# Time to dissolve?



## mikeinkaty (Dec 26, 2012)

I don't think I've seen anywhere here how much time it should take to dissolve Sterling with Nitric acid. Hoke doesn't seem to get very specific on this point.

Soon as my nitric (70%) gets here my first use will be to dissolve 5 toz. What temp should I keep it at and how long should it take? I will use 200ml of dh20 and start with about 80ml hno3.

Once I get this first test done I'll start increasing volume.

Mike


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## etack (Dec 26, 2012)

The questions is how much surface area and what is the temp of the acid? those two things will effect your time the most. The more of each the faster the reaction. make your sure our container is large enough for the reaction.

Eric


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## goldsilverpro (Dec 26, 2012)

There are so many variables involved, your question is impossible to answer. In general, the higher the surface area of the silver, the higher the temperature, or the stronger the nitric in solution (up to about 60/40), the faster it will go. However, there are practical limitations on these to prevent such things as foamovers.

A problem you seem to have is that you want exact answers on everything and, generally, that is impossible. You will learn more by actually doing it. Just learn as much as you can and then dive in with your best shot. Take your time and think about what you're doing. This is something you'll soon develop a feel for.

I've run a lot of high silver material, such as sterling or coin silver contact points, and have never found it necessary to provide additional heat. The reaction, itself, will provide plenty of heat, if you do things right. I covered the material with the calculated distilled water and then fed the nitric in increments, allowing the reaction to die down somewhat before making another nitric addition. When all the calculated nitric had been added, I covered the container and let it work overnight. I did this in plastic buckets in a fume hood. If there was still some undissolved silver in the morning, I poured off the solution, put the silver in a beaker and dissolved it with heat, as in the process below.

A faster way would be to use heat. With an uncovered beaker, it should take from 220-235ml, at most, of 70% nitric + an equal amount of distilled water to dissolve the 5 oz, depending on the amount of copper in the sterling. With a rig to capture and provide perfect refluxing of the fumes, the nitric could be half this figure. With a watch glass on the beaker, the nitric usage would be somewhere between these extremes. You don't know how much nitric it will take until you do it. 

To use heat, put the 5 oz of sterling in at least a 1500ml beaker, place the beaker in a pyroceram Corning Ware dish, in case the beaker breaks or the solution foams over, and place the dish on the hotplate inside of a fume hood. Cover the material with the distilled water and heat at a fairly low temperature setting (say, low to med-low) until you see some steam coming off - about 130-140F, or so. Add a small amount of nitric - maybe 20ml, or so. You should see an immediate reaction and red-brown fumes. When the reaction slows down, give it a gentle stir, add a little more nitric, and repeat until a small addition creates no increased reaction and then stop adding. Cover the beaker with a watch glass between additions - each time you lift off the watch glass, hold it over the beaker and rinse it into the beaker with a very small amount of distilled water from a squirt bottle. This may take less (or, more) nitric than what you've calculated. If a small amount of silver remains undissolved, turn up the heat a bit but don't ever allow it to boil. After 10-15 minutes, if silver still remains, add a smidgin more nitric. The dissolving of 5 oz shouldn't take much more than an hour unless the sterling is in large chunks - heavy rings, for example.

Let it cool. If crystals form (probably won't happen), add some distilled water and heat gently to dissolve the crystals. Let it cool. Filter to remove any dirt. Cement out the silver with copper bus bar.

About the only fly in the ointment, that I can think of, would be the presence of tin. This could happen with such things as solder on the sterling foil from knife handles. Tin will convert to the slimy, insoluble, voluminous, metastannic acid from the nitric. If there's just a little bit, it should filter out OK. If there's a lot, Harold_V has often posted an incineration/HCl method for handling it. Search.


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## mikeinkaty (Dec 26, 2012)

Thanks GSP. I thought I had said the sterling was in cornflake form. Guess not. But an estimate helps. I had no idea if it was 10 minutes, 10 hours, or 10 days. Nothing I found gave much of a hint.

I picked through the sterling pretty well looking for things like tin and solder and other not-sterling stuff. All of this batch is plate ware, spoons, forks, large bowls, and other solid stuff like heavy rings.

I guess my engineering background is showing. OK State, 65', BSEE.

Mike


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