Porosity in Silver Casting

The problem of "...round cavities, visible only after the first polish and grow larger inside" is inclusions, likely air inclusions.

The source of the problem may be a little harder to discover. Some of your post is confusing.

The first image appears to have issues besides air inclusions. Are you perhaps waiting too long to spin?

"... last time I used recast (scrap)... Is this batch of reject rings cast from this recast? If yes, from where did the metal come? Are you certain of the alloys and percentages?

What are you doing differently today than you did when you had success?

We have many members who makes silver jewelry, likely someone will stop by and visit more with you.

https://www.youtube.com/results?search_query=casting+silver+alloy+rings

Good Morning !!!!!

Solar Eagle

I read the article by Richard V. Carrano. this article has very relevant information on porosity.

We put an argon cylinder in the centrifugal casting machine and

a preliminary analysis gave us a positive result.

I hope to post more information soon !!!

Thank you in advance !!!!

Att

UserBrSP

I don't know a lot about this, but have you looked at the defects with a microscope to see if it's an impurity or a void?

Hi UserBrSp,

Welcome to CR4!

I use centrifugal casting machine, silver with 30% Silver Alloy, is melted in a graphite crucible at 1040° Celsius (1904° Fahrenheit) by induction, last time I used recast (scrap), the flasks have 450° Celsius (842° Fahrenheit) when casting.
Ambient temperature was Temp Ambiente=26°. Goldstar investment.

It is unclear what you are comparing here. Here is what I take from this paragraph of yours:

A mixture of 70%Ag + 30%Ag alloy is taken to 1040ºC where it is (I assume) melted.

In a previous batch, 100% scrap was used (unclear what this actually is), and taken to [only] 450ºC.

What is the significance?

I know there is something here, but you must provide more details!

• What are the exact compositions of your melts?
• Are you sure your casting machine is working properly?
• Have you called the manufacturer of the casting machine?

Have you checked temperature calibration on the flask temperatures of the casting machine?

Oh, and why was the last sentence: "Goldstar investment." included?

"Oh, and why was the last sentence: "Goldstar investment." included?"

Gold Star makes investment casting powder.

http://www.goldstarpowders.com/products/ip-xl

It is necessary to distinguish between macro-porosity and micro-porosity. No matter how well you act, it is not possible to avoid a minimum porosity, invisible to the naked eye, called micro-porosity. The expert hand, in the centrifugal lance to lost wax, obtains reproduction without macro-porosity, that is to say the visible one

The shortage of borax or boric acid can favor porosity. If the temperature of the molten metal has a value that is too high at the moment of casting, gases are absorbed and then released when solidification occurs.
The increase of the centrifugation speed can contribute to reduce the porosity, to make the production complete and compact.

"The minimum pore size required to allow the in growth of mineralized tissue seems to be in the order of 50 µm: larger pore sizes seem to improve speed....."

The photos look like you have two issues.

One is trapped air. You probably want to add a vent or a pop off at the end of the casting. The vent is a length of wax wire connected to the far end of the casting from the sprue and run back up to the top of the mold independent of the sprue. When you burn out the mold it opens an air path to get air out of the cavity. A pop off is a small chamber attached to the far end of the mold cavity. It provides a space for trapped air to go and also allows the first metal into the mold to flush through the cavity and into the additional chamber. The slug of metal in the pop off can then be removed and remelted.

The second is dissolved gas in the silver. Using induction melting means that you pick up less hydrogen from an acetylene flame, but it's still important to make sure everything is clean and dry going into the induction melter. Degassing helps, but sometimes metal becomes so contaminated you just have to send it to a refiner. If you can get it into a vacuum casting setup, that also helps as the vacuum promotes the gas to come out of the metal. Your other option is to take your scrap and add only small amounts to new metal to dilute the problem down to a level that won't show up in the work. Another alternative is to ingot the material and roll or draw it for use as wrought silver.

Hello,
I need help from this forum.
Lately in my silver casting, I had some porosities and

defects present in silver pieces (Big Rings). The defects, in

the form of round cavities, are visible only after the first

polish and grow larger inside. In some cases with impurity

inside and in others not.

I use:
centrifugal casting machine with induction,
-silver with 30% copper alloy and zinc,
-melted in a graphite crucible at 1040 ° Celsius (1904 ° Fahrenheit),
-Lots of manufacture with 24 flasks, where we reuse the

material (scrap),
-The flask have a final temperature of 450 ° C (842 ° Fahrenheit) when casting the molten metal.
other information :
-Elimination of steam wax.
-ambient temperature was Ambient Temp = 26 ° C.
-Investment of Goldstar.

I am sending photos of these rings with porosity

Thank you in advance

Att

450 C is a typical flask temperature for sterling, but this alloy has 30% copper and zinc which could drive the necessary melt temperature up depending on ratio of copper to zinc.

If this temperature worked well for production in the past, perhaps the copper-zinc alloy is out of balance - more copper than previously vs zinc.

Molten silver absorbs oxygen from the air. Solid silver does not contain any dissolved oxygen = any dissolved O2 is forced out of the metal matrix and shows up as inclusions/voids on cooling. Thus you must melt in an oxygen free crucible.

There are several ways to do this.

https://www.google.ca/search?q=molten+silver+oxygen+exclusuion&oq=molten+silver+oxygen+exclusuion&aqs=chrome..69i57.13655j0j8&sourceid=chrome&ie=UTF-8

In my opinion: needs higher speed (rpm) and/or higher heat (temperature).

I'm new to the forum and happy to see a beautiful input here.

Why are you using. Ariana Grande's picture?

I'm a big fan of her and also copy everything of her.

As a precious metal, we use silver jewelry like rings, earrings, and so many and those are available in a jewelry shop.