s a pitty
that our productors do not participate in this kind of discussion,
that could be very profitable for them and of course, for us. Do you in
the States have problems with silver stains in casted silver stuff?
Are you obliged to cover them with a silver coat or rhodium coat?
Daniel, could you tell me where you are exactly? can I try to cast my
pieces with you? Do you work with silver? Best regards


#14

The RI casting company that I work with just purchased a new casting
machine that produces castings without firescale in sterling.
Probably uses an inert gas, but I haven’t asked for details. Pits are
more a problem of proper sprue placement, as well as melting and
casting at the proper flask temperature. Daniel knows his stuff,
hopefully he can help you. Rick Hamilton

Gold and Platinumsmithing
CAD/CAM
Jewelry Photography


#15

Hi Daniel,

I agree 100% with you. All my position is about cost-benefits. I don’t
have mass production as well Canan, I supose. All my experience with
induction casting machines came from a bigger 18 k gold mass
production company I was partner few years ago. Today I have my own
small company and I produce 18 k gold design oriented jewelry and I
control 100% of the production process. I have a small magnetic
polisher and I dream everynight with a laser spot welding machine as
well other tools, but my reality is fight agaist costs and all the
problems we have living and working in the third world(I know You
understand me!). If I can’t have today all those new fantastic and
expensive machines and tools, I put all my investment possibility in
the human capability , hiring and training unemployed young people. By
this way I try to reduce some social problems of my country joining
hand work with design and producing a 50% hand made and 100%
hand-finished 18k gold fine jewelry to Brazilian market and few other
countries( USA, Japan and Europe). When I tell you this I believe you
understand my point of view, and I also believe that most of the
Orchid members that has small production or just mand-made production
will understand too. Sorry if my English isn’t good enougth to turn my
thoughts more clear. Best wishes,

Christo Kiffer
christokiffer@christokiffer.com


#16

Hi again: I am getting alhzeimer…

I forgot about melting temperature, very important: no more than 1000

  • 1020 C Also a good vaccum system. And he told me something about some
    models having a thin part and another part very thick. He says the
    upper side, always gets porosity due to escape of gases. I can see
    there are a lot of things that makes a good casting.

I apologize for posting two messages of the same subject.

Regards, Daniel Mischelejis


#17

–Hi All This is a problem all casters get from time to time. One
point that as not been mentioned is the condition of the burn out
furnace.If using gas or oil it is very easy to set the atmosphere
inside during the burn out, since most casters now use electric
furnace it is much more difficult . In the first part of the burn out
an oxygen rich atmosphere is required ,this is easy to do with a well
vented furnace( uses a lot of wasted electricity but this can be got
round) In the second part (dwell) a carbon rich atmosphere is required
, this is the part that most casters do not do. It is easy to do and
costs cents. First block all vents and then place charcoal in the
furnace this will give a carbon rice atmosphere.

Regards
David Sheard


#18

Hi Christo: I understand you and also agree with you and your point of
view. I also wish to have a laser spot welder in the future, it costs
too much. When I was talking about the continuous casting furnace is
for melting and getting a plate for rolling, cuting and stamping. I
don�t know anything about lost wax casting. I asked to a friend who
makes a lot of lost wax, about your problem. He told me that he
prevents porosity by using good quality wax and investment. Investment
he gets from local production makes a porosity casting. He only uses
investment imported from USA or Germany. Also uses electrolitic copper
for the alloy.

Another tip: after wax is totally out (inverted position) , he place
the flask upwards to let the wax gases go out of the investment. This is
done during 15 minutes, at the same temperature, 650 - 680 Centigrade
degrees, before casting. When he casts gold, the flask temperature is
set to 625 C. When he casts silver, the flask temperature is set to
550 C. I write here exactly what he told me. I repeat that I have no
experience with lost wax casting. I wish this can help you.

Regards from, Daniel Mischelejis


#19

Hi again

I have a “worse” problem now!

Some of the “large size” rings came back from the customers …They
have black patches at inside …These are the rings casted mostly
from the same period of time which came out with lots of porosity…

Is this problem coming from the sulfuruzation of the investment?

This week end I will start doing some investigation! I ordered 4
different type of investment powder, I will use same silver alloy for
each of them with keeping the other variables same for all…

At the end if I decide on one of them then just using that investment
I will play with temperatures…

Then I will try to control gases by charcoal and etc…

Its gonna be a long month!

Thanks
Ms.Canan Onural


#20

Once read that surface porosity is probably caused by incomplete burn
out which leaves bits of carbon in the mold. Porosity that shows up
as the metal is polished away is probably caused by metal/mold
combination being too hot. The silver absorbs oxygen. The shape of
the sprue button may indicate if the metal is too hot. If the
surface of the button is convex the temperature combination is ok,
If the button is convex the combination of mold/metal temperature is
too hot.

The best method I have found for melting silver in a small shop is a
handy melt furnace with a controller. You can set the temperature of
the metal and consistantly hold it. If the first castings have a
problem the temperature of the burn out oven or the furnace can be
adjusted. Plus the furnace has a graphic cruicible which reduces the
oxygen around the metal.

good luck
Lee