# I cannot get the nitric acid out of my silver cement.



## WillGreen

Dear All, 

Let me tell you about a problem which causes me sleepless nights. 
It’s all very simple. 
I start by putting fine and sterling silver in a flask and then I pour nitric acid on it (and a bit of water). 
The silver dissolves. When the reaction is done, I drop the silver by putting copper in the acid. After everything is done, I end up with some deep blue nitric acid and silver cement at the bottom. 
All of this is elementary. 
Now comes the part that I have been doing consistently wrong. 
I let the silver sink to the bottom, then I start pouring off the nitric. I pour off as much as I can, then I fill the flask with water. And so on. I do this until the liquid in the flask has become completely colourless, assuming that, at that point, all the nitric acid has disappeared. Actually, it’s not that simple, because nitric acid is heavier than water, so it will stay at the bottom. Anyway, I used perhaps 400 ml of nitric acid to dissolve the silver and I think that I used close to 30 litres of water to get all the acid out of the flask. 
When the liquid is really colourless, I pour off as much as I can and I heat the flask up to dry the silver. 
When the silver is dry, I put it in a crucible and start the melt (with an oxy-propane torch). 
I smell nitric acid when I melt the silver and the same thing happened over and over again: a crust forms on top of the silver as well as at the bottom of the crucible (which is ruined). It is impossible to pour an ingot. To get the silver out of the crucible, I keep it almost upside down, using a much higher flame than necessary to melt silver. The silver forms tears and eventually drops out of the crucible, drop by drop. 
It’s obvious that the silver is not clean. It still contains nitric acid. I have no clue how to get the nitric acid out of the silver. The last time I used so much water, I was certain that I was overdoing it. And it still didn’t work. Can someone please explain the proper technique to me? 

Thank you very much, 


Will


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## martyn111

how much silver are you trying to dissolve in your 400 ml of nitric acid?
To quote Butcher in another thread, 2.8ml of 70% nitric with 3ml of distilled water will dissolve 1 gram of silver.
Calculate how much nitric you will need to dissolve the silver you are trying to refine (2.8ml / gram of silver) and add the nitric in increments to the silver and distilled water until everything dissolves, this method will reduce the amount of acid used, lower your costs and reduce you waste stream.


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## NobleMetalWorks

You should be exhausting your Nitric Acid when you dissolve the silver. I dilute to 50/50 Nitric by adding nitric acid to water, not water to nitric acid. I then pour this into a beaker I already have silver I want to dissolve in. If the acid becomes exhausted and there is still silver left, I make small additions of nitric acid. If all the silver is dissolved, i take a weighed silver bar and add that to the solution until the acid is exhausted and no longer is attacking the silver bar. Then I weigh the silver bar to figure out how much silver was dissolved, for my records to keep track of yields, etc. In this way you exhaust all the nitric acid in your solution and are left with no more free nitric acid, it's all being held so to speak by the silver nitrate in solution.

If you are not exhausting the nitric acid in your solution before you attempt to cement out with copper, the nitric acid will attack the copper making copper nitrate. Which would turn your solution blue. When you get a chance you might want to do a test with the types of metals you are dealing with, and try dissolving them in Nitric, Sulfuric and Hydrochloric acids individually. Then when you observe different colors you might have an idea of what is happening and where you can improve your process.

The other more obvious introduction of copper was your sterling silver. As you dissolved your sterling silver into solution you were dissolving silver and copper. That's probably where you got the first initial blue color. If you were observing this process, and noticed that the blue color became more intense after you cemented with silver, you would know if you had free nitric acid in solution or not. Regardless, if you make sure all the nitric acid is exhausted when you dissolve the silver, you will not no issues other than the copper already dissolved into solution as sterling silver.

Instead of washing it by adding large volumes of water, since the silver is in crystal form and not powdered, you can filter it off and washing it in the filter itself. You use far less water and you wash much better.

Scott


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## goldsilverpro

When rinsing something in a beaker, I always stir well after adding each addition of rinse water and then let it settle again before decanting. If you have fines that don't settle well, this can take a long time. If you don't stir it, you can easily retain acid and copper solution in the cement. For slow settling stuff, you can speed this up by not waiting until the fines are 100% settled. You can pour all the rinses into a bucket as best you can, let that settle, and worry about it later.

About the only time I rinse in a beaker is on stuff that is heavy and settles very quickly, like high quality gold powder. Otherwise, I find filtering to be faster and the rinsing to be much more efficient.

With silver cement, I first test the solution with a drop of dilute HCL to make sure 100% of the silver has been cemented out of the solution. The drop of HCl should produce no white cloud, at all. When testing, it is best to put a few drops of the solution coming from the stem of the funnel in a beaker and then add a drop of dilute HCl to that. I then let it settle until clear and carefully siphon off as much of the clear solution as I can without disturbing the sludge. I then filter this solution in a gravity filter. When the solution has filtered, I add the sludge to the same filter. After this has filtered, I rinse (hot water works the fastest) until the drips coming out of the filter stem show no blue and have a pH of about 4 or higher. I let each rinse run through until it is barely dripping. When adding rinse water, I only use enough to cover the sludge. Most rookies make the mistake of using too much rinse water - they fill the entire funnel with water .Each time, I also rinse down the entire inside surface of the filter paper with a squirt bottle. If you are 100% sure that all of the silver has been cemented, you can use tap water for rinsing. 

Of course, a vacuum filter will work faster (usually - not always) than a gravity filter.


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## WillGreen

Thank you very much people. 
I really appreciate. 
I know what to do now. 

Kind regards, 


Will


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## WillGreen

One last question: I assume that it does not make a difference is there is a bit (less than 5 %) of nickel silver in the silver that I am dissolving.
Am I right? 
I sometimes use nickel silver for mokume gane, but very little. I assume that the nitric acid will dissolve it and that it will not be dropped. 

Thank you all again. 

Best regards, 

Will


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## Geo

you do understand that nickel silver is not silver at all. 

from wiki :

Nickel silver, also known as German silver,[1] Argentan,[1] new silver,[1] nickel brass,[2] albata,[3] alpacca,[4] or electrum, [5] is a copper alloy with nickel and often zinc. The usual formulation is 60% copper, 20% nickel and 20% zinc.[6] Nickel silver is named for its silvery appearance, but it contains no elemental silver unless plated.


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## kadriver

To dissolve/digest sterling/925 silver first weigh the metal and use this weight to calculate the approximate amount of nitric needed.

Then clean the silver mechanically with hand tools to remove stones and I use a small toothbrush sized wire brush to remove grime.

Then heat each piece to redness with a MAPP torch (propane takes too long for me).

Place all the silver in a suitable heatable container, then add DISTILLED WATER to the incinerated silver - I add 1ml DH2O for every gram of sterling/925 to digest.

Then place on medium heat and start adding small amounts of concentrated nitric acid about 30 or 40 ml at a time.

I have found that 1ml of concentrated nitric acid will dissolve (approximately) 1 gram of sterling/925 silver, and about 0.7ml nitric will dissolve 3N fine silver.

This is not to dispute what butcher has described, this is just my experience when it come to digesting 925/sterling silver.

When the silver is nearly completely dissolved, I allow it to stand for a few hours (overnight is better) then draw off the silver (and copper) nitrate solution and filter it.

I like to have a small amount of undissolved silver left in the container. 

This lets me know that there will be little to no excess nitric to be concerned about.

Excess nitric will consume copper real fast and put out brown fumes, plus the cement silver will re-dissolve as fast as it is cemented and sometimes causes it to float, causes the silver to be as fine as pluff mud making it difficult to rinse and clean.

On the other hand, sometimes the nitric acid gets used up so completely that when the copper is inserted it just sits there and no silver will cement.

If this happens, I have learned (from other forum members) to just add a few drops of nitric acid and stir, then the silver will begin to cement.

Also, if the silver nitrate is highly concentrated, then it can form a hard crust of silver metal on the copper and cause the cementing to stop.

If this happens, I dilute the silver nitrate with an equal amount of distilled water and try again. This usually solves the problem.

Here is a video of the technique I use to completely rinse my cement silver free of excess nitric and copper.

If one is careful and removes the copper (and adding fresh pieces) before it begins to drop tiny bits of copper in the cement silver, combined with this washing technique, Then there will be little to no copper in the cement silver and it will be very close to 3 nines fine.

Adding a few drops of ammonia to the wash water will reveal any dissolved copper by turning the wash water blue (after adding the ammonia).

[youtube]http://www.youtube.com/watch?v=94Z2cNjixrY[/youtube]

Hope this helps!

kadriver


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## goldsilverpro

kadriver said:


> I have found that 1ml of concentrated nitric acid will dissolve (approximately) 1 gram of sterling/925 silver, and about 0.7ml nitric will dissolve 3N fine silver.



When dissolving silver *in an open vessel with no refluxing*, half of the nitric goes up into the air and is lost. Here's the overall equation for this after the NO gets oxidized to NO2. The NO2 is the brown fume.
2HNO3 + Ag = AgNO3 + NO2 + H2O

Calculating from this, the ml of nitric needed per gram of silver is:
70% nitric - 1.181 ml/g
68% nitric - 1.220 ml/g
67% nitric - 1.245 ml/g

Copper would take 4.15ml/g with 68% (1.22 x 3.4). For 90/10, Ag/Cu, it would take 1.513ml/g with 68% [(0.9 x 1.22) + (0.1 x 4.15)].

If one were able to reflux these fumes with 100% efficiency with a condenser, these figures would be cut in half. Even with just a watchglass, some refluxing occurs, but I would guess not much. The use of hydrogen peroxide can increase the efficiency. I think that using weak solutions and keeping the solution cool can improve it. There are probably other ways that I haven't thought of.

Everyone uses different techniques and that's probably the reason people report different nitric usage. In my case, I usually had huge scrubbers and was able to dissolve in open containers, which I definitely prefer for several reasons. Nitric has always been cheap for me. Using 68% nitric, hot, with a 50/50 dilution, I have proven to myself while dissolving 10s of 1000s of oz of all sorts of silver alloys, that the 1.22ml/gm works almost every time, at least with an open container. That's why I always give that figure in my posts. I guess I should qualify that in the future for those that are doing things differently.

So, kadriver, you must be doing something to increase the efficiency. Otherwise, your numbers would be impossible. Can you clue us in as to how you are accomplishing this?


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## Anonymous

This has been a very interesting thread. It's always a pleasure reading the threads on this forum from some of the world's greatest minds.


Kevin


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## MysticColby

probably just a watch glass. That's what I use, and it looks like almost no red fumes escape. there is also a respectable amount of dripping from the watch glass, despite the solution only being warm. I'm sure it's not as efficient as possible, but I would estimate the refluxing is much better than you made it seem

Also, if you add all your water to the reaction container first, then add small amounts of nitric to that and heat, the nitric concentration is rather small, giving a lot of water to re-dissolve the NOx fumes before they even leave the liquid.


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## goldsilverpro

MysticColby said:


> Also, if you add all your water to the reaction container first, then add small amounts of nitric to that and heat, the nitric concentration is rather small, giving a lot of water to re-dissolve the NOx fumes before they even leave the liquid.



That's how I did it and it took 1.22ml/g. In fact, I'm the one that introduced that method and the similar method for aqua regia to the forum. Doing this isn't that intuitive. There are still jillions of people in the world that pre-mix these acids before adding them to the material. Some have pre-mixed all their lives. All the books have you pre-mixing the acids. If I hadn't introduced this, I would bet that everyone would still be pre-mixing the nitric and AR. I joined the forum about a month after the forum started and everyone was pre-mixing their AR. I introduced and re-introduced, over and over, what I call the 2 acid method (cover with HCl, heat, add HNO3 in increments until no reaction) and, for about a year, the only one that had tried it and was using it was Lazersteve. Now, most everyone has seen the light and these methods have become the norm.

The dissolving reaction actually produces NO gas and not NO2. The NO gas isn't soluble in that solution, at least in the sense that it dissolves and becomes part of the solution as ions. As a lot of gases are soluble in liquids (e.g., SO2), it is slightly soluble but it essentially remains as NO gas in the solution. Only when it escapes the solution and is oxidized by the oxygen in the air to NO2, is it able to truly dissolve in water and form ions. When H2O2 is added to the solution, it oxidizes the NO produced to NO2, which dissolves and converts back to HNO3. This enables a higher efficiency of nitric usage and mostly eliminates the evolution of NO2, the red-brown fumes (the BFRC, as some have put it). 

If you look up the solubility of NO in water, you will find that it is much more soluble in cold water than in hot water. This is true with most gases because the heat forces the gas out of the solution. On the other hand, most chemicals, like table salt, that actually dissolve and form ions, are more soluble in hot water than in cold water.


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## NobleMetalWorks

goldsilverpro said:


> MysticColby said:
> 
> 
> 
> Also, if you add all your water to the reaction container first, then add small amounts of nitric to that and heat, the nitric concentration is rather small, giving a lot of water to re-dissolve the NOx fumes before they even leave the liquid.
> 
> 
> 
> 
> That's how I did it and it took 1.22ml/g. In fact, I'm the one that introduced that method and the similar method for aqua regia to the forum. Doing this isn't that intuitive. There are still jillions of people in the world that pre-mix these acids. Some, all their lives. If I hadn't introduced this, I would bet that everyone would still be pre-mixing the nitric and AR. I joined the forum about a month after the forum started and everyone was pre-mixing their AR. I introduced and re-introduced, over and over, what I call the 2 acid method (cover with HCl, heat, add HNO3 in increments until no reaction) and, for about a year, the only one that was using it was Lazersteve. Now, most everyone has seen the light.
> 
> The dissolving reaction actually produces NO gas and not NO2. The NO gas isn't soluble in that solution, at least in the sense that it dissolves and becomes part of the solution as ions. As a lot of gases are soluble in liquids (e.g., SO2), it is slightly soluble but it essentially remains as NO gas in the solution. Only when it escapes the solution and is oxidized by the oxygen in the air to NO2, is it able to truly dissolve in water and form ions.
Click to expand...


GSP,

I am glad you introduced and re-introduced this method. I started learning chemistry on this board, I never did anything but the most basic chemistry in high school. Taking the MIT courses I have learned that textbook chemistry, and real life chemistry are two totally different things. Chemistry is about exact measurements, and correct additions of, but it's also about observing and correcting. I prefer making small additions, and observing the reactions, etc instead of following a recipe. It's what I consider part of the craft work that makes each individual refiner different from another. It's being far more intimate with what you are doing, it's more dynamic, more intuitive. It's probably saved me a boat load of money in chemicals.

Anyway, thank you GSP. I consider this to be one of the most important things I have learned, while learning chemistry. The act of observing and adapting dynamically.

Added: The MIT courses I am taking are free online courses. I didn't want to make it seem I was attending MIT. 

Scott


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## goldsilverpro

Thank you, Scott,

I didn't invent that AR method. I may have come up with the nitric method for silver
, but it is basically a spin-off of the AR method. I learned the AR method about 32 years ago from a guy I worked for (see my signature), who was the best and most innovative refiner I've ever known. If I ever had a mentor, it was him. However, I have worked for or have been around many large refineries in my life and they all pre-mixed the acids. Maybe that's changed since they now have access to the forum.


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## kadriver

goldsilverpro said:


> kadriver said:
> 
> 
> 
> I have found that 1ml of concentrated nitric acid will dissolve (approximately) 1 gram of sterling/925 silver, and about 0.7ml nitric will dissolve 3N fine silver.
> 
> 
> 
> 
> So, kadriver, you must be doing something to increase the efficiency. Otherwise, your numbers would be impossible. Can you clue us in as to how you are accomplishing this?
Click to expand...


GSP, My figures are just estimates. I have used 1.22ml nitric per gram of sterling/925 silver to digest and I always had excess nitric.

When I added copper I would get brown fumes and the cement silver would tend to float - plus the soluiton got hot and consumed the copper at a rapid rate.

I used to just add a couple of spoons of urea to kill the excess nitric, but even then the solution would heat up and produce brown fumes.

I cut back the amount of nitric from 1.22ml per gram to 1ml nitric per gram and I have not had a problem with it since.

Another benefit is the cement silver has a nice looking crystaline structure.

With excess nitric present the cement silver was like very fine powder and was much more difficult to rinse.

Keep in mind that I am doing tiny batches (about 800 to 1000 grams per batch) compared to the volumes you were working with.

Also, there is usually some undissolved silver left over after I am done - as nearly all the nitric acid is consumed by the reation.

I am not trying to dispute your numbers, just reporting what I have observed based on my experience.

Of course this is just an estimate and I could be totally out to lunch with my claim of 1ml nitric to 1 gram 925/sterling!

I think Oz posted a thread on this somewhere on the forum that gives detailed amounts of nitric used to dissolve silver.

 Edit: I am also very careful to keep a watch glass or cover on top of my reaction vessel at all times.

kadriver


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## kadriver

I used to pre-mix my AR to dissolve gold, and pre-mix nitric with distilled water to dissolve silver.

Incremental nitric additions, along with incinerating before disgestion, have been two of the most valuable tools taught to me by this forum.

Good stuff - all of it - we are really lucky to have all this and to have Pros like Goldsilverpro, 4metals, Oz, Lou, and Harold - plus countless others.

They have all been very helpful to me and my refining education - and I am still learning.

kadriver


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## Palladium

I always wondered about them numbers you posted Chris. I guess the number really never bothered me because i'm not a numbers guy. What i mean is that the refining techniques i have learn have all been from everyone here. So i got to see trial and error at it's finest before i even started practicing refining. I learned from the best! I use numbers to get estimates of where i want to go with it and then i drive by observation. Actually i do almost all my refining this way because i learn through observation to make up for my lack in other areas. I do all most all my silver and gold filled in open stainless and your estimates seem quiet right. I also notice when i shift to a beaker that the efficiency seems to go up also. Your teaching goes a long way on this form. My techniques of 5 gallon buckets, measuring cups, pyroceram, and hot plates comes from you. The business model belongs to Harold. The wisdom of professionalism, 4 Metals, and the list goes on and on. If you weren't mentioned it wasn't because i haven't learned something from you because chances are somewhere along the line i did. This forum is a great thing!


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## MysticColby

thanks for the technique, GSP. I bet it's the ultimate in flattery to have someone tell you about your own process 

I should do my own nitric:silver experiments to see how much I actually use.
In practice, I tend to just put a kg or so of silver in a 2L beaker, fill to 500 ml with DI then slowly add 500 ml nitric. reaction stops, then fill to 2 L with DI, pour off liquid, then repeat until silver is gone.
Knowing what I do about steric hindrance, I would bet than the amount of nitric needed to dissolve 1 g of silver is less if you have excess silver available (add 1 ml nitric to 1 g silver vs. add 1 ml nitric to 100 g silver). I'll try both scenarios!


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## WillGreen

Hello, 

Two little reactions: 

1) I know, of course, that nickel silver does not contain silver; for the rest, except for its composition, I know nothing about nickel silver, so I wasn't even sure that it would dissolve in nitric acid to begin with and that dropping the silver with copper would not also drop the nickel silver - this may sound stupid, but I didn't know. 
In the meantime I know: nickel silver dissolves in nitric acid and dropping silver with copper does not drop the nickel silver. 

2) I do not want to contradict people who know infinitely much more about all of this than me, but - and I didn't say this in my last post - in order to get rid of the excess nitric acid I 1) calculated the how much nitric I would need, using the figure of 2.8 ml per gr. of silver and 2) I had some silver ready to throw into the solution in order to saturate it. I am pretty sure that I used as much silver to saturate the solution as I used silver to begin with, therefore I have no doubt that the figure of 2.8 ml is much too high and I promised myself to use 1.4 ml per gr. next time. Now that I read these posts, I will use 1.3 or 1.25 and we will see. 

Again, it is certainly not my intention to criticise anyone. I am just letting you know what I did and what I found out. 

With best regards, 

Will


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## SynKronos

Can't believe what I am reading. 
If using .999 you would not have a "cement".

If creamy white fine though precipitating separately then mostly likely tin or lead has been added to make 925. Most likely lead. Thermally separate then repeat the process.


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## goldsilverpro

WillGreen said:


> Hello,
> 
> Two little reactions:
> 
> 1) I know, of course, that nickel silver does not contain silver; for the rest, except for its composition, I know nothing about nickel silver, so I wasn't even sure that it would dissolve in nitric acid to begin with and that dropping the silver with copper would not also drop the nickel silver - this may sound stupid, but I didn't know.
> In the meantime I know: nickel silver dissolves in nitric acid and dropping silver with copper does not drop the nickel silver.
> 
> 2) I do not want to contradict people who know infinitely much more about all of this than me, but - and I didn't say this in my last post - in order to get rid of the excess nitric acid I 1) calculated the how much nitric I would need, using the figure of 2.8 ml per gr. of silver and 2) I had some silver ready to throw into the solution in order to saturate it. I am pretty sure that I used as much silver to saturate the solution as I used silver to begin with, therefore I have no doubt that the figure of 2.8 ml is much too high and I promised myself to use 1.4 ml per gr. next time. Now that I read these posts, I will use 1.3 or 1.25 and we will see.
> 
> Again, it is certainly not my intention to criticise anyone. I am just letting you know what I did and what I found out.
> 
> With best regards,
> 
> Will


For 67% Nitric, it takes about 1.22ml plus 1.22ml of distilled water, per gram of silver. A gram of copper takes about 4.15ml.These would be true in an open top container. With a watch glass covering the beaker, it might take a bit less.

I usually just remember that a gallon of nitric will dissolve 7 pounds of silver or 2 pounds of copper. Or, you might remember a gallon of nitric will dissolve 100 troy oz of silver. All these, although not exactly the same, are quite close enough for practical work. This is not rocket science.


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## jccoleman

I have this problem. I left a silver dime in the nitric acid until no reaction. So all the nitric was used. Then I cemented the silver out of solution and filtered. Here's my problem...on the filter paper along with the silver looks to be very fine copper 'mud'. I don't understand why this is happening. What could be causing this?


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## jimdoc

jccoleman said:


> I have this problem. I left a silver dime in the nitric acid until no reaction. So all the nitric was used. Then I cemented the silver out of solution and filtered. Here's my problem...on the filter paper along with the silver looks to be very fine copper 'mud'. I don't understand why this is happening. What could be causing this?



What are you using to cement it? One dime is a tiny experiment.

Jim


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## Palladium

Did you just leave the dime in the acid without heating?


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## jccoleman

Actually 2 dimes, but still small. I have used no heat, other than garage heat at 100F, on the dimes. The solution was originally used to dissolve silver from some jewelry. It was quite some time ago, so I was checking it for silver because I could not remember if I cemented all the silver out. When some appeared, I left solid copper wire in solution. I began filtering the silver as it fell off the wire. As the solution filtered the copper looking sludge accumulated on the filter. To 'clean up' the filtrate, I put a few drops of nitric in the solution and circulated it though the filter. It did get rid on the brown sludge but I wasn't sure if it dissolved some silver. So, to use up this Nitric I used the dime. When no bubbles were evident, I stared with the wire again. It appears, if I try to recover all the silver then I get into this copper sludge. I haven't read of this. 

I


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## g_axelsson

How do you know it is copper?

Göran


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## jccoleman

Copper is the preferred and predominant choice in the electrical industry because of its high conductivity, both electrical and thermal. In order to obtain the required properties, unalloyed high purity copper is almost always used.......There are approximately four dozen different wrought alloys that contain a minimum copper content of 99.3 weight percent .....Unalloyed pure copper, rather than its alloys, is used almost exclusively as electrical wire conductors.


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## butcher

I doubt it is copper (mud).
Most copper salts are soluble, especially nitrate salts.

It sounded like the acid was used previously on other materials, if so you could have several other metals in solution besides the metals in the dimes.

Copper will displace the metals which are less reactive than copper (valuable metals).
If this acid was used on other materials it could have other metals or salts of acids which were used in the previous processes.

The black precipitate could be dirty gold or some other metal (possibly some copper with gold).


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## jccoleman

I'm pretty sure the solution was only used on a 10kt gold necklace and some silver coins. I wanted the gold. The dimes were used because I was pretty sure I had unreacted nitric. I cemented some silver out then and never saw this 'copper' mud. I sat it aside, probably a year, in a bottle. 

What would happen if the solubility limit was reached; the copper salts would precipitate out correct? It seems to only happen when I, my term here, over cement. 

I don't have anything black. It's been silver or this brown mud.


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## Palladium

Put a few drops in a black plastic spoon. Add a few grains of table salt. If you still have silver in solution then it will turn white, cloudy, or both.


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