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
