Silver melting (really, I've searched first...)

[MGH]

I’m not sure if my silver is coming out as it should after melting. I see a little bit of off-white coloration on parts of the top surface. I am cleaning the buttons with hot 5% Sulfuric Acid. Also, I’m getting the same kind of off-white coloration in my flux. This has happened with two different melting dishes. The first one I had previously used to melt two small batches of refined gold powder as well as melting about 40g total of inquarted gold. After all that, my flux was still clear with just a little bit of pink/green color. After melting this silver, it turned off-white and hazy. Same thing happened with a new clean dish.

The source material is clean silver wool as in the picture. For the second melt (the larger of the two buttons), I cleaned the wool with an ammonia wash, followed by two DI water rinses before melting. Not sure if that actually accomplished anything. The silver looked clean before and after.

I’m melting in a 3 inch dish with borax flux, using MAPP gas, and quenching in water. Any ideas why I have this discoloration in my silver and flux? Sorry, no pictures of the flux at the moment. You can most easily see the discoloration on the larger piece of silver on the right side. Even after a good long soak in hot 5% Sulfuric Acid, this color remains. Not sure if it's the same thing going on with the bottom sides of the buttons.

 

[NobleMetalWorks]

Some people use sulfur to discolor coins, they are generally called "coin doctors". One of the methods to do this is to use weak sulfuric acid. It's done to trick -people into believing the coins developed coin toning over a long period of time. Naturally toned coins command a premium. Now days, some people are selling doctored coins and metals and actually stating they are doctored, and still commanding a premium, specially high purity silver bars that show their crystals and have been toned.

Here is a primer on the subject:

http://news.coinupdate.com/png-adopts-coin-doctoring-definition-1321/

Here are what some toned coins look like:

So if you are starting with pure silver wool like it seems you are, with very little impurity, I would suspect two things are happening. First, the really different coloring is actually because you are pickling in weak sulfuric acid. Sulfur is one of the favorite ways to cause forced toning of silver, some people even burn a cigar under a silver coin to do this, some use weak sulfuric acid. It can also be done with sodium hydroxide and sulfur like in this video:

[youtube]http://www.youtube.com/watch?v=VrA7Jyw1uOs[/youtube]

The other thing that it looks like your silver has going on is some oxidation. You can prevent a lot of oxidation by simply keeping a torch on the silver as you pour it.

Scott

 

[Lino1406]

Does your borax flux include KNO3?

 

[MGH]

The other thing that it looks like your silver has going on is some oxidation. You can prevent a lot of oxidation by simply keeping a torch on the silver as you pour it.

Scott

Thanks Scott. Probably do have those two phenomena going on there. But oxidation can be white colored? I thought it would be dark.

I’m not pouring into a mold. I’m just melting in a dish, allowing to cool enough to solidify, and then plucking out the button to quench in water. Next time I’ll try keeping the flame on it from farther away to still allow it to cool, but hopefully prevent oxidation.

Does your borax flux include KNO3?

Not that I know of. I’m using “20 Mule Team Borax” straight from the Box. Are you saying that I should have KNO3 in my flux when melting silver, or that it might be a cause for this discoloration?

I experimented a bit to see if I could clean these up a little. I placed them back in some fresh 50/50 Nitric Acid/DI Water for about one minute. It certainly did take the shine off (as expected) and now you can see the crystal structure better too. I had some pictures, but having trouble posting at the moment.

Thanks all,
-Matt

 

[Lino1406]

KNO3 is "whitening" both silver and gold
by removing iron and metal oxides etc.

 

[Anonymous]

I'm not sure if I've read your 1st paragraph correctly. If not, please correct me, but are you (did you) melting/melt your silver in the same dish as your gold? If so, that may be part of your problem.


Kevin

 

[MGH]

I'm not sure if I've read your 1st paragraph correctly. If not, please correct me, but are you (did you) melting/melt your silver in the same dish as your gold? If so, that may be part of your problem.


Kevin

Yes, I did, so I suppose I deserve a scolding if that's my problem. Can you tell me why this would cause the discoloration in this scenario.

 

[Anonymous]

I'm not sure if I've read your 1st paragraph correctly. If not, please correct me, but are you (did you) melting/melt your silver in the same dish as your gold? If so, that may be part of your problem.


Kevin

Yes, I did, so I suppose I deserve a scolding if that's my problem. Can you tell me why this would cause the discoloration in this scenario.

It's called contamination. I am not that experienced to explain it to the "T", but what you've done is mixed whatever contaminants that may have been mixed in with your gold into your silver when you remelted into the same dish.

Buy a couple of melting dishes and mark the bottom of each one, (Ag) on one, then (Au) on the other. Or simply write gold or silver on them. From that point on, NEVER, EVER , NEVER EVER melt another metal that is not marked for that dish. That will eliminate the possibilities of other precious metals in the melt.

I wouldn't be surprised if you have gold melted in with the silver. And because I'm not sure, and I'm sure you're not sure, I would redissolve all the silver and start over. You might even recover some gold too.

Kevin

 

[MGH]

It's called contamination.

Actually, I should have remembered, but I did say originally that this also happened when I used a brand new dish. I understand that I should have dedicated dishes for different metals, but even at the risk of being argumentative, I don't think this is a case of cross-contamination. Contamination from oxides or incompatible flux maybe, but I have the same thing happening with a new dish as with a used one, so I'm looking for what else might be going on. Of course I still must consider the other possibilities discussed in the posts above too.

I've been trying to find more info on the mechanism by which KNO3 acts as a whitening agent. It's entirely possible I just haven't searched hard enough yet, but does anyone else have more detail on that?

 

[Anonymous]

Are you washing the powder good enough with distilled water? Preferably hot or boiling distilled water. It's possible that could happen if the acids aren't rinsed completely out of the powder.. I think.

Kevin

 

[MGH]

Are you washing the powder good enough with distilled water? Preferably hot or boiling distilled water. It's possible that could happen if the acids aren't rinsed completely out of the powder.. I think.

Kevin

It was never powder. I melted the wool pictured above directly. So no, I cannot say with certainty that the source material was pure to begin with. I did wash the material of the second button with ammonia, but can't say that made a difference either way. It just seemed that this discoloration was something I was causing, and I wanted to be able to prevent it next time.

 

[Anonymous]

Are you washing the powder good enough with distilled water? Preferably hot or boiling distilled water. It's possible that could happen if the acids aren't rinsed completely out of the powder.. I think.

Kevin

It was never powder. I melted the wool pictured above directly. So no, I cannot say with certainty that the source material was pure to begin with. I did wash the material of the second button with ammonia, but can't say that made a difference either way. It just seemed that this discoloration was something I was causing, and I wanted to be able to prevent it next time.

Oh yeah,, I forgot about the steel wool looking pile in the picture.

My guess is that isn't pure silver. What I would do is like I said before, put it in a 50/50 Nitric/Peroxide bath and let it dissolve it and then add double the amount with distilled water, hang a clean piece of copper in it and watch it start dropping the silver. After that, pour through a filter and give it at least 3 hot Distilled water rinses, let it dry, then put it in a CLEAN melting dish. I wouldn't use those other ones for this, not unless you know how to clean the dish out. There are some threads that talk about cleaning out the melting dishes, but I never tried it yet.

Kevin

[UPDATE]
If that wire roll is pure silver, then I would guess that it has contamination from possible oils from handling and possibly other stuff.

 

[MGH]

Just an update: I re-melted the smaller of these two buttons to at least get a nicer shape, and also to see if I could avoid the discoloring by keeping the torch on the metal longer and allowing it to solidify while still under a flame. The physical shape is nicer, but I still got the discoloration, maybe to a lesser degree but it’s hard to say.

I figured I’d try some simple polish – it’s the purple colored cream you see in the background. It did remove the discoloration pretty easily, so I think I can safely assume this is just on the surface as opposed to contamination throughout. The only drawback is that this is probably an abrasive polish and I probably lost a little silver. If I do this again I’ll be sure to weigh the pieces before and after.

I’ll be looking for a graphite mold or two in the future.