# Problem with melting refined silver and gold in a crucible



## WillGreen

Dear All, 

I am new here, this is my first post. 
I am a goldsmith from Ireland. I make mokume gane. 
I am having a problem that I do not understand. I refine my own silver and gold. It was never a problem. To stay with silver, I refine silver by putting it in nitric acid and I precipitate it by saturating the solution with copper. I end up with silver cement, which I wash with water several times. I boil the silver in water with some baking soda in it. The last time I tried, I washed the silver with hydrochloric acid. I used a new crucible to make an ingot. After this one melt, my crucible is pretty much destroyed. There is black muck all over it. It takes enormous heat to get to melt the silver - I can't really pour it anyway. The black gunk sits on top of the metal, producing a shield. The silver melts under the gunk. It drives me pretty insane by now, because I tried everything I can think off and I still did not get rid of this gunk. I destroyed 6 or 7 crucibles by now ... I have made hundreds of ingots. I really know how to do it. There is something wrong here, but I do not know what it is. Can someone please help? 

Thank you for reading and with kind regards, 

Will


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## butcher

Will, 

You probably burned off all of the silver as a white chloride smoke; silver becomes a volatile gas as a chloride in the melt.

You should not try and melt silver chloride you will loose silver in white fumes.

In your melt if any silver remains it is not converting to metal in the melt, adding soda ash in volumes may help, (a carbon source like flour may assist also, or you may have to resort to using a metal collector and reducing flux in the melt).

If you have to melt silver chloride (AgCl), it should first be converted to elemental silver metal powder or an oxide of silver before melting.

Silver chloride can be melted with a whole lot of sodium carbonate (soda ash) (washing soda), but this is not a good process in my opinion.

After cementing silver from a silver nitrate solution, the silver is converted to metal silver and can be melted after washing traces of copper from the powders, no reason to add baking soda or anything else.

Making silver chloride should be avoided if possible.

There are several methods for converting silver chloride to metal before melting.
NaOH and Karo syrup (sugar) method.
HCl acid and aluminum method.
Iron and sulfuric acid method.
The last two methods form elemental silver metal powders as the aluminum or iron dissolve into solution taking the chloride from the silver.

P.S. gold chloride will also form a volitile gas and burn off in the smoke as a yellow gas, that can form yellow or violet stains on cold sufaces where fumes condense, gold also should be converted to metal before melting, or any salt NaCl or other chloride salt removed from gold powders before melting.


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## WillGreen

Dear Butcher, 


Thank you for your informative answer. 
I do not want to contradict you, as you seem much more knowledgeable about this than me, but I would like to raise some points anyway. 
As said, I refined the silver in nitric acid, precipitating it with copper. 
You say that I probably produced silver chloride, but there is no Cl in HNO3, so this is not possible. 
I only washed the silver in HCl the last time I tried. 
There shouldn’t be any problem with what I am doing, as it is a very simple process. 
After I get the silver cement out of the nitric acid, I wash it with distilled water over and over again, until the water is completely clear. One should assume at this point that all the nitric acid is gone, but this does not seem to be the case, because if I boil the cement and the water, the solution turns blue once again. And I may be wrong, but I think that this is the problem: I am melting the silver while there is still nitric acid attached to it (even if it is completely dry). In my opinion, this explains the black gunk - unless I am completely wrong. If I am right, the question is how to get rid of the nitric acid. 
It is bizarre that the literature does not deal with this. There are videos on how to refine silver on You Tube; no one mentions this problem. But I cannot solve it. I lost weeks of work because of it by now. 

With best regards, 

Will


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## MysticColby

I was also confused by butcher. I don't believe he has silver chloride at all - he made metallic silver

I'm not sure why you washed the silver with baking soda, I haven't heard of that before. I have heard of the HCl wash.
With cemented silver, there is still copper nitrate trapped inside the cemented silver, so you will never get above about 99.4% pure. washing with HCl will dissolve as much copper as possible, which is why you get the blue color. You should keep washing until all the blue is gone.

Now for the melting problem. you added copper and silver did cement and stay cemented. there won't be any nitric remaining after that. plus, nitric would evaporate/boil long before silver melts. I believe that most remaining nitrates will decompose into metals and nitric acid when heated (like that copper nitrate trapped inside the silver), the nitric then boils away - which is why it's safe to do this outside.

black gunk that destroys crucibles... what are the crucibles made out of? I assume you're using borax as a flux? borax will look like a black gunk and float on top of the silver, but shouldn't destroy crucibles unless something is added to it like sodium carbonate, and then it takes a while to completely destroy it.
have you tried using a graphite rod to remove the gunk? (dip it in, stir briefly, pull it out, scrape gunk off rod onto a brick, let rod cool when it gets too hot)
you've gone through several crucibles, so I assume you've had time to let the gunk harden. well, for that matter: is it liquid when the silver is melted? can you describe any observations you have on this gunk?


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## WillGreen

Dear Mr. Colby, 

Thank you for your answer. 
I refined some silver again and was just going to filter it and wash it with HCl, but let me answer your questions. 
I think that I read about boiling silver cement in distilled water with a teaspoon of baking soda in Ian Ferguson’s book on Mokume gane. 
The crucibles are used are the common types. They are made of fused silica. I do not use borax as a flux, because I consider the silver to be pure (as good as); no flux is necessary since there will be no oxidation. The crucibles are seasoned, of course. I have never seen such black gunk before - it certainly is not borax. As I said before, it floats on top of the silver, creating a shield. The silver melts under it, after a lot of trouble - I almost need to keep the crucible upside down to get some of the silver out! It looks like black, dirty enamel and it actually behaves that way. This answers another of your questions: it never really melts, it stays very sticky. I have no clue what it is. I bought silver sheet in the meantime because I cannot make a simple ingot - which I know how to do, of course. 

With kind regards,

Will


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## Palladium

What is the source the silver is coming from? What kind of copper are you using for precipitation? What is your filtering technique?


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## WillGreen

Hi, 

The silver comes from Burger Edelmetale in Germany. It's pure silver. 
The copper is 99.7 pure, the rest being oxygen for the most part - that is what they say. It contains no phosphor. 
I use a Buchner funnel with a vacuum pump for filtration. 

Best, 

Will


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## Palladium

WillGreen said:


> Hi,
> 
> The silver comes from Burger Edelmetale in Germany. It's pure silver.
> 
> 
> Best,
> 
> Will




This is the silver your trying to melt?
If it's pure then why are you dissolving it and dropping it with copper?


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## qst42know

If your cement silver retained traces of nitrates, adding HCL would form aqua regia converting your silver powder to silver chloride.

Read this thread and pay attention to Manuel to get your silver back.

http://goldrefiningforum.com/phpBB3/viewtopic.php?f=50&t=8184&p=99747&hilit=silver+chloride#p99747


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## butcher

I could be wrong but I still believe you made some silver chloride when you used the HCl wash on the cemented powders, or brought chloride salts into the melt from its use.

I would not wash the silver nitrate in HCl, even if HCl did not make silver chloride powders before the melt, which they probably will somewhat by forming a light passivated coating of silver chloride on your silver metal powders (this is a lot of surface area so the chloride content could be a lot even if the silver powder did not change colors or show signs of the passivation, as qst42know describes from the aqua regia formed, in solution, or just from the silver reacting with the HCl acid, any chloride salt formed in the powders from the wash in HCl, these chloride salts would form silver chloride when in the melt.

Lets look at a chloride salt like ammonium chloride (NH4Cl) and another salt like ammonium nitrate (NH4NO3), these will not form aqua when dry, but can in a fusion or a melt where the high temperatures can make a chemical reaction to occur that may not normally, if gold was added to these salts and the temperature raised to where the salt melted to a syrup (fusion), gold would be dissolved into this salt solution (heating too strongly would vapor off some gold chloride as fumes), if we dilute these salts with water and a little HCl we can get a solution that will test positive for gold in solution with a stannous chloride test, here the salts and high temperature chemically reacted with the gold in the high temperatures.
Here we see where the salts form and an acidic solution in the fusion of the salts, the chloride and nitrate chemically react in the melt to act as aqua regia and dissolve gold.

Cemented or precipitated metal powders will contain salts of the acids we used these salts cannot be completely washed away by rinsing them, this is why we incinerate before changing acids, example precipitating a metal from an HCl solution will leave chloride salts in the powders, adding a nitric acid to a chloride salt will form a form of aqua regia, this is why we incinerate before changing acids when we do not want to form aqua regia and dissolve gold, the previous acid used will leave salts of that acid, the new acid will donate hydrogen to these salts and oxidize these previous salts back into the acid that formed them.
NaCl + H --> HCl acid
Then 
HCl + HNO3--> aqua regia

This is similar in principle how the saturated salt solution and nitric acid leach work to dissolve gold, or the poor mans aqua regia which is formed using HCl and sodium nitrate salt and used to dissolve gold.

The cemented silver powders would not benefit from a wash in HCl, copper nitrate is very soluble and rinsing will wash away as much as possible, washing the silver powders in HCl would form a surface coating of silver chloride on the very fine silver metal powders, HCl will react with silver, and form a passivated coating of silver chloride on the silver, and no more silver will be attacked by the acid, these fine silver powders would develop this coated shell of silver chloride when washed in HCl, although the bulk of silver would not convert, this cemented silver is no longer silver nitrate, although it can hold nitrate salts in the powders or copper nitrate salts, or even some nitric solution if damp, and as qst42know stated any chloride or nitrate salt when acidified or heated strongly would make aqua regia and thus can also form silver chloride in solution or in the melt.

Heating the silver powders cemented from a silver nitrate solution with copper, will retain some nitric or nitrate salts, these nitrate salts will vapor off as NOx gases in the melt long before silver even begins to melt, they will not interfere with the silver forming a molten metal.

I also agree with MysticColby, it does sound like there may be something you are using in the flux could that be what is dissolving your crucibles, unless you are just burning them away with the oxidizing flame of the torch trying to get the molten flux to give you silver metal with a hotter flame, also I do not understand why you would have much that much flux in the dish.


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## qst42know

If you did not rinse away all the baking soda (sodium bicarbonate), baking soda when heated becomes soda ash (sodium carbonate). Soda ash attacks crucibles causing them to melt. 

Skip the HCL and skip the baking soda and see if your problem goes away.

If you want pure silver you might want to look into a small silver cell. It will remove the trace contaminants that can build up in over used silver.


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## maynman1751

> This is the silver your trying to melt?
> If it's pure then why are you dissolving it and dropping it with copper?


I have the same ?????? as Palladium!!!!?????


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## WillGreen

Palladium: I make mokume gane. For some rings I only use silver and copper, for others I use different sorts of golds, for others I use gold and platinum or gold and palladium. 
If I make a ring with silver and copper, I end up with a lot of lemel, copper and silver together. I put it in nitric acid to get the silver back.
I also refine my own gold for gold only rings - not for combinations of gold and Pt or Pd; I find that too complicated. 
Butcher and qst42know: thank you for your information. I am sure you know much more about this than I do, but this morning I made an ingot which turned out to be completely fine (after weeks of torture). I dissolved the silver in nitric acid, brought it back with copper, washed the cement with water, then distilled water, then with Hcl, then with distilled water again - there is no longer any gunk in my crucible. You are right that I will create silver chloride if I use Hcl; however, temperature plays a role here: I cannot give you exact temperatures, but when your solution is hot, you will create much more silver chloride than when your solution is cold - if it is really cold, the amount of silver chloride will be negligible. 
I still do not know where all the gunk came from and it is still bugging me.


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## goldsilverpro

WillGreen said:


> Palladium: I make mokume gane. For some rings I only use silver and copper, for others I use different sorts of golds, for others I use gold and platinum or gold and palladium.
> If I make a ring with silver and copper, I end up with a lot of lemel, copper and silver together. I put it in nitric acid to get the silver back.
> I also refine my own gold for gold only rings - not for combinations of gold and Pt or Pd; I find that too complicated.
> Butcher and qst42know: thank you for your information. I am sure you know much more about this than I do, but this morning I made an ingot which turned out to be completely fine (after weeks of torture). I dissolved the silver in nitric acid, brought it back with copper, washed the cement with water, then distilled water, then with Hcl, then with distilled water again - there is no longer any gunk in my crucible. You are right that I will create silver chloride if I use Hcl; however, temperature plays a role here: I cannot give you exact temperatures, but when your solution is hot, you will create much more silver chloride than when your solution is cold - if it is really cold, the amount of silver chloride will be negligible.
> I still do not know where all the gunk came from and it is still bugging me.



I still think it could be silver chloride. From what you've said, I see no other possibility, except maybe copper nitrate (see below). All you have is silver, copper, nitric, water, and, of course, HCl. It takes a lot of rinsing to remove all of the nitric/copper nitrate. If these are not totally removed, the addition of HCl produces aqua regia, which will produce silver chloride. The second batch was maybe rinsed better. Assuming there is nothing else in the mix, other than what I mentioned above, all this could be avoided by eliminating the HCl. Why you're using it is a mystery. It is unnecessary. If you insist on using an acid, use some 10% sulfuric. However, that would also be unnecessary. I would just rinse the cement a number of times with hot water, were it me. 

Are you diluting the nitric 50/50 with water. If not, when it cools, you could have some copper nitrate crystals mixed in the silver. This could be removed but it probably would take more rinsing. It not totally removed, it could be the gunk.


Here's how I do it:

(1) Assuming the concentrated nitric you're buying is about 67-70%, dissolve silver in nitric solution diluted 50/50 with distilled water, preferably as below - all the water first and then the nitric in increments. About 2.4 ml of 50/50 nitric will dissolve 1 gram of silver. For copper, it takes about 8.2 ml/gram. 

I usually calculate how much distilled water and nitric I need, put the silver in a bucket under a hood, and add just the water. In a 5 gallon bucket, I don't do more than 2 pounds of silver at a time because, when the solution is more than about 2"-3" deep, the dissolving gets too slow. Also, this gives plenty of room for any foaming that occurs. Then I add about 1/4 of the nitric. When the fizzing slows down considerably, I stir it slowly and then add another 1/4. This is repeated until all the nitric is in - takes about 30-60 minutes to get it all in. It gets hot. To keep it hot, don't let it cool much between additions. I then cover the bucket and let it work overnight. In the morning, I pour it off carefully into another bucket. If there is any silver left in the bottom, I dissolve it with some fresh nitric and water in an adequate sized beaker and add the liquid to the main solution. 

(2) Filter solution to remove dirt, etc.

(3) Cement silver with clean pure copper. Bus bar is best, if you can get it. New copper tubing is second best but you might trap some silver inside it. Solid, heavy, copper wire is OK if it's in single straight pieces and not all twisted together. Whatever you use, make sure the pieces are long enough (sticking an inch or two out of the solution) so you can easily remove them. The more copper surface area you have, the faster the silver will cement. If you have excess unreacted nitric in the solution, it will dissolve copper until the nitric is used up. Only then will the silver cementation start permanently forming. Stir occasionally and let it sit overnight. 

Stir, let it settle a bit, and test for completion by first putting a drop or two in something dark, like a black plastic spoon. Add a drop of salt water or weak HCl. If it turns cloudy at all, give the solution a stir and let it react longer with the copper until the test shows no white cloud. Remove the copper and scrape and/or rinse (squirt bottle) any attached silver into the container. Although most of the silver falls off during cementation, it sometimes speeds things up to knock the cement off the copper a couple of times during cementation. Occasionally stirring helps do this also.

(4) Filter and rinse very well with a number of hot water rinses. Let each rinse drain almost completely through before adding more hot water. Be attentive and don't let the silver dry out. I usually stir (break up) the silver somewhat with a stainless lab spatula between rinses - try not to burnish the silver and don't puncture the paper with the spatula. The lab spatula I like for most anything is called a Scoopula.
http://www.sciencecompany.com/Scoopula-Stainless-Steel-6-inch-P16134.aspx

(5) Dry

(6) Melt


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## WillGreen

Dear Chris, 

Thank you very much for explaining to me how you refine silver - I very much appreciate.
Next time I refine silver I will follow your procedure to the letter. 
The nitric acid I buy is 69 - 70 % pure. I do not add any water to the solution. I didn’t know that this was necessary. I also never measure. Occassionally, I use a bit of hot water to get the reaction going. I also let it work overnight. In the morning, I often find crystals in the solution. They disappear when I add a bit of warm water. Perhaps this is the copper nitrate that you are referring to. 
I will refine some silver without using Hcl - indeed, why use it if it is not necessary? 

Thank you very much again. You enlightened me. 


Will


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## maynman1751

WillGreen said:


> Dear Chris,
> 
> Thank you very much for explaining to me how you refine silver - I very much appreciate.
> Next time I refine silver I will follow your procedure to the letter.
> The nitric acid I buy is 69 - 70 % pure. I do not add any water to the solution. I didn’t know that this was necessary. I also never measure. Occassionally, I use a bit of hot water to get the reaction going. I also let it work overnight. In the morning, I often find crystals in the solution. They disappear when I add a bit of warm water. Perhaps this is the copper nitrate that you are referring to.
> I will refine some silver without using Hcl - indeed, why use it if it is not necessary?
> 
> Thank you very much again. You enlightened me.
> 
> 
> Will


If you follow Chris' directions, there will be no problems. His avatar says it all......he's a pro! 8)


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## MysticColby

qst42know said:


> If you did not rinse away all the baking soda (sodium bicarbonate), baking soda when heated becomes soda ash (sodium carbonate). Soda ash attacks crucibles causing them to melt.


After reading everyone else's responses, my money's on this theory.

And HCl wash is, at the most, optional to begin with (I am under the impression it is to dissolve any copper oxide that would of been on the surface of the copper used to cement the silver). Since you're having problems, I agree you should skip the HCl wash as well. Even small amounts of borax (seasoning) should grab this copper oxide.

Everyone I've seen here urges ~30% nitric for dissolving silver (1:1 dilution of 65-70% concentrated), then maybe even dilute again (1:1) just before adding copper to cement. Too concentrated and the nitrate salts will crystallize out of solution (as you described).


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## goldsilverpro

MysticColby said:


> qst42know said:
> 
> 
> 
> If you did not rinse away all the baking soda (sodium bicarbonate), baking soda when heated becomes soda ash (sodium carbonate). Soda ash attacks crucibles causing them to melt.
> 
> 
> 
> After reading everyone else's responses, my money's on this theory.
> 
> And HCl wash is, at the most, optional to begin with (I am under the impression it is to dissolve any copper oxide that would of been on the surface of the copper used to cement the silver). Since you're having problems, I agree you should skip the HCl wash as well. Even small amounts of borax (seasoning) should grab this copper oxide.
> 
> Everyone I've seen here urges ~30% nitric for dissolving silver (1:1 dilution of 65-70% concentrated), then maybe even dilute again (1:1) just before adding copper to cement. Too concentrated and the nitrate salts will crystallize out of solution (as you described).
Click to expand...

I'm being picky but, believe it or not, with 70% nitric diluted 1:1 with water, you end up with 41% nitric and 65% diluted 1:1 yields about 38%.


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## Alentia

WillGreen said:


> I also let it work overnight. In the morning, I often find crystals in the solution. They disappear when I add a bit of warm water. Perhaps this is the copper nitrate that you are referring to.
> Will



The crystals you are finding in the morning are pure silver nitrate salts. (I hope you never threw them away...) Silver nitrate salts forming when solution is oversaturated. Adding water, as you do, dissolves silver nitrate salts and silver is ready to be cemented.


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## mikeinkaty

Was this problem ever resolved? Could he have had a bad batch of crucibles?

Mike


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## MGH

I’m also wondering if this was ever resolved. I’m having similar trouble. Here’s the background with my situation:

I can’t claim to have vast experience, but I have melted silver in a melting dish before - starting both with clean sliver wire and cement silver that I had processed myself. The buttons all came out looking decent, needing to have the flux cleaned off with a soak in hot 5% sulfuric acid, but then being nice and white.

I then processed some scrap sterling silver jewelry (from several different sources), about 8 ounces of silver. I tested each piece with Schweter’s solution prior to processing. I dissolved the material in 50/50 nitric acid/DI water, filtered to a clear blue solution, then cemented with clean copper pipe. I rinsed the cemented silver 7-8 times with hot DI water, testing the last two clear rinse waters with ammonia to ensure that no more copper was present. The remaining water was evaporated away on a hot plate, with the silver in a 1000mL beaker.

I obtained what I thought was reasonably pure silver powder, but upon melting the first half ounce, my melting dish turned into a black (and brown, green, and red) mess. I used a MAPP gas torch for the melt, and also used a brand new dish. The problems didn’t become immediately apparent until I started to let the button cool before plucking it out to quench it in some water. At a certain point after taking the heat off, a sort of crust formed across the surface of the button. I figured this was a sign that it was cool enough to grab with the pliers, but the button was still molten and it deformed when I tried to grab it. I waited just a bit more, then grabbed it, but it seemed the flux had hardened quite a bit by that time and it didn’t want to come out. I repeated the melt a few times to see if I could improve my timing, but the same thing kept happening. The only thing I can do is grab the metal at a semi-solid state; otherwise the flux (just using Borax) solidifies and I can’t get the button out. There’s also some metal left stuck in the dish. After quenching the mangled button in water, it has the same discolored crud on it as the melting dish.

I started with a brand new dish, seasoning with Borax before adding any silver powder. A few pictures are attached. I’m using no other acids or reagents other than nitric acid, deionized water, solid copper pipe, and Borax. Any other ideas? Could PGMs in the silver cause these kinds of trouble in the melt?

Thanks,
Matt


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## lazersteve

Here's a typical borax color chart:

Borax Colors in Dish

Steve


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## MGH

Thanks Steve for the link and the chart. I need to study this for a bit, then I'll post back here when I make some progress.


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## muconium

MGH, did you have any resolution to your problem? I'm having the same situation with silver cement from karat-scrap. I used coin silver to inquart. The gold came out great. But the silver melt wasn't and I ended up with a chocolate brown borax residue in my crucible. The flux discolored the silver button as well. I can deal with both with dilute sulphuric, just wondering what the source of the discoloration is. It covers the button when cooling and makes plucking up the button with tongs difficult, as you mentioned.

According to Lazersteve's chart, it's molybdenum? Or nickel?


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## MGH

I still haven’t really circled back to this yet. Been thinking and trying to answer some of my own preliminary questions - especially what to consider as the nature of my MAPP torch flame. My searches tell me this is a “cold” reducing flame; not strongly reducing, but more reducing than it is oxidizing because I’m not mixing any oxygen or even air into the fuel.

So under those conditions, my guess about my contaminants is as follows:
Copper, opaque red
Iron, pale green (green might also indicate Chromium or Vanadium)
Nickel, opaque grey
Cobalt (blue)

Muconium, if you’re using a MAPP gas torch like me, then I would guess that your chocolate brown color is actually the mixture of opaque grey from nickel mixed with some red form copper. Molybdenum seems too exotic to expect it in either of our scenarios.

I suppose all these colors simply indicate insufficient washing of my cement silver. My next step will be to re-dissolve an ounce or so of my silver powder and then re-cement. I’m still puzzled though. I stirred at least a half dozen times over three days while this silver cemented, and then washed many times with DI water, even testing the last two washes with ammonia (and obtaining no color change). We will see…

Please feel free to correct flaws in my conclusions or logic. I’ll update again once I get back to the refining bench (Spring is finally here! Time to do some gardening too…). Thanks.


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## MGH

I finally got back to some hands-on refining in the last few days. I think my problem with this batch was an incomplete cementation or even just dirty copper. I’ll try to find a good bus bar for the next batch.

Rather than starting over (dissolving in nitric acid again), I washed the cement with hot 5% sulfuric acid. The solution turned blue, and then I rinsed several times with water. I did another wash with more sulfuric, but the second wash did not [apparently] leach out any more contaminant. The melt improved remarkably - resulting in a clean button and clean melting dish. I know this step shouldn’t be necessary; my goal for the next batch will be to do it right the first time. :|


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