Now that silver is so very expensive, more and more of us are trying out the base metal clays. One of the questions that comes up regularly is about firing gemstones embedded in copper and bronze clays.
For the most part, any stone that can be fired in place in fine silver clay using a 1650°F/2 hour firing schedule can be fired in place in carbon. Quite a few stones that don't do well in an open shelf firing at high temperatures can be successfully fired when buried in carbon. In a recent class, I had a several students try to fire CZs and lab grown spinel in place in copper and bronze clays. The stones in bronze came out beautifully, but quite a few of the ones fired in copper came out with a frosty haze on them. Intense cleaning removed some of the haze, but we couldn’t get the stones back to their original sparkly state. While we were able to clean the top surface of the stones, even with a clean-out opening behind the stones, I could not remove enough of the coating on the faceted sides of the stones to bring back their original sparkle.
This happened to almost every piece made from pure copper clay in January and February 2011. Repaired and re-fired pieces always ended up with cloudy stones. I ran some alternate test firings. I tried firing hotter, lower, shorter, and longer, but there was no combination of firing that eliminated the problem. I tried firing them in batches of copper only, in batches mixed with bronze pieces with much the same results. The stones in the bronze were beautiful, but the stones in copper were cloudy. I tried using new carbon, old carbon, coconut carbon and coal carbon with no change in result. I tried two different brands of copper clay with no change in results.
I am fairly certain that the issue has to do with the pure copper clay—it, or some other byproduct of firing copper in carbon, may be adhering (fuming?) onto the stone surfaces. To test this, I tried replacing some stones using bronze to make the repair and applying a bronze wash over the copper to blend in and even out the surface color. These stones came out beautifully!
Not many people seem to have been firing stones in the copper clay, but if you do try it and you have a problem with cloudy stones, you can try to fix them by replacing the stones and applying a bronze wash to the piece and re-firing.
I just did some re-testing (May 2011), using the same carbon, container, kiln, and firing schedules. Pieces fired at 1470°F and 1560°F came out fine. The piece fired at 1700°F did come out cloudy, so temperature may be a factor.
I put out a query to the metal clay yahoo group and several people reported problems, while others reported no problems. It may have to do with temperature and atmospheric conditions--more moisture in the air promoting the fuming/coating. Jokes about phases of the moon and dancing around in a ritual manner come to mind--except that I didn't do any dancing around any of the batches. This may just remain a kiln mystery until more successes and failures display some pattern.
Sadly, this raises more questions than answers, so I am sending out an appeal: Please let me know if you have success or failure when firing gemstones in copper clay and maybe we can find a pattern and figure out what is going on. Respond here or to my email firstname.lastname@example.org.