Tips and Information about making jewelry

With this blog, I hope to share my knowledge, successes, trials and errors, student's work, tips, and information about making jewelry.

Friday, April 27, 2012

Sterling Silver Metal Clay Testing Results with Casting Investment (Ring Plugs)

In September I posted the results from testing the sterling silver clay in August. In this test I found that using an investment plug causes a reaction with the sterling silver clay.

Someone told me that I could place this back into the kiln and fire it again to remove the black.
I did just that and found that it does not go away! I also tried pickleing it in Sparex, it still didn't fade away!
 The only way to remove it is to sand! 

So, fair warning to everyone. Do not use an investment plug to control the ring size. Wrapping it with fiber paper didn't work either.  For the sterling silver metal clay, create the ring three sizes up from the wanted finished size allowing it to shrink down close to the size needed.

Until next time!

Janet Alexander


  1. Janet, thanks for posting this. I've gotten this reaction before as well but not as much as in your photo. Did you refire in carbon? or open air. I was successful with torching it so as to completely cause firescale (something we try to avoid) then using pickle. Worked great. However the pieces where not as bad the one in your photo. Give that a try, nothing to loose!:-)

    1. Hi Patrick,

      I heated it up with a torch and then quenched it in pickle and still have the black. I repeated this several times and still have the black. I have basically depleted the ring leaving a fine silver coating everywhere except where the black stuff is.

  2. Hi Patrik, I fired it in carbon, only one stage, to 1500 F. I also tried pickling it in Sparex, on luck. I will try heating it with a torch and then pickle.

  3. I'm glad you posted this, as I had the same problem and couldn't figure out what happened. I was able to take off most of it by using an abrasive 3M bristle I know not to use investment! Luckily I just invested in a set of ceramic ring sizers from Rio Grande and they don't seem to cause the discoloration.

  4. I see this is an older thread so I am coming in to the conversation late but:)...
    I am really struggling with this issue too. I make rings to order in my shop, and I really do not find the shrinkage with PMC sterling to be at all consistent, it seems to vary wildly from one package to the next. I have had rings shrink anywhere from one size (I've actually had two packets of PMC now that produced rings which shrank only 1-1.5 sizes when fired) and up to four sizes. Frustrating to say the least, when you have customers waiting for their order!
    I did try the Hattie's Patties and found as you did, the black discoloration was impossible to remove.
    I really don't want to go back to PMC3, I love everything about the sterling except this shrinkage problem.
    Have you tried anything else recently that gave you better results? I would so appreciate any ideas.
    Thanks! Joanne

  5. Hi Joanne,
    I have found that if I make rings that are formed around ring mandrels that they shrink on the average of 3 sizes. Contrary to that, if I make rings by rolling the clay out 8 cards thick and then cut a hole in the center, that I get inconsistent shrinkage. I taught a class using that technique, and all rings in the class shrunk 3 to 5 sizes smaller! It was unreal. The same thing happened in the next day's class. All rings were fired together at the same time for each class. Needless to say I don't teach that class anymore. I have had better results using the mandrels, and wrapping the thinner clay around it. I am guessing the thicker the clay the higher the shrinkage, as far as the center hole goes.
    When I create the thinner rings I roll them out 3 cards thick and get on the average of shrinkage of 3 sizes down. I don't know how thick you are rolling yours. What I have done for the ring stoppers is to use the thicker fiber paper wrapped around the investment plug. Rio Grand sells these, #116-477 and then I place them inside a screen box so the carbon doesn't get between the paper, investment and ring.

    I hope this helps.


  6. Hi Janet,
    Thanks so much for your reply. I always use a mandrel for rings, and I roll them out by feel, I make my own molds so this just seems to work best. I am about to order some of the shrinkage stoppers you recommend from Rio, but I can't find a screen box anywhere, so I assume I will have to make my own. Do you have any advice on the type of screen to use? There is really no information out there on this subject, so I really feel like I'm in the dark here.
    Thanks again,

  7. Hi anonymous,

    Yes, I did make my screen box. You can find steel screen in most hardware stores. Don't use aluminum screen due to its low melting point.