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Deoxidizing sterling silver

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Billybobjoe

Sun Apr 04 2010, 04:45AM

Registered Member #396 Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176

I've gotten into lost-wax casting using plaster molds and sterling silver. Its generally used to make jewelry because of its ability to reproduce incredible detail in wax models.

I've been getting excellent results however recently I've noticed symptoms similar to those described in this article

as gas porosity. Its barely noticeable until the final piece is to be polished, in which case its impossible to obtain a mirror finish.

From what I understand, this can be caused by dissolved oxygen in the molten metal. I am melting the sterling silver in a graphite crucible using a very small gas forge with a reducing atmosphere (the exhaust gas coming out of the top vent hole can be ignited). But during the short pouring process in which the metal is removed from the forge and poured into the plaster mold (I'm working on eliminating this atmospheric exposure) apparently the metal is absorbing atmospheric oxygen (also when I peek in the forge by removing the lid to check the melting progress). This short exposure probably isn't a problem, but it is exacerbated when the unused metal (the stuff for sprues) is remelted and used again.

I've heard from a few sources to use at least 50% "new" clean metal in each melt, but silver isn't exactly free, and I'll keep having to buy large quantities of new silver ($$$), a problem which is compounded by the fact that during each cast, about 20% of the metal is actually used, the rest makes up the sprues (channels for molten metal) and is cleaned to be remelted.

My question is how can I "deoxidize" sterling silver which has been remelted multiple times and has absorbed some oxygen? For each melt I use a borax and boric acid flux but from what I understand this only... (actually I'm not quite sure what flux does in this case... there's really no slag to collect...)

IntraWinding

Sun Apr 04 2010, 05:21AM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

This might help.

Also, perhaps you could develop a technique for 'degassing' your old silver back to 'as new' condition. I imagine this would involve heating under vacuum and/or with a suitable reducing agent.

Perhaps a good thick layer of molten flux floating on top of the melt would prevent oxygen reaching the metal?

Alan

Billybobjoe

Sun Apr 04 2010, 11:28PM

Registered Member #396 Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176

I did see that article and I will definitely try to keep the metal covered with a flame when pouring (which seems to be the only thing I'm doing incorrectly, and would be easy to fix if I had four arms...maybe I need to find some friends to help haha...).

Degassing is exactly what I'm looking for. A vacuum came to mind but I'd like to get some feedback on the effectiveness of that approach because while it should be possible for me to do, 1600F molten metal won't exactly be as straightforward as vacuuming epoxy.

I did just find this forum thread

It outlines a few techniques in the posts:

â€¢ "Other deoxidizers include silicon and zinc"
â€¢ "I would recommend forcing Nitrogen or Argon through the molten silver to draw out any latent oxygen"

Also on this page

â€¢ "a small addition of lithium is helpful in removing dissolved oxygen"

Any thoughts?

john bunsenburner

Mon Apr 05 2010, 10:23AM

Registered Member #1937 Joined: Sun Jan 25 2009, 12:28PM
Location:
Posts: 53

From the research I conducted cadmium was used to removed the bubbles however is now banned (for obvious reasons), though you might still have a look into that. You might want to have a look around here. As for the silver lost in sprues, I recommend you give a centrifugal casting machine a shot if you want to be serious about casting. Good luck with your endeavors.

Proud Mary

Thu Apr 08 2010, 11:22AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Floating a carbon disk on the molten metal will attenuate the formation of Copper (I) and copper (II) oxides, and hence gas porosity (but not shrinkage porosity) and the Cu2O inclusions with which it is associated.

Billybobjoe

Thu Apr 08 2010, 11:57PM

Registered Member #396 Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176

Proud Mary wrote ...

Floating a carbon disk on the molten metal will attenuate the formation of Copper (I) and copper (II) oxides, and hence gas porosity (but not shrinkage porosity) and the Cu2O inclusions with which it is associated.

The carbon disk was another method I heard and will give a try but doesn't do much to eliminate O2 in the metal I already have. And shrinkage porosity isn't the problem I'm having as my spruing is set up properly.

I should have mentioned that I'm doing vacuum assisted casting (didn't want to throw the word vacuum around to avoid confusion but essentially in this process a vacuum "pulls" the metal into the plaster mold through the porous plaster). This is different than vacuum casting where everything (melting/casting) is done in an actual vacuum chamber.

Putting a crucible of molten silver (quickly!) into a vacuum chamber is feasible and I will give that a try (when I'm home from school this summer).

I've also heard of bubbling argon through the metal which sounds feasible, but then I've also heard that argon can be absorbed (seems like silver has a much greater affinity to O2 than Ar though...)

IntraWinding

Fri Apr 09 2010, 12:27AM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

If the oxygen is released on solidification then use this effect to remove it. Melt the silver in a reducing atmosphere (don't make things worse!) and allow the melt to slowly crystallise from the base of the crucible up towards the top. Hopefully the oxygen will concentrate towards the top and be released. It might also separate some of the alloy components, but they'll get mixed back in on the next melt. I'm not sure how you'd arrange for the crystallising front to move slowly upwards though.

Alan

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