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Sunday, October 27, 2013

I Begin the Metal Clay Firing Tests in Earnest and An Online Free Calculator for Metal Clay Ring and Pendant Sizing

I decided to go back to square one and run some tests.  My intuition (and a few isolated incidences) tells me some of my larger pieces may be only sintered superficially.

I think if I find the lowest temperature at which the piece appears to be sintered, the inside could still be very unsintered and therefore pose a potential problem in the long run.

So I want to know what’s the highest temperature I can fire at without showing signs of over-firing. 

The test I do for sintering is to sand the piece.  But this could only be showing me that the outside of the piece is sintered.

I recently had a two-part (onlay) piece that was, to all appearances, fully sintered.  I sanded it (120, 240, 400) then polished it and gave it a patina.   

A few days later, the onlay fell off, revealing unsintered clay on both halves of the piece.

I’ve decided to do some tests.  The more I read on the subject (granted it’s not all from one manufacturer/distributor… I’m reading about all kinds of metal clays), the more I realized there are so many factors that could contribute to an unsintered outcome.

Still, I have to try.

Here are some of the things I’m willing to change for my testing (and showing what my first test will be):
Ramp                     500°F/hr
Holding Temp       1420
Carbon Under       ½ inch
Carbon Over         ½ inch

I’m starting off with Hadar Clay Brilliant Bronze.   I’m firing inside carbon, then letting the pieces cool down to room temperature, then firing inside carbon a second time.

I’m not going to experiment with holding times.  I’m also going to fire in one bowl only (unless it gives out, then I’ll see what I have that’s similar).  I’m not trying different carbons.  I am, however, going to start storing my carbon in an airtight container rather than as it is now which is about five different bowls of it open to the elements (under a porch).

So here are the pieces I decided upon for my testing.

  • 2-cards thick ¾ x 1½ inch rectangle
  • 4-cards thick ¾ x 1½ inch rectangle
  • 6-cards thick ¾ x 1½ inch rectangle
  • 8-cards thick ¾ x 1½ inch rectangle
  • 12-cards thick ¾ x 1½ inch rectangle
  • 20-cards thick ¼ inch square

This is what they look like before firing.  I stamped each one with its card-thickness to make sure I know what's what when I'm testing for strength and sinteredness.

It makes sense to me that difference thicknesses (and/or different combinations of clays) may require differences in firing schedules/methods.  I hope this testing leads to some conclusions.  Otherwise I’m wasting a lot of clay.

Phase one is finished and I'm letting the pieces cool (overnight).  Check back tomorrow to see the results after the phase two firing.

Did you know that although metal clay is a relatively new technique to the contemporary jewelry maker (starting in 1990), the ancient Incas made jewelry from precious metal powders?

BTW, here’s a little internet tool that you can use to help determine size/shrinkage of various metal clays for pendants and rings.  Click here


  1. Ok, so the carbon should be kept in an air tight container?? How did I miss that? Are you familiar with "magic carbon"? Have you used that and which do you recommend?

  2. Hi Brenda,

    I don't know how crucial it is that the carbon be in an air tight container, but I was reading an article that mentioned that and I'm thinking, "Geez, my carbon is out there in the elements, gathering moisture (and bugs)." I figure it can't hurt to keep it locked up and if anything it may help.

    I've heard of magic carbon, but I haven't used it. I use just one kind and keep it that way to minimize the number of changing factors in the equation.

    I use the carbon Hadar recommends. Let me see if I can find out the name... BRB