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Big Silver ingots


#1

I’m trying to do some big silver 925 ingot to make sheet. We use 1Kg
to make each one. 60mm X 8mm X 200mm.

The problem is that we always get bubbles, oxidation and irregular
surface along the center of the ingot.

I’m using a grain refiner 925 alloy at 1014 C and 250,300,350 C ingot
(vertical closed)

Thanks.
Pablo Colmenares


#2

Hi Pablo;

The problem is that we always get bubbles, oxidation and irregular
surface along the center of the ingot. 

My guess is that you are pouring the metal into an ingot mold that
is flat, like a square pan. This would mean that the metal would cool
from the edges towards the center and in the center of the slab, you
have shrinkage. If you were to construct an ingot mold that was
upright, so that you were pouring metal into a slab that we standing
on end, the shrinkage would happen on one end of the slab and could
be cut off. Molds of this type are available, but may not make an
ingot quite as large as what you need. But my theory is based on my
guess about the position of your mold. I suppose you could get
problems doing it my way too. I’ve not had problems as my mold stands
the ingot up on end, but the ingots I’ve made are a little smaller
than yours. I also have used a sand mold to cast ingots and have had
good results. I used a small version of the old “cope and drag” mold
and caster’s sand. This system is less expensive than the "Delft"
system that jewelry tool suppliers sell and is able to cast larger
articles. I’ve seen it in a number of supplier’s catalogs. Best of
luck.

David L. Huffman


#3

2 things to try:

  1. when pouring, keep the metal in a reducing atmosphere, so even if
    you are using an electric melting furnace, also use a torch to keep
    the metal coated in a gas atmosphere that is oxygen free.

  2. increase the thickness of you casting

  3. try a hotter mold

Mark Zirinsky
denver


#4

Hi David, thanks for your reply

"My guess is that you are pouring the metal into an ingot mold
that is flat, like a square pan" 

We have been trying to do the ingots in many ways, at many moult
temperatures too.

The Vertical closed way seems to be the best to avoid metal
contraction, but we are still having problems.

Still many bubbles,oxidation along the center of the ingot. I would
like to understand better the way that silver acts during
solidification.

Thanks.


#5

I have no direct experience with your problem. There’s a book called
something like “The U.S. Navy Foundry Manual” - Lindsay Books has or
had it. In casting big bronze and brass, they use gates and risers
and
sand cast. What that does is put the casting in the middle of the
flow, instead of at the end. When a casting is the end of the line,
there’s no place for gases to go, unless it’s investment. Even then
the porosity of the investment (or sand) only goes so far. Using
risers puts the casting in the middle of the flow, and gives an
outlet for gases. Just a thought…

http://www.donivanandmaggiora.com


#6

I am assuming that you are talking about a pipe or porous semi hollow
inside the ingot, not on the ingot surface although you might see
some porosity there also.

If so you are having a problem with dissolved oxygen in the melt.
Molten silver can dissolve tremendous quantities of oxygen which
come out of solution on solidification.

If so you will need to correct your melt and pouring practices. This
may not be easy to do.

The melting temperature should be kept low and you must not "stew"
the melt. Melting should be done in a vacuum or inert or reducing
atmosphere. You can help by floating carbon over the melt to keep
oxygen out. You have to be careful not to add oxygen during the pour
and not carry carbon bits into the mold.

The mold should be hot and and you must control the cooling so it
occurs from the bottom upward with the sprue the very last to cool
and solidify. An insulating cap will help will help keep the cooling
somewhat controlled. This last portion will probably be porous -
hopefully you can cull this. It used to be a practice to deoxidize
with a bit of cadmium but this produces dangerous fumes.

jesse


#7

Hi this is Rajendran form Tanishq India,

We are doing a large number of silver ingots weighing 10 kgs each.
An inert atmosphere is very useful for silver Ingots. If you are
familiar with the pouring of gold ingots, silver should be poured
much faster rate.

Vertical mould with relief angles should be used to easy release of
the ingot, by controlling the speed of pouring with respect to the
speed of solidification you can minimise the top shrinkage cavity.
Mould design should be a narrow square at the top to so that the
wastage due to shrinkage can be further minimised.

Regards,
Rajendran G