Sterling Silver Centrifugal Casting Problems — Firescale/Brittleness

As a non-expert, if your casting processes/methods/timing have not changed, I'd look at the raw material purity.

that was of course my first shot. i used different suppliers of metal but i dont seem to avoid the previously described problems. the extent to which i find these problems varies from time to time but i dont seem to manage to avoid it completely, which result in great risk, specially when casting original models. my guess is i am oxidizing the metal too much but my currents skills prevent me from finding a bold solution to this. thanks.

excellent artice!

What would you say is the most effective way of reducing oxygen pickup while attempting centrifugal casting?

as I said I use a propane/oxygen mix.

If you look at the top of the linked page you will see ← Previous Next → this will turn the pages of this on-line book which contains the info you seek,,and then some....

To help you out this is a link to the 'next' page...

Is there a way to oxygenate the silver prior to pouring? That will eliminate alot of other metals from the batch. Silver can dissolve the most oxygen of any metal, so, if you oxygenate the molten alloy, you can float out the impurities.

Otherwise, you will have to eliminate oxygen from the batch and perhaps pour at a lower temp or use more flux in your batch and scrape it off more carefully before you pour.

Solar Eagle refers to "oxygen pickup". Oxygen from the air dissolves into molten Silver.

As the Silver hardens, this silver is forced out of solution and often damages the casting - rough surface, trapped voids etc. It additionally can oxidise the copper component - as the SE article states.

Oxygen in Silver is a well known problem and this might help oxygen problem casting silver

and this similar search might provide more leads.

casting silver oxygen

Thank you for the feedback.unfortunately the links you posted only redirect me to a google search page, which is the way I found thid forum.

Solar Eagle's reference is great as to determining the causes and possible outcomes of oxidized molten sterling silver but it doesnt quite go deep as to how to prevent it in an open cruicible melting operation previous to a centrifugal casting.

i've read articles that refer on how to prevent oxidization by furnace melting and other variables appliable to vacuum casting experiences, but havent found consistent tips as to preventing this oxygen pickup while attempting centrifugal.

any tips?

I assume you failed to read some of the links, your Royal Highness.

Solutions to this problem lie in following links and forums untl you find a solution...

Once again I thank you.

But are there any articles in particular that you would recomend from the aproximately 372,000 ones the search engine throws?

This is an honest question, not a condescendent one.

i found a few amazing papers but havent yet found a pragmatic-on hands tip that succeeds in bringing light to my obvious ignorance.

This is an honest remark, not a condescendent one.

Anyhow I appreciate you investing your time in aiding my quest.

The sieving of the details is part of the game.

repeat the search and add several - minus signs after the main terms.

Molten Silver oxygen uptake +forum and so on

the minus terms exclude whatever you decide you want to prune

pruned

Years ago I worked in a smelter and we used washed Willis Quartz fines for purifying precious metals prior to casting.

This was done by adding a small amount on top of the batch in the oven, allowing a period of time to pass, then skimming the quartz off the top just before pouring began.

The purification time interval was determined by the trial and error method of dipping small samples out of the batch then pouring it into small molds and observing the result until the process was proven.

It worked well on pure gold, silver, & copper as well as mixtures of these with other metals when we made brass and other exotic metal alloys.

Don't they use crushed lime too?

I seem to remember seeing that on one of the "How it Works" TV programs?

Look here

Here is a CR4 posting from a while ago

Here is a cool PDF on silver casting

Have fun and good luck!

thank you for your answer.
how exactly did you carry out the skimming procedure?

Would doing the work in a low oxygen environment work? Heating and casting under a flow of something like nitrogen should be practical given the small volume of molten material.

I've read stuff about creating a reduced oxygen atmosphere by flowing for example nitrogen.
I just dont know how to manage this nitrogen flow throughout the process.
what wquipment would one use?

Thank you for the feedback Spinco. Actually I pour into a refractory plaster mould (otherwise known as investmente mould). I am quite certain that the copper I use is electrolytic copper and should be 99% pure. I am wondering if the quality of the investment can have an influence in the result of the casting in terms of oxydation. I tried two new different investments in the past months. Both of them were significantly cheaper . i didnt see a substantial difference as to the quality of the castings in terms of superficial defects or cracking of the moulds. but I am not very knowledgeable as for chemical processes occuring in the investments when heated or when carrying out the casting sequence.

You use the term "refractory plaster". Is this a water based plaster or an ethyl silicate based investment? I would not recommend the use of a water based plaster at the high temperatures needed for copper or silver. Plasters will lose their bond above 1800 degrees F and revert to dry plaster with the water released into the metal causing porosity issues. What are your new investments and methods of coating? Spent 40 years in Foundry industry and am familiar with several investment methods as well as Sand and Permanent Molds.

Interesting blog (usual on CR4 I find).

My ex in the UK is a registered silversmith in the UK, and I was her "HOT" man for years....but I never saw this problem before. But we did not use a centrifugal unit either...

My thoughts, over and above all the excellent posts already made, is has anything changed in your methods? If it worked before, it should work now I feel.....

Is the moulding plaster (as a name to use!) the same type and quality as you used before?

Is it the same wax and wax supplier?

Have you changed the silver temperature? Do you even measure it, or is it done by "experience"?

Is the gas burner, burning all the oxygen? Do you heat the silver surface, or just the crucible?

Is the (lost) wax being fully removed?

Is it a different wax supplier?

Are you heating the mould sufficiently before casting? Do you even heat it before use?

Have got the same problem when casting but without the centrifugal unit?

Are the sprue's maybe too small and slowing the metal down too much? Try increasing the width of the sprue's.

I am sure that I have not covered everything that may have been changed, but spend a moment or two reflecting upon what could be going wrong for you, just in your methods......maybe only some small thing has been changed....the flux or the flux supplier.....

Best of luck, keep us informed.

You've probably seen this but just in case you haven't and if this your method it may be of some benfit: https://books.google.com/books?id=e_09Enaf4tIC&pg=PA103&lpg=PA103&dq=melting+with+a+reducing+flame&source=bl&ots=YrbVkzdR2L&sig=E80E13q4MesKl1KKNczE4WT0LbA&hl=en&sa=X&ved=0CC4Q6AEwBGoVChMIyoXR6drfxgIVy82ACh0PbACx#v=onepage&q=melting%20with%20a%20reducing%20flame&f=false

https://books.google.com/books?id=e_09Enaf4tIC&pg=PA103&lpg=PA103&dq=melting+with+a+reducing+flame&source=bl&ots=YrbVkzdR2L&sig=E80E13q4MesKl1KKNczE4WT0LbA&hl=en&sa=X&ved=0CC4Q6AEwBGoVChMIyoXR6drfxgIVy82ACh0PbACx#v=onepage&q=melting%20with%20a%20reducing%20flame&f=false

What temp are you casting at. It sounds like you are pouring too hot.

Hi silvCrow. To be honest I dont measure the temperature of the molten metal.
My equipment is quite rudimentary if something. It works well but of course when it comes to determining details or facts like these it makes it harder to be able to adjust the methodology.
I try to melt slowly and with a minimum of oxygen flow through the torch. but it has happened to me a few times that the metal got solid too fast and the cast came out incomplete, so there is always the fear of this happening again when letting the centrifugal arm free.