Restrictions on firing gemstones in PMC

     My previous posting with Rio Grande's chart on survivability of gemstones when exposed to the heat of a PMC firing has sparked a response from Bridgette Rallo of The Greenwoods Studio. Ms. Rallo is a jeweler, educator, Heart of Stone Studio professional customer, and expert on PMC. She says:

You know, Judy, I really question exactly how the stones they fired at Rio looked after they came out of the kiln...I have never heard of a stone that can survive a firing at the temperatures needed for PMC 3, never mind PMC+ or Standard. The only exception is calibrated hematite, which can take 1100 degrees for 30 minutes. I had an assistant who put a bezel into the kiln without removing a Mexican lace agate and I noticed it after about 5 minutes...the temp had reached 1030 degrees and I was able to salvage the stone but it did darken quite a bit. When Rio says the stone survived, they mean it didn't blow up in the kiln. But I will guarantee you that it had considerable color change. I get color change even on pieces of sea glass, which is why I make pre-fired bezels for them now.

    So, it appears that great care and discretion needs to be practiced when planning to include gemstones in any PMC firing. I am listing some other approaches worth considering:

--Firing only synthetic gemstones and CZs (Cubic Zirconias), although there is no absolute guarantee that these stones won’t change color or at least darken, in the kiln.

--Pressing the gem into plasticine to make a mold, then filling that mold with plaster of paris, popping it out when dry, and then substituting that for the gemstone during firing. After firing, carefully drill or chip out the plaster and you have an automatic seat for the stone. (I suggest drilling a hole through the bottom of the metal seat so that light gets into the stone from the back.) However, unless you’ve set up a bezel or other way to trap the stone in place, you’d have to glue it in, something that you wouldn’t want to do with an expensive stone because it is certain that it would eventually fall out.

--A third option that I’ve thought of but don’t know if it would work with PMC, is to purchase a pre-made prong setting (that fits the stone) and fire the bottom of it right into the clay of the piece, making sure that a bit of clay anchors the setting in permanently. Then you can set your gemstone safely into the setting after firing. Might be worth experimenting with...Any other comments or tips are welcome.

Next posting, my thoughts about the importance of cut in faceted gemstones…