I use Sparex #2 and have never seen the sludge people are discussing here, however it has turned a couple of sterling silver pieces of mine dark grey. Since they are from rock salt casting, they are very hard to clean. Does anyone know what might be causing it and how to clean them? Is it possible that I burned them some how?
I have left Sterling I’m Sparex II long enough to dissolve some solder joints. I would not be at all surprised, depending on the concentration, at the possibility of eroding the surface of the metal it self.
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Hmmm. Sodium casting. Sodium is bad for silver. It likes to turn it black and sometimes brittle. “Salt disease” is an issue for those of us who have done silverware holloware and flatware restoration. Pickle aka sparex is also bad for silver if left in too long.
My sweetie Timothy Green left a large silver casting in the pickle for at least five days. I kept warning him. “Get that out of there”. But then we’re married so of course he didn’t listen to me. When he finally pulled it out it was like a sponge. Pickle will also dissolve or weaken solder seams. The lower the temp solder the faster it will weaken and dissolve. So I never leave my silver in the pickle longer than oh say 20 mins.
I’m betting though that the salt is the culprit. Next time you try a salt casting be sure to remove the casting as soon as you can and clean it thoroughly in a sonic before you pickle. Then pickle for only 20 mins. If the silver is still black…time to try a new casting technique.
Have fun and make lots of jewelry.
Wow, thank you so much. After about 20 pieces these two were the only ones that turned black in the pickle. I left them in longer to try to clean them, so that was probably a huge culprit but I have an ultrasonic cleaner and never even thought to put them in there before pickling them. Thank you so much for that advice and for answering my question! Have a great day.
Thanks for your reply Holly,
It is sterling silver. What ended up happening, after I posted this question, was that I finally decided to just put it in Liver of sulfur. It came out the color of bronze instead of black. I added a little more soldering work to it and that time when I put it in the pickle it turned white! I guess re-heating it under the torch cleaned it somehow and the pickle worked. I can’t explain it. It sounds like I overheated when I melted it then, right? I feel like it was black right after I poured it into the rock salt. Reheating it with flux definitely did the trick.
It does. Thank you so much, I finally have an answer!
Even if you do not overheat it too much sterling gets oxidation when you cast it. It is the copper in it coming to the surface, it just will be a lot more if you do overheat it a lot, same as when you are soldering (ie: Firescale), but even uglier!
It is not copper coming to the surface, it is copper oxide and silver sulfide. The copper oxide is not soluble in the molten sterling, it is formed during melting and floats in the melt. As the metal solidifies it is forced out in front of the solidifying metal and much of it ends up on the surface of the casting and the rest ends up in the interstitial space between the sterling crystals. Also the investment is allowing oxygen to freely get to the cooling metal further oxidizing the copper near the surface. This is why every casting has a layer of fire stain present over the whole surface. These oxidation problems can only be solved by using a completely inert atmosphere during melting and casting. The second black compound is silver sulfide formed when the investment begins to break down from the heat of the molten metal and release sulfur dioxide gas. This is the likely culprit when castings will not come out of the pickle white because silver sulfide is not soluble in most pickle compounds where at least the copper oxide is and will result in a somewhat pink surface on some castings. but like all fire stain it will not remove the subsurface oxides and the purple haze associated with it.
By re-heating to annealing temp you are essentially beginning to depletion guild it or bring up the fine silver, and this burns of much of the surface oxidation. I also prefer to anneal my castings anyway, it helps to make them a little less brittle.
You cannot “burn” off the oxidation with a torch, you can only produce more of it, but it will definitely assist in the surface depletion process where the copper is oxidized and then copper oxide is dissolved from the surface by pickling.
Salt casting is not a real good idea because as Jo mentioned sodium chloride attacks silver. The chlorine in salt will react with silver forming silver chloride which turns black with exposure to light and is not water soluble so it resists removal by pickle. Silver chloride is soluble in ammonia so you can remove it from the surface by soaking the silver in ammonia solution.
Thanks for stepping in and clearing things up Jim.
Hi James–delighted to see you back! Hope to see more of you!
Wouldn’t there also be some silver oxide as well (although, granted, much less than copper oxide)?
I haven’t done any casting in about thirty years and was really surprised to hear that investment contains sulfur! Why would they put the daddy of all oxidizers in a casting material??
The few times I did cast way-back-when, I had them use 960 silver instead of sterling, and the pieces all came out white instead of black…:-)… The material was only marginally more expensive (quite insignificant at those old silver prices!), and, needless to say, it saved a tremendous amount of work…
Here is an old technique which could conceivably be thought of as ‘burning off the oxidation with a torch’ using an extreme reducing atmosphere:
I started out doing Yemenite filigree using the traditional benzine can for the fuel (not to be confused with benzene!) and foot bellows for air/oxygen. After soldering, we would often bend the air tube like this ^ with our free hand so that no air was added (by the bellows) to the flame beyond the ambient air in the room. This created an extremely soft, highly reducing flame (yellow/orange) which we would pass over the piece. Any sign of black turned to white (prior to pickling)!
Janet in Jerusalem